ORNL-TM-4174.txt

 

ORNL-TM-4174

 

POSTIRRADIATION EXAMINATION OF
MATERIALS FROM THE MSRE

H. E. McCoy
B. McNabb

 

 

 

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This report was prepared as an account of work sponsored by the United
States Government, Neither the United States nor the United States Atomic
Energy Commission, nor any of their employees, nor any of their contractors,
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed, or
represents that its use would not infringe privately owned rights.

 

 
 

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ORNL-TM-4174

Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

POSTIRRADIATION EXAMINATION OF MATERJIALS FROM THE MSRE

H. E. McCoy B.McNabb

DECEMBER 1972

 

 

} NOTICE
" This report was prepared ss an account of work
sponsored by the United States Government. Neither

* ; the United States nor the United States Atomic Energy
Commission, nor any of their employees, nor any of
their contractors, subcontractors, or their employees,
makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, com-
pleteness or usefulness of any information, apparatus,
product or process disclosed, or represents that its use i

- would not infringe privately owned rights, l

 

 

 

f ;
OAK RIDGE NATIONAL LABORATORY fiy
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION

~ PISTRIBUTION OF THIS DOCUMENT IS UNLI ITED

 
 

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CONTENTS
ADSIIACE . ot ittt ittt ittt it aa e e it et 1
Introduction . .............. @t e e et e e, 1
The MSRE and Its Operation .. ... ..c.ovuiiniiiininrnneneoeneeasosenenrasascasosoannsns 1
DS P O . L o ettt et it e i ie e e e 2
HStOTY « ot iee et et ie ettt aas it as i et 2
Examination of a Graphite ModeratorElement ........ ... ... ... il 6
Description of Graphite Elements ......... ..o oo 6
Visual Examination of Element 1184 . ...... e e et e e e 8
Chemical Analysis of the ModeratorElement .. ......... .. ..o, 10
Summary of ObServations .. ..........uneinitmiiiiinti it 14
Examination of the Graphite and INOR-8 Surveillance Specimens . .. ...............ocoiivinnn. 14
Examination of INOR-8 Control RodThimble ......... ... i iiiiiiiiiiiann. 15
Physical Description .. ........coiiiieiiininiiniinnannn e ittt 15
UndeformedSamples ...........oiiiiiiiieiiininnann, et etare e 18
Deformed Samples ... ..ovvttiii i i et 23
Summary of Observations . ............uururanniinnenneaoiiiiteteeeiiaaaaes 31
Examination of Freeze Valve 105 ... ... ittt ittt rrieectaonnansrtonnsnnns 31
Physical Description ..............c.ouann. e e e ee e e e 31
Visual and Metallographic Examination ... ..... ... oottt 31
Mechanical Property Tests .. .....ccuiiitiiiirnnnroieascientnnnntnrsoensaosaecsensas 39
Summary of Observations ..............ccceiviiirnnnn et eeesaiaeeceae e 44
Examination of the Sampler Assembly ... ..........oiiiiiiiiiiinon.ts e 44
Physical Description ........ S P 44
11 ) (3 o 021 S R R RN 47
Mist SHield .. .ovie ettt iii e iiettiaeeasaaeaaar st aaa e 54
Summary of Observations ................. Versanen e Cernesieetsanaacesaues 62
Examination of a Copper Sample Capsule ............ e ettt ieree it 65
Physical Description . ... ... e et P 65
Examination...........ccoiiiiiiiennones ettt ia et aanee e 65
Summary of Observations . ... ....uuuiiiniieentor oottt 69
Examination of the Primary Heat Exchanger ....................... I ... 69
Physical DesCrpPtON ... vvvves vt teninteeseneeenmersonsstrtaneranenaaateessuneanns 69
Examination......... e e e ettt et 70
Summary of ObServations .. .........iieeeirenreiae ittt 79

iii
 

Examination of the Coolant Radiator .................... e ettt iteiataea e 79
Physical Description . ... c.iiitiuiitiniiiii ittt ieatarettaratanrensaoasansnacnans 79
DS VAt OIS . o o v ottt eet e ittt ittt et e st e et e 80
Summary 0f Observations .. .....ciiiiiiiadaiieriititireetaataateacataanantrssaaanns 92
UMY L.ttt et ittt i ieseeeeeoaaaasocneasasosusnsnsosnnensnensasosensssnaasanss 92
Acknowledgment ... ... .. oo i i i e i it i e e s 95

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POSTIRRADIATION EXAMINATI'ON OF MATERIALS FROM THE MSRE
H.E.McCoy B.McNabb

- ABSTRACT

The Molten-Salt Reactor Experiment operated very successfully. The fuel loop was above 500°C
for 30,807 hr and contained fuel salt for 21,040 hr. A surveillance program was active during
operation to follow the property changes of the graphite moderator and the INOR-8 structural
material. After operation was discontinued in December 1969, several components were removed for
examination. These included a graphite moderator element from the core, a control rod thimble,
freeze valve 105, the sample cage and mist shield from the fuel salt pump bowl, a copper sampler
capsule, tubes and a portion of the shell of the primary heat exchanger, and tubes and two

" thermocouple wells from the air-cooled radiator,

Examination of these materials showed excellent mutual chemical compatibility between the
salts, graphite, and INOR-8. The INOR-8 exposed to fuel salt formed shallow intergranular cracks
believed to be due to the ingress of the fission product tellurium. The INOR-8 was also embnttled by
exposure to thermal neutrons, and this was attributed to the formation of helium by the ! B(n,a) Li
transmutation. ‘

INTRODUCTION

The Molten-Salt Reactor Expenment was a unique fluid- fuel reactor' It operated at temperatures
around 650°C for more than 30 ,000 hr between 1965 and December 1969. Operation was terminated in
1969 because the technical feasibility and promise of molten salt systems had been demonstrated, and the
operating funds were needed for development work associated with advanced concepts of molten-salt
reactors. ' S |

Surveillance samples of graphite and INOR-8 were removed periodically during operation of the MSRE,
and these were examined in detail. After operation, parts of several components were examined. The details
of the examinations of the various sets of INOR-8 surveillance samples were reported, 25 and some of the
observations made on the various components were discussed in a topical report® dealing with intergranular
crackmg of INOR-8. The present report consolidates the observations made on the surveillance samples and
the components. The components include a control-rod thimble from the core, a freeze valve that isolated
the reactor vessel and a fuel drain tank, the salt sampler cage and mist shield from the fuel salt pump bowl,
a portlon of the shell and several tubes from the primary heat exchanger; two thermocouple wells from the
coolant circuit, and several tubes from the air radiator i in the coolant c1rcu1t :

* THE MSRE AND ITS OPERATION

The MSRE and an account of most of its history are described by Haubenreich and Engel,! and Robert-
son’ has described in detail all components and systems. Because these references are widely available, the

 

1. P. N. Haubenreich and J. R. Engel, “Experience with the MSRE,” Nucl. Appl. Technol. 8, 118 (1970).
-2, H. E. McCoy, An Evaluation of the Molten-Salt Reactor Experiment Hastelloy N Surveillance Specimens — First
Group, ORNL-TM-1997 (November 1967).
- 3. H. E. McCoy, An Evaluation of the Molten-SaIt Reactor Expenment Hastelloy N Surveillance Specimens — Second
Group, ORNL-TM-2359 (February 1969).
4, H. E. McCoy, An Evaluation of the Molten-Salt Reactor Expenment Hastelloy N Survetllance Specimens — Third
Group, ORNL-TM-2647 (January 1970).
5. H. E. McCoy, An Evaluation of the Molten-Salt Reactor Experiment Hastelloy N Survetllance Specimens — Fourth
Group, ORNL-TM-3063 (March 1971).
6. H. E. McCoy and B. McNabb, Intergranular Cracking of INOR-8 in the MSRE, ORNL-4829 (November 1972).
7. T. C. Robertson, MSRE Design and Operations Report, Part I — Description of Reactor Design, ORNL-TM-728
(January 1965)..
 

 

description here is restricted to that necessary for the reader to understand the function of each component
- and the significance of the postoperation examination.

Descnptron

The parts of the MSRE with which we will be concerned are included in the flowsheet in Fig. 1. The

- MSRE consisted basically of the primary circuit mcludmg the reactor vessel, a fuel pump, and an
intermediate heat exchanger a coolant circuit including the tube side of the intermediate heat exchanger, a
coolant pump, and an air radiator; and several auxiliary components associated with fuel and coolant salt
storage and fission gas- processing. Al metallic parts that contacted salt were made of a nickel-base alloy
known as INOR-8 and now available commercially under the trade names of Hastelloy N and Allvac N. This
alloy, developed at Oak Ridge National Laboratory® specifically for use in fluoride salts, has the nominal

' composition Ni—16% Mo—7% Cr—5% Fe—0.05% C. The graphite moderator was made of a special
low-porosity graphite, grade CGB, to exclude salt from the. graphite pore structure.’ The graphite was
produced in the form of bars 2.5 in. square by 72 in. long. These were machined to 2 in. square with a
channel on each face for fuel salt flow.

The fuel salt composition was LiF-BeF, -Z1F 4 -UF,4 (65-30-5<1 mole %); the coolant was LiF—34 mole
% BeF, . At full power the 1200-gpm fuel stream normally entered the reactor vessel at 632°C and left at
654°C; the maximum outlet temperature at which the reactor operated for any substantial period of time
was 663°C (1225°F). When the reactor was at low power, the salt systems were usually nearly isothermal at
about 650°C. During extended shutdowns the salt was drained into tanks, where it was kept molten while
the c1rculat1ng loops were allowed to cool. Plugs of salt frozen in flattened sections of pipe (“freeze
valves™) were used to isolate the drain tanks from the loop. The liquidus temperature of the fuel salt was
about 440°C and that of the coolant salt was 459°C, so the loops were heated to 600 to 650°C with
external electric heaters before the salt was transferred from the storage tanks Hehum (sometlrnes argon)
was the cover gas over the fuel and coolant salts.
~ During operation, samples of fuel salt were obtained by lowering small copper buckets (capsules) into
the pool of salt in the pump bowl. The pump bowl served as the surge space for the loop and also for
separation of gaseous fission products from a 50-gpm stream of salt sprayed out into the gas space above
the salt pool. To protect the sample bucket from the salt spray in the pump bowl, a spiral baffle of INOR-8
extended from the top of the bowl down into the salt pool. A cage of INOR-8 rods inside the spiral baffle
guided the sample capsule in the pump bowl. | o
The fuel system was contained in a cell in which an atmosphere of nitrogen containing from 2 to 5%

oxygen was maintained. This containment atmosphere was recirculated through a system that provided
cooling for the control rods and the freeze valves. Most of the coolant piping was exposed to air.

History

‘The history of the MSRE during the four years in which it operated at significant power is outlined in
Fig. 2. Construction was finished and salt was charged into the tanks late in 1964. Prenuclear testing,
including 1100 hr of salt circulation, occupied January through May 1965. During nuclear startup
experiments in May through July 1965, fuel salt was circulated for 800 hr. The salt was dreined_, and final

 

H. E. McCoy, “The INOR-8 Story,” ORNL Review 3(2), 35 (1969).

8 .
9. H. E. McCoy and I. R. Weir, Materials Development for Molten- Salt Breeder Reactors, 0RNL-TM-1854 p. 46 (June

1967).

-
«)

N

  
 
 

 

ORNL-DWG 65- 114108

LEGEND

" FUEL SALT
— COOLANT SALY
sevsnsencsnss HELIUM COVER GAS

RADIOACTIVE OFF -GAS

 
 

 

 

 

 

 

 

 

 

 
  

 

 
  

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

i ——
i
!
-1
. OFF-GAS A
N T A It DT Y (R LR
ABSOLUTE
FILTERS
BLDG.
ra’vtmurm
STACK  FAN b emrtfimetsans 0
: : X FROM ! FREEZE VALVE (TYP)
~tign COOLANT ; . L g
F" SYSTEM t"
¢ __.fi;r;-:
i -
. y ¥ o
1 1 ABSOLUTE
g -
P o WATER STEAM Zo_ b #o FILTERS
WATER STEAM L : s
t i goerdliesd
ODOLANT
DRAIN
TANK

 

 

 

 

 

  
   

 

 

 

 

 

 

 

 

 

 

Fig. 1. Design flowsheet of the MSRE.

 
 

 

 

INVESTIGATE
OFFGAS PLUGGING

REPLACE WALVES
AND FILTERS

RAISE POWER
REPAR SAMPLER
ATTAN FULL POWER

CHECK CONTAINMENT

FULL ~POWER RUN
=— MAIN BLOWER FALURE

‘REPLACE MAIN BLOWER
MELY SALT FROM GAS LINES
REPLACE CORE SAMPLES
TEST CONTAINMENT

RUN WITH ONE BLOWER
> WNSTALL SECOND BLOWER

ROD QUT OFFGAS LINE
CHECK CONTAINMENT

30-doy RUN
AT FULL POWER
}REFLEEARLNE
- DISCONNECTS

- SUSTAINED  OPERATION
AT HIGH POWER

 

REPLACE CORE SAMPLES
© TEST CONTAINMENT

 

 

 

02 4868 89
POWER {Mw)

Fig. 2. Qutline of the four years of MSRE power operation.

 
 

SALT N
FUEL LOOP POWER

 

 

S
—

 

 

 

    

ORNL~DWG 69-.T253R2

XENON STRIPPING

INSPECTION AND

REPLACE CORE SAMPLES

TEST AND MODIFY
FLUORINE DISPOSAL

PROCESS FLUSH SALT
PROCESS FUEL SALT
LOAD URANIM -233
REMOVE LOADING DEVICE
233y 7ERO -POWER
PHYSICS EXPERMENTS
INVESTIGATE FUEL

SALT BEHAVIOR

CLEAR OFFGAS LINES

REPAR SAMPLER ANO
CONTROL ROD DRIVE
233, DYNAMCS TESTS
INVESTIGATE GAS
W FUEL LOOP

HIGH-POWER OPERATION
10 MEASIRE B3y o /e,

INVESTIGATE COVER GAS,
XENON, AND FISSION
PRODUCT BEHAVIOR

ADD PLUTONIUM
IRRADIATE ENCAPSULATED U

MAP F.P. DEPOSITION WITH
GAMMA SPECTROMETER
MEASURE TRITIUM,
SAMPLE FUEL

REMOVE CORE ARRAY
PUT REACTOR IN STANDBY

%, .
 

)

ORNL-DWG T0-2164

120
Ho

100

  
     

8

CHROMIUM (ppm)
3

40

 

 

30

RUN 8 1

FLUSH & ¢ tie 2
DJFMAMJJASONDJFMAMJJASONDJFMAMJJASONDJFMAMJJASOND
19€9

1966 1967 1968 -

Fig. 3. Corrosion of the MSRE fuel circuit in 235y and 233y power operations, as measured by chromium
concentration in the fuel salt.

preparations for power operations were made in the fall of 1965. Low-power experiments in December led
into the history covered in Fig..2 (see Haubenreich and Engel’° and MSR Program semiannual progress
reports for more detail).

The nuclear fuel was 33%-enriched 23U, and the UF, concentration in the fuel salt was 0.8 mole %
until 1968. Then the uranium was removed by fluorination and 2*2UF, was substituted. The UF,
concentration required with 223U was only 0.13 mole %. The composition of the fuel salt was observed by
frequent sampling from the pump bowl.!! Aside from the 23U loading and periodic additions of small
increments of uranium or plutonium to sustain the nuclear reactivity, the only other additions to the fuel
salt were more or less routine small (~10-g) quantities of beryllium and, in two or three experiments, a few
grams of zirconium and FeF,. The purpose of these additions was to adjust the U(III)/U(IV) ratio, which
affects the corrosion potential and the oxxdatlon state of corrosmn-product iron and nickel and fission
product niobium. '

The primary corrosion mechanism in the fuel salt system was selective removal of chromium by

-

2UF, + Cr(in alloy) = 2UF; + CrF,(in salt) ,

and the concentration of chromium in salt samples was the primary indicator of corrosion. Figure 3 shows
chromium concentrations observed in the MSRE fuel over the years of power operation. The step-down in
chromium concentration in the salt in 1968 was effected by processing the salt after the 235U fluorination.
The total increase in chromium in the 4700-kg charge of fuel salt is equivalent to leaching all of the
chromium from the 852 ft*> of INOR-8 exposed to fuel salt to a depth of about 0.4 mil.

Since the coolant salt did not contain uranium, the corroslon rate was extremely low. During operation,
the chromium content of the coolant salt remained at 32 ppm, w1th1n the accuracy of the analysis.

 

10. P. N. Haubenreich and J, R. Engel, “Experience with the MSRE,” Nucl. Appl. Technol. 8, 118 (1970).
11. R. E. Thoma, Chemical Aspects of MSRE Operation, ORNL-4658 (Decembér 1971).
 

EXAMINATION OF A GRAPHITE MODERATOR ELEMENT

Description of Graphite Elements

The properties of the grade CGB graphite used to fabricate the moderator elements are given by McCoy
and Weir.!2 It is basically a petroleum needle coke that was bonded with coal-tar pitch, extruded to rough
shape, and heated to 2800°C. High density and low permeability were achieved through multiple
impregnations and heat treatments. The product was well graphitized and highly anisotropic. |

The graphite was produced as bars 2.5 in. square by 72 in. long. These bars were machined to several
configurations, but most of them had the geometry shown in Fig. 4. These elements were assembled as
shown in Fig. 5 to form the core. The elements fit together to form channels for salt flow. Four moderator
blocks were left out to leave spaces for three control rod thimbles and a surveillance assembly. The five
elements “enclosed” by the four spaces had INOR-8 lifting lugs so that they could be easily removed for
examination.

 

12. H. E. McCoy and J. R. Weir; Materials Debelo_pment for Moiten-Salt Breeder Reactors, ORNL-TM-1854, p. 46
(June 1967). - ' :

ORNL~LR-DWG 56874 R

    
   

TYPICAL MODERATOR STRINGERS

SAMPLE PIECE

NOTE: NOT TO SCALE

 

Fig. 4. Typical graphite stringer arrangement.
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Fig. 5. Reactor core block assembly plan, drawing D-BB-B-40416.

 

 

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Visual Examination of Element 1184

One of the five removable graphite core moderator elements (1184) was examined in the
High-Radiation-Level Examination Laboratory (HRLEL) after completion of the operation of the MSRE.
Visual examination revealed machining marks still plainly visible in the fuel channels and on surfaces not
exposed directly to flowing fuel salt, indicating little if any change during operation. Figure 6 is a
photograph of the fuel channel facing the center of the reactor, and it shows the excellent condition of the
core block. The small thumbnail flaw in the fuel channel about 39 in. from the top is likely a small crack
that was in the bar during fabrication, since tool marks are on the top of the flaw and skip over an area at
the end of the flaw. The few scratches on the surface are not very deep and were likely caused by handling.

- There was a crack in the graphite around the INOR-8 lifting stud that was screwed into the top of the
bar. This crack extended about halfway around the bar, as shown by the short arrows in Fig. 7. The long
arrow indicates the same channel in the different views. The small punch mark visible in the top view was
made to indicate the fuel channel toward the center of the reactor, with a control rod on one side and the
surveillance sample basket on the other side, The crack in the graphite did not affect the integrity of the
bar, since it was on the end, and several threads remained to secure the lifting stud. The crack was pro‘bably
caused by the approximate twofold difference in thermal expansion of graphite and INOR-8.

Length, width, and bowing were measured. There were no significant changes — all measurements were
within the original tolerance for fabrication of the core block. The measured length of the approximately
rectangular portion of the bar was 64.50 £ 0.03 in. Thus, there was no measurable change in length. Width
measurements across the flat parallel surfaces were made by reading 1- to 2-in. micrometers through the
Kollmorgen periscope. The original dimensional tolerances were 1.998 T%,9%% in. The measurements shown
in Table 1 were made at approximately 1-ft intervals from top to bottom of the side facing the center of
the reactor. Measurements were also made near the top, middle, and bottom at 90° to those listed in Table
1 on the sides that had 0.0375 in. relief on each side for flow between the channels starting 2 in. from
either end. Values obtained were: top = 1.9226 in., middle = 1.9231 in., and bottom = 1.9230 in. All
measurements were within tolerance, so we believe that there were no significant changes or observable
trends, even though no actual measurements of the individual bars were recorded before operation.

The bowing was measured by stretching a string tightly between two C-clamp fixtures at either end of
the bar and measuring the distance between the bar and the string with a cathetometer, then rotating the

7 PHOTO 0370-71

 

Fig. 6. One of the removable graphite moderator elements after operation in the MSRE, showing the fuel channel
facing the center of the reactor. Machining marks are plainly visible in the fuel channel, showing excellent condition of the
core block. e :

 
 

 

 

 

 

 

i
i
P
t
I
i

 

b

 

Fig. 7. Top of the graphite moderator element. The short arrows show the crack originating at the INOR-8 lifting stud
and extending partway around the graphite block. The long arrow indicates the same channel in the different views. The
crack was probably caused by the approximate twofold difference in thermal expansion between the INOR-8 and the
graphite during thermal cycling of the reactor.

Table 1. Dimensional measurements
of graphite moderator element

 

Measured width?

 

" Location (in)
“Top . 19947+
1ft - 1.9954
2ft _ 1.9944
3ft 019948
4 ft _ ' 1.9942 - _ o ;
Bottom 1.9971 '

 

"Dimensional tolerances . - were
1.998+0-909 in, so that the width _
could vary tom 1.998 to _1.993 in. .

bar 90°, sighting on another bar surface, and repeating the measurement. The maximum obsen}ed bowing
was 0.012 in., with the string being closér to the bar in the center, possibly indicating slight sagging of the
string. We concluded that there was no appreciable ‘bowing. No tolerances for bowing were included in the
specifications, and the bar was not supported on a flat surface; absolute measurements were not made or
considered necessary, since the bar was removed easily from the MSRE core. ’
Figure 8 shows the bottom of core block 1184 as it appeared in 1964 during the assembly of the core
and (inset) as it appeared in the HRLEL after operation for the full life of the MSRE (including 22,000 hr

 
 

 

10

 

Fig. 8. Comparison of the bottom positioning stud on the graphite moderator element when initially installed in the
MSRE and after operation. The large picture was made before the reactor was operational, and the small picture in the
lower right corner was made in the hot cells after the moderator element had been at temperature for over three years.

exposure to molten salt). There is no apparent change, as the small cracks and the small chip missing from
the bottom are apparent in both photographs, and the numbers are just as legible as before operation.

A cross section of the INOR-8 lifting stud in contact with the graphite is shown in Fig. 9. There appears
to be little if any reaction with the graphite. There is a surface modified layer about 1 or 2 mils deep similar
to that observed on surveillance samples and attributed to cold working of the surface and subsequent heat
treatment during service, There appears to be very little difference in the side of the thread in contact with
the graphite and the root of the thread, which probably was not in intimate contact.

Chemical Analysis of the Moderator Element

Small samples were core-drilled from various locations of the moderator element for chemical analyses
by the spark-source mass spectrograph and by Auger spectroscopy. Small electrodes (% in. diam) were used
in the spark-source mass spectrograph. Small amounts of the electrodes were vaporized and the
concentrations of various atomic masses measured. In Auger spectroscopy the surface of the sample is

bombarded with electrons. Vacancies are created in the inner electron shell of some atoms, and some of the

excited atoms revert to the ground state by emitting an electron as a result of a radiationless
transition.!3~1$ This Auger electron has an energy that is characteristic of the parent atom. Only the atoms
in the first one to three layers are involved, and excellent sensitivities are attainable.

 

~13. L. A, Harris, J. Appl. Phys. 39, 1419 (1968).
14. L. A, Harris, J. Appl. Phys. 39, 1428 (1968).
15. R. E. Clausing, MSR Program Semiannu. Progr. Rep. Feb. 28, 1971, ORNL4676, pp. 143-45,
 

 

o«

wi

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11

PHOTO 0372-71

 

Fig. 9. INOR-8 thread that was in contact with graphite for about three years at 650°C. The modified layer is likely
due to working effects from machining the thread. Reduced 35%. '

. The results of the mass spectrographic analyses are given in Table 2. These data are quite useful, but
several factors limit their accuracy. The penetration of the spark into the graphite was likely a hemisphere,
so some new surface continued to be included as the penetration depth increased. The calculated depths are
estimates of the maximum depths sampled, and the sensitivity of the analysis increases with increasing
current. The size of the area sarripled is also of concern, particularly at the lower penetration depths. This
graphite has pores about 0.1 um (1000 A) in diameter, and the compositions of the first samples (taken
from areas a few angstroms in diameter) could be quite dependent upon the location. These factors make it
impossible to obtain a quantitative concentration profile, but it appears that the fission products are
concentrated at the surface and decrease rapidly with depth.

Auger analyses were performed on several samples from the MSRE. The results are summanzed in Table
3. Sample 6, the first analyzed, was taken from a core surve:llance.specunen removed from the MSRE in
April 1968 at the conclusion of operation with 2 3§_U fuel. The other samples were all 0.3-in.-diam plugs
taken by core drilling various parts of the graphite moderator element 1184. The data in Table 3 are
normalized relative to a carbon peak of 100, and peak heights are given in arbitrary units. The data are not
quantitative due to lack of suitable standards, and because the sensitivity is different for different elements
and for different peaks for the same element, peak height cannot be equated to concentration.
 

 

12

Table 2. Concentration of elements on surface of MSRE graphite stringer at reactor midplane

 

Concentration (wt %) at various maximum penetration depths (A)

 

 

| Mass Element ~25 ~25 800 900 2000 3000 4000
i Fission Products Detected
87 , Rb : , ' 0.0018 '
89 Y : _ 0.0044
90 Sr 0.0067 :
Fission% Mo 36 2.4 0.59 0.09 0.26 0.30 0.24
93 Nb , 0.003 o 0.002 0.001
99 Tc 0.57 0.39 - 0.09 0.02 0.05 0.05 . 0.04
' 101,102,106 Ru 14 . 0.95 0.19 0.04 0.11 0.09 0.09
103 ~ Rh 0.34 022 0.055 - 0.01 0.03 0.02 0.02
110 | Pd ‘ 0.11 0.053 0.01 0.02 0.015 0.01
109 Ag . - 0.0073 S 0.003
114 ‘ cd T . 0.0046 0.015 0.007
125 Sb . . 001 - 0.0057 0.005 0.004
130 Te 041 - 030 0046 . 0.009 0.02 002 001
133 . . La T 0.004 o 0.006 0.003 0.002
138 Ba 0.005 L ‘
140 Ce , T 0.007 001 0.006 0.005
141 Pr 0.003 _ 0.003- . 0.003
144 Nd 001 0.01 - 0.006
147 Pm 0.0008
Nonfission Elements Detected .
Mo 0.87 0.58 0.20 0.03 0.02 0.11 0.07
Ni 0.9 :
Fe 0.3
Cr 0.05
Mn 0.005

 

ZMass numbers of molybdenum fission products.

Samples 5, 7, and 10 were taken from the flow channel: sample 5, 3 in. from the top; sample 7, in the
center; and sample 10, 3 in. from the lower end of the element. Samples M6 and 13 were taken from the
edge of the graphite moderator element (outside the main flow channel), with sample 13 about 3 in. from
the bottom of the element and sample M6 about halfway along the element.

All the samples examined had substantial amounts of Mo, Tc, S, and Rh. Iron, nickel, and chromium
were present on most of the samples, with the two samples from the bottom of the moderator element
(samples 10 and 13) having the largest concentrations. Tellurium is probably present on all the samples and
is discussed briefly below. Peaks that may be associated with antimony were detectable on samples 10 and
13, although the peaks are not listed in Table 3. An unresolved pair of peaks tentatively identified as due to
palladium were detected at 330 eV on nearly all of the samples.' Tellurium, chromium, and ruthenium
concentrations are difficult to determine because of interference from strong lines of oxygen, carbon, and
molybdenum. The largest amounts of tellurium and chromium are probably similar in quantity to the
largest amounts of iron. We ‘believe that ruthenium is probably present, but the lines are not nearly as
strong as those for molybdenum. Low-energy Auger peaks like that at 20 eV for tellurium are frequently
very strong; thus the large amplitudes listed in Table 3 need not be associated with high surface
concentrations of tellurium. Pure tellurium would have several strong peaks between 400 and 500 eV but
our data do not disclose strong lines on any of the recorded spectra at these energies. c

i1}
 

 

 

o) w - " o
Table 3. Comparison of relative Auger peak heights obtained from surfaces of graphite samples removed from the MSRE core®
‘ Es(tlimit‘ltled Relative peak heights for various energies and indicated elements
: ep .
. Sample (atom 206V, 150 eV, 182eV,  197eV, 302eV, 330eV, 383eV, 510eV, 650eV, 850eV,
~ layers) Te Te, Mo, S Te, Mo Nb Rh Pd N 0 Fe Ni
: -0 . na 29 9 2 na na na na na na
6 {surveillance specimen) 10 ‘na- 5 23 4 na na ‘na na na na
- 25 na 4 16 3 na na na na na na
. , -0 na na na na na na na na na na
7 (flow channel, center) 15 na 19 40 nd 9 na 12 25 na na
: 30 na 26 50 nd 11 na 17 26 na na
o 0  na 13 5 w 4 w. 2 14 nd w
5 (flow channel, top) 15 - 1,600 35 16 w 5 1 1‘3 37 2 1
- ‘ .. 0 na na na na na na na na na na
10 (flow channel, bottom) 16 4,000 63 13 w 2 w 13 96 3 2
. - -0 na 15 21 w 1 w 8 24 nd nd
M6 (edge, center) 15 nd 24 15 w 2.5 w 5 19 nd nd
: 0 4,300 13 14 2 6 2 15 64 5 1
13 (edge, bottom) 15 16,000 131 24 5 8 w

17 97 10 11

 

Gy = wéak, nd = r_ldt detected, na = not analyzed.

€1

 

 
 

14

Summary of Observations

One graphite moderator element that had been in the MSRE during its entire operation was removed
and was examined in the hot cells. The element was in excellent mechanical condition, and no dimensional
changes were detectable. One small crack, which appeared to have been present in the element before
installation, was noted in one of the fuel channels, and a small crack was formed adjacent to an INOR-8
lifting stud by differential expansion during thermal cycling.

Some samples were taken from the moderator element and from one surveillance assembly for chemical
analysis. Techniques involving the spark source mass spectrograph and Auger spectroscopy were utilized.
These techniques were not sufficiently developed and the number of samples was too few to warrant
sweeping conclusions. However, most of the fission products and the elements in INOR-8 were noted near
the graphite surface; penetration into the graphite was very limited.

EXAMINATION OF THE GRAPHITE AND INOR-8 SURVE[LLANCE SPECIMENS

Two surveillance facilities were available in the MSRE. One was in the core in the lower-eft open
position shown in Fig. 5 and contained INOR-8 and graphite specimens that were exposed to the fuel salt.
(The other three positions contained control rods.) Another surveillance facility was outside the core,
where INOR-8 was exposed to the cell environment of 2 to 5% O, in nitrogen. Samples were removed
periodically from both facilities. They were examined visually, and the INOR-8 samples were subjected to
mechanical property tests and metallographic examination.

The details of the visual examinations were reported previously.! ™! ® The first assembly removed from
the core suffered some damage because relative movement of the graphite and INOR-8 parts was
restricted.!® Minor design modifications eliminated this problem. The graphite in each surveillance
assembly was in excellent condition; machining marks were clearly visible, and we found no evidence of
chemical reaction with the fuel salt. The peak fluence received by the graphite was about 1 X 10%!
neutrons/cm? (>50 keV), and the resulting dimensional changes should have been less than 0.1%.2° These
dimensional changes should not have resulted in significant changes in the mechanical properties of the
graphite, so no tests were run on the samples. Numerous samples were used to study fission product
deposition and penetration into the graplnte, and the results of these studles have been summarized
elsewhere.?

INOR-8 specimens were removed from the surveillance facilities inside and outside the reactor vessel.
These specimens were examined visually and metallographically and were subjected to various mechanical
property tests. The details of these examinations are presented in other reports??2™25 and will only be
summarized here.

16-

 

16. W. H. Cook, MSR Program Semiannu. Progr. Rep. Aug. 31, 1966, ORNL-4037, pp. 97—-102.
17. W. H. Cook, MSR Program Semiannu. Progr. Rep. Aug. 31, 1967, ORNL4191, pp. 196-200.
18. W. H. Cook, MSR Program Semiannu. Progr. Rep. Aug. 31, 1968, ORNL-4344, pp. 211~-15.
19. W. H. Cook, MSR Program Semiannu. Progr. Rep. Aug. 31, 1969, ORNL-4449, pp. 165—-68.
20. C. R. Kennedy, MSR Program Semiannu. Progr. Rep. Aug. 31, 1968, ORNL-4344, pp. 233--35.

(1972). :
22. H. E. McCoy, An Evaluation of the Molten-Salt Reactor Experiment Hastelloy N Surveillance Specimens — First
Group, ORNL-TM-1997 (November 1967).

23. H. E. McCoy, An Evaluation of the Molten-Salt Reactor Expenment Hastelloy N Surveillance Specimens — Second
Group, ORNL-TM-2359 (February 1969).

24. H. E. McCoy, An Evaluation of the Molten-Salt Reactor Expenment Hastelloy N Surveillance Specimens — Third
Group, ORNL-TM-2647 (January 1970).

25. H. E. McCoy, An Evaluation of the Molten-Salt Reactor Experiment Hastelloy N Surveillance Specimens - Fourth
Group, ORNL-TM-3063 (March 1971). :

21. M. W. Rosenthal et al., The Development Status of Molten-Salt Breeder Reactors, ORNL-4812, pp. 116-35

4

X
 

o}

#)

15

Specimens outside the reactor vessel were above 500°C up to 17,483 hr and received a peak thermal
fluence of 2.6 X 10'° neutrons/cm?. They were oxidized, and metallographic examination revealed that
the oxide penetrated about 5 mils. The specimens from the core were exposed to fuel salt and were only
slightly discolored. They were above 500°C for periods up to 19,136 hr and received thermal fluences up to
1.5 X 10?! neutronsfcm?. Metallographic examination revealed only minor changes in microstructure. A
shallow layer of modified structure less than 1 mil thick was noted. This layer etched more rapidly and
contained some lamellar product. The amount of this modified layer did not vary systematically with time
or temperature of exposure. Experiments produced evidence that the effect was likely due to cold work
from machining, causing carbide to precipitate more rapidly near the surface during h1gh -temperature
service. This structural change is not of significant importance.

The postirradiation mechanical property tests on the INOR-8 samples showed that the high?t_emperature
fracture strain of the alloy was decreased by irradiation. This is attributed to the helium produced by the
10B(n,a)?Li transmutation. However, considerable progress has since been made in developing an alloy
with modified chemical compositiori that has improved resistance to helium-induced embrittlement.?®

Metallographic examination of the deformed surveillance samples revealed shallow intergranular
cracking, particularly in samples deformed at 25°C. This type of cracking indicates intergranular
embrittlement, and considerable evidence shows that the embrittlement is due to the inward diffusion of
the fission product telturium.?” Although the cracks were only a few mils deep in samples from the MSRE,
there is considerable concern over how cracks would propagate by this process in a power reactor over a
30-year operating period.

o EXAMINATION OF INOR-8 CONTROL ROD THIMBLE

Physical Description

The MSRE used three control rods fabricated of Gd,0; and Al,0; canned in Inconel 600.2® The
control rods 0peiated iriside' thimbles made of 2-in.-OD X 0.065-in.-wall INOR-8 tubing. The assembly
before insertion in the MSRE is shown in Fig. 10. Thimble 3 was examined in some detail.

The detailed shop drawings for the thimble are shown in Fig. 11. The thimble tubing was made from
INOR-8 heat Y-8487, and spacers were machined from INOR-8 heat 5060 (see Table 4 for compositions).
The spacers were joined to the thlmble by beads of weld metal that were deposxted on the tubmg through
clearance holes in the sleeves.

The exterior of the thlmble was exposed to fuel salt and the intefior to the cell enwronment of N, +
O,. The tubing was above 500°C for 30,807 hr and received a peak thermal fluence of 1.9 X 10?1
neutrons/cm?. The lower portion of the control rod thimble was severed by electnc arc cutting and moved
to the hot cells for examination. Flgure 12 shows the electric arc cu_t at the left (about at the midplane of
the core),-the spacer sleeves, and the end closure (located at the bottom of the core). The fuel salt appears

“to have wet the INOR-8 near the center of the core, where the tempereture and flux were highest (as shown

around the left and_;center;sp'acer 'sleeves), but not at the bottom (as shown on the _right sleeve). Some of

 

26, M. W. Rosenthal P. N. Haubenrelch H. E. McCoy, and L. E. McNeese, “Recent Progress in Molten-Salt Reactor
Development,” At. Energy Rev. 9(3), 60150 (September 1971).
27. H. E. McCoy and B. McNabb, Intergranular Cracking of INOR-8 in the MSRE, ORNL-4829 (November 1972).

28. G. M. Tolson and A. Taboada, MSRE Control Elements: Manufacture, Inspecnon Drawings, and Specifications,
ORNL-4123 (July 1967).
 

Fig. 10. Control rod thimble assembly before insertion in the MSRE,

Table 4. Heats of INOR-8 examined after exposure in MSRE

 

| PHOTO 74410 §

 

 

 

 

. Content (%)
Heat Location
Mo Cr Fe Mn C Si S P Cu Co Al v Ti w B
Y-8487 Control rod thimble 168 73 41 03 0.05 0.17 0.0075 0.004 0.03 0.1 0.16 0.25 0.007
5060 Rod thimble sleeve 164 7.05 39 045 0.06 0.52 0.006 0.001 001 0.07 001 0.28 0.01 0.005
5094 Freeze valve 105 163 7.1 3.8 0.52 0.07 0.76 0.007 0.001  0.01. 0.08 0.02¢ 0.39 0.05 0.004
5059 Sampler cage 169 6.6 39 035 007 0.59 0.003 0.001 0.07 0.01 0.21 0.01 0.04
5075 Sampler mist shield 164 6.6 40 046 0.07 0.58 0.006 0.003 0.01 006 0.02 0.26 0.02 = 0.09 0.001
5068 Heat exchanger shell 16.5 6.45 4.0 045 0.05 0.58 0.008 0.03 002 0.1 0.01 0.27 0.01 _ -
N2-5101 Heat exchanger tubes 164 6.9 39 045 0.06 0.60 0.009 0001 001 0.1 0.01 0.33 0.01 0.06 0.006
5097 Radiator tubes 16.2 7.0 42 047 0.06 0.62 0.01 0.001 0.01 0.18 0.02¢ 0.33 0.20 - 0.02
24717 Thermocouple wells 163 7.1 43 004 0.057 0015 0.003 0.008 0.10 0.14 0.055 - 0.10 047
Y-8699 Radiator header 169 638 32 030 0.06 0.13 0.003 0.03 0.20 0.002 0.24
" 8Al plus Ti.
‘e o« H « "

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Fig. 11. Control rod bottom thimble assembly and details, drawing BB-B-56346E.

 

 

 

 
18

 

Fig. 12. Portion of control rod thimble examined after operation of MSRE was terminated. The thimble is made of
2-in.-OD X 0.065-in.-wall INOR-8 tubing (heat Y-8487). The electric arc cut on the left was near the center line of the
core. :

the variations in light and dark areas are due to uneven lighting for the several photographs that make up
Fig. 12. |

The first saw cut was made through the sleeve and thimble nearest the electric arc cut. The spacer sleeve
had been machined with ribs 0.100 in. high and 0.125 in. wide to position it relative to the graphite

moderator, and the sleeve had four drilled holes through which weld beads were deposited on the thimble

to hold the sleeve in place. According to the shop drawings, the minimum and maximum diametral
clearances between the thimble and the sleeve were 0.000 and 0.015 in, respectively. Thus salt would likely
enter this annular region and be in contact with most of the metal surfaces.

Undeformed Samples

Samples of the control rod thimble and the spacer sleeve were cut and examined to determine their
condition at the end of service. Typical photomicrographs of the inside of the thimble tube are shown in
Fig. 13. This surface was oxidized to a depth of about 2 mils by the cell environment of nitrogen containing
2 to 5% 0, . The oxidation process modified the microstructure to a depth of 4 mils, likely by the selective
removal of chromium. o ‘_ |

Photomicrographs of one of the weld beads are shown in Fig. 14. All of the surface shown was exposed
to flowing fuel salt. Some grains were dislodged near the surface, and grain boundaries are visible in the
as-polished condition to a depth of 1 to 1.5 mils. | |

Photomicrographs involving the interface between the thimble and sleeve are shown in Fig. 15. Figure
15(2) shows the annular region with a separation of about 7 mils and some salt present. Few surface
irregularities are visible at a magnification of 100x, indicating that they are considerably below 1 mil.
Figure 15(b) is a 500x view of the thimble and shows some surface cracks to a depth of 0.3 mil. The
outside of the sleeve is shown in Fig. 15(c); a few grain boundaries to a depth of about 1 mil are visible.
Additional photomicrographs of the sleeve are shown in Fig. 16. The sleeve material does not appear to
have received much work, since the carbide is very inhomogeneously distributed. Grain size is larger than
usual away from the stringers. The inner and outer surfaces both have modified structures. A higher
magnification view of the inner surface [Fig. 16(b)] shows that much of the modification is a high density

of primary carbide. The outer surface [Fig. 16(c)] has a shallow layer of small grains, likely due to a

working operation,

 

 

   

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- Fig. 13. Inner surface of INOR-8 control thimble in as-removed condition. This surface was exposed to cell
environment of N, containing 2 to 5% O,. (@) As polished;_ (b) etched; (¢} etched, typical microstructure. Etchant: aqua
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Fig. 14. Outer surface of INOR-8 control rod thimble showing weld deposit made to hold spacér sleeve. The weld .
surface was exposed to flowing salt. (@) As polished; (b) etched with aqua regia. o _ .

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Fig. 15. As-polished surfaces of INOR-8 control rod thimble and sleeve. (@) Annulus between thimble and sleeve;
thimble surface is in upper part of picture. (b) Outside surface of control rod thimble, showing presence of some salt and
shallow surface cracking. (¢) Outside surface of sleeve; surface exposed to flowing fuel salt.

 
 

 

 

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Fig. 16. INOR-8 control rod sleeve. Etched with aqua regia. (@) General microstructure; inner surface is on left and
outer surface is on right. (b) Inner surface of sleeve. (c) Outer surface of sleeve. Reduced 30.5%.

 

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A second cut was made away, from the sleeve, and typical photomicrographs are shown in Fig. 17. This
part of the thimble was exposed: to flowing salt. Some of the grain boundaries are v1s1b1e in the as-polished
condition. to a depth of about 4 mils, and there is some surface modxficatlon [Fig. 17(a)] Etching [Fig.
17(b)] delineates more of the grain structure and shows the shallow surface modification.

Microprobe scans were run on the thimble samples that were taken from under the sleeve and outside
the sleeve. In the first case the thimble wall was exposed to almost static fuel salt, and no gradients in iron
and chromium concentration could be detected within the 3-um region of uncertainty near the surface, The
sample outside the sleeve, which was exposed to flowing fuel salt, was depleted in chromium to a depth of
almost 20 um and in iron to a depth of 10 um (Fig. 18). Thus the amount of corrosion that occurred varied
considerably in the two regions. Measureménts were not actually made, but we would expect similar results

for the spacer sleeve. The inside surface was exposed to almost static salt and was likely not corroded

detectably. The outside surface was exposed to flowing salt and probably was depleted in iron and
chromium.

Deformed Samples

The next step was to deform some of the thimble and the sleeve to determine whether surface cracks
were formed similar to those noted in the surveillance samples. Since the product was tubular, we used a
relatively quick and cheap ring test. The fixture shown in Fig. 19 was made by (1) cutting % in. through a
1-in.-thick carbon steel plate with a 2.in.-diam hole saw, (2) cutting out the partially cut region with ample
clearance around the hole, (3) cutting the plate in two along the diameter and removing 1% in. of material
on each side of the cut, and (4) tapping a % -in.-diam thread into the two pleces Then rings % in. wide
were cut from the thimble and the sleeve. They fit into the groove and were pulled to failure, with the
resultant geometry shown in Fig. 19. The initial loadmg curves include strain associated with the ring
conforming to the geometly of the grip, so we cannot tell precisely how much the sample is deforming.
Thus the yield strength and the elongation are only relative numbers, but the ultimate tensile strength and
reduction in area obtained from these tests are true values. Obviously, this type of test is deficient in giving
good mechanical property data, but is sufficient for deforming the matenal and observing the mcxdence of
surface cracking. : "

The tensile properties of the rings are shown in Table 5. The test results show the following important
facts: - '

1. All the unannealed specimens of heat Y-8487 tested at 25°C have a “yield stress” of 52,000 to
61,000 psi, with these values appearing to be random in the variables examined.
2. At 25°C the crosshead displacement was about 1.2 in. for the unirradiated tubing and O 4 to 0.5 in.

-for the irradiated tubing. Again the variations from 0.4 to 0.5 in. appeared random.

*3. The yield stresses at 650°C were about equwalent for irradiated and unirradiated tubing. However
the crosshead displacement before fracture decreased from about 0.4 in. to 0.1 in, after irradiation. This
was due to embrittlement from helium formed from the ' *B(r, @)’Li transmutation.

.4. No material of heat 5060 (sleeve matenal) is available for unirradiated tests, but the vendor’s
cé;tification sheet showed a yield stress of 46,300 psi, an ultimate tensile stress of 117,000 psi, and a
fracture strain of 52%. The values that we obtained show higher strengths and lower ductility.

Several of the specimens that had been strained were examined metallographically to determine the
extent of cracking during straining. Photomicrographs of the control rod thimble, which was exposed to
flowing salt, are shown in Fig. 20. The inside surface was oxidized, and the oxide cracked as the specimen
was strained. The oxide should be brittle, but it is important that these cracks did not penetrate the metal.

 
 

 

 

 

 

 

 

 

 

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Fig. 17. INOR-8 control rod thimble Where it was exposed directly to flowing salt. (@) As polished

 

 

 
 

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Fig. 18. Electron mlcroprobe scan of INOR-8 sample from control rod thimble. Thimble had been exposed to fuel salt

for 21,040 hr and had been above 500°C for 30 307 hr.

Table 5. Tensile data on rings from control rod thimble 3 (heat Y-8487)

 

 

- o .  Postirradiation Test Crosshead  Yield  Ultimate Crosshead Reduction
Specimen Condition anneal temperature . speed stress? stress travel in area
"’ (C) - (in./min)  (psi) (psi) (in.) (%)
1 Unirradiated o 25 . 0.05 52,000 114,400 1.20 - 445
2 Unirradiated o 25 0.05 - 56,900 117,500 1.17 © 46.0
3 Unirradiated S S 225 005 58,000 124,300 1.18 - - 430
4 . Unirradiated o - 650 - 0.05 - 39,100 76,700 0.37 - 287
5 _ Unirradiated ' 650 0.002 40,700 62,300 0.20 20.6
3 Irradiated - None 25 0.05 - 54,400 105,100 0.54 23.2
T Irradiated .. None -2 . 005 53,300 102,500 ~ 0.51 29.7
8. Irradiated None _ .25 005 . 60,500 110,800 042 28.5
9 Irradiated None 650 - 0.05 38,300 51,200 0.099 12.1
10 © Irmradiated . ~ None ' 650 - 0.002 34,200 38,200 0.061 - 9.7
.11 Irradiated .=~ 8 hrat 871 C f 25 0.05 -49,300 - 100,500 0.36 = 349
12 ‘Irradiated . 141 hrat 871°C 25 0.05 48,700 104,900 0.39 344
13 Irradiated? None 25 - 005 51,700 98,600  0.45 - 329
14 L

Irradjated® None . . =25 - 005 - 42400 123,000 020 18.0

 

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Fig. 19. Fixture for testing rings of control rod thimble and spacer sleeve. A tested sample is shown on the left.

However, the side exposed to the salt exhibited profuse intergranular cracking. Almost every grain
boundary cracked, and the cracks generally propagated to a depth of one grain, although there are several
instances where the crack depth exceeded a depth of one grain. Several photomicrographs were combined
and rephotographed to obtain Fig. 21. This sample had 192 cracks per inch, with average and maximum
depths of 5.0 and 8.0 mils respectively. Many of the cracks shown in Figs. 20 and 21 have blunt tips,
indicating that they did not propagate into the metal as the specimen was strained.

A similar sample was cut from the thimble under the sleeve, where the salt access was restricted.
Photomicrographs of a specimen tested at 25°C are shown in Fig. 22. The cracks are quite similar to those
in Fig. 20 for a sample that was exposed to flowmg salt. The composite photograph in Fig. 23 is quite
similar to Fig. 21 of the sample exposed to flowing salt. The sample in Fig. 23 had 257 cracks per inch,
with average and maximum depths of 4.0 and 8.0 mils respectively. The accuracy of our statistics and
possible sampling inhomogeneities lead us to conclude that the severity of cracking is equivalent in the
samples exposed to flowing and almost static salt. Thus flow rate over the range represented by these two
sample locations does not appear to be an important variable. |

Examination of the spacer sleeve gave us another opportunity to determine whether salt velocity and

cracking were related. The photomicrograph in Fig. 24 shows cracks on both sides, with more being present
in the field photographed on the side where the salt was rapidly flowing. However, the composite
photograph in Fig. 25 shows that cracking is about equivalent on both sides. Cracking statistics on the side

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Fig. 20. INOR-8 control rod thimble specimen eprseci to fléWing salt and tested at 25°C. (@) Fracture — dark edge is
oxide on inner surface of tube; cracks formed on outer surface; etched. (b) Cracks in outer surface of tubing as polished.
(c) Cracks in outer surface of tubing; etched. Etchant: aqua regia. Reduced 30.5%.
   

 

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Fig. 21. Deformed section of INOR-8 control rod thimble. Fracture is on the left. The upper surface was in contact
with flowing salt; the lower surface was exposed to the cell environment of N, plus 2 to 5% O,. The sample is about 0.06
in. thick. '

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Fig. 22. INOR-8 control rod thimble exposed to fuel salt under a spacer sleeve and tested at 25°C. (¢) As-polished view
of fracture. 40X. (b} As-polished view of cracks. (c) Etched view of cracks. Etchant: aqua regia. Reduced 30.5%.
 

 

 

 

Fig. 23. Composite photograph showing density of cracking along edge of portion of deformed INOR-8 control rod
thimble. 9.5X. The sample is 0.58 in. long. The fracture is on the right, the upper surface was exposed to fuel salt, and the
lower surface was exposed to the cell environment of N3 plus 2 to 5% O,.

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Fig. 24. Section of INOR-8 spacer sleeve tested at 25°C; as polished. The upper surface was exposed to almost static
fuel salt, and the lower surface was exposed to rapidly flowing fuel salt.

 

 

Fig. 25. Composite photograph comparing density of cracking on sides of INOR-8 spacer sleeve exposed to almost
static salt (top) and to flowing salt (bottom). 11X. The sample is 0.52 in. long. Reduced 4 3%.
31

. exposed to rapidly. flowing salt revealed 178 cracks per inch, with a maximum depth of 7.0 mils. The side

exposed to restricted salt flow had 202 cracks per inch, with a maximum depth of 5.0 mils. The average
depth was 3.0 mils on both sides. These observatlons also indicate that salt flow rate is not a significant
factor in mtergranular cracking.

Summary of Observations

The control rod th:mble was in excellent phys:cal condition, with only slight discoloration and
chromium depletion to a depth of 0.8 mil (20 um) observed. Mechanical property tests showed that the
thimble was embrittled slightly at 25°C and to a greater extent at 650°C. The reduction in ductility at 25°C
was likely due to carbide precipitation and that at 650°C to helium from the ! °B(n,&)” Li transmutation.

- The examination of the control rod thimble spacer revealed rather uniform cracking of all surfaces
exposed to fuel salt. This points out two important characteristics of the cracking. (1) Irradiation of the
metal was not solely responsible for the cracking. The flux attenuation across the wall of the thimble would
have been very slight, but cracks only formed in the metal on the fuel side (Fig. 20). Thus, irradiation alone
does not cause the cracking, but contact with the fuel salt is required. (2) The severity of cracking was not
sensitive to salt flow rate over the flow range experienced in a fuel channel and under a spacer with
restricted flow.. ' |

EXAM[NATION OF FREEZE VALVE 105
Physncal Descnptlon

Freeze valve 105 falled during the final thermai cycle mvolved in terminating the MSRE. The valve was
used to isolate one of the fuel drain tariks and was frozen only when salt was in that drain tank. The line
was filled with salt from the drain tanks, so the fission product concentration would have been relatively
low. The line was filled with salt at temperatures above 500°C for about 21,000 hr, and the salt was static
~ except when the reactor was being filled. | -

Detailed fabrication drawings for FV-105 are shown in Fig. 26. The MSRE used several freeze valves. In
each a section of tubing was aircooled to freeze a salt plug that prevented salt or gas flow through the pipe.-
The tubing was usually flattened and a thin housmg for the air coolant was welded around the flattened
section. The portion of the housmg parallel to the tubing was kept thin so that it would accommodate the
differential expansion between the pipe and the housmg when the valve was being thawed or frozen. Freeze
valve 105 involved a field modification to obtain more coolant. This modification, shown in detail in Fig.
27 and schematically in Fig. 28, resulted in the housing becoming too rigid to expand freely. The inability
of the inside to move freely relative to the coolant housing led to a fatlgue fallure in the inside pipe. The
~ location of the-valve is shown in Flg 29, o '

Visual and Metallographic Examination

| The salt was partially removed to reveal the crack shown in Figs. 30 through 32 The crack begms at the
spot on the weld and proceeds about 1 in. parallel to the weld.

A metallographic sample was taken from the cracked area: An as-polished view is shown in Fig. 33 and

an etched view in Fig. 34. The weld was on the outside of the tubing and supported the *;-in. end plate of

the cooling shroud. The crack began near the weld outside the cooling shroud and penetrated the tube wall.

 

 

 

 

 

 

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

   

 

 

 

 

 

 

 

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A\ A ; NO. | DWGNO. | oo | DESCRIPTION (NAME, SIZE, E7C.) MATERML
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— = ‘ e DWG, E-NN-B 55449 i £,
1o f FUEL PROCEJHNG_QEL._L._ ' .
T - 5 : B ' | A“"' A FOR FV, 104, /05, dma- SEE
—_—— — = — =YYV | F&%EI%J%ATYS}QVE —— ] / Owa.£-65-0-52395
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‘m‘fmcouFuj (Fes) smdee ”5: v _ ’ REFERENCE DRAWINGS NO.
. ATTACHED N dccomEAnce WirH : . RIDGE NATIONAL LABORATORY
I  MSR-G3-940; ENTITLED Onx o':*mm o
; ‘PEOCEDURE FOR WeELDNG THERMO COUpLES o UNioN CARBIDE NUCLEAR COMPANY
_ . ! O MSRE FPrrinG f COMponenTS" : S _ DIVISION OF UNION CARBIDE CORPORATION
i TAreD Mo /132, 1963 OAK RIOGE, TENNESSEE
| ;
{ B | maufb see Den 30 AT ' LTS ON DIMENSIONS UNLESS BLDG.
_ _ o , GENERAL SPECIFICATIONS _ | MSRE NO. 72, -
Ald 7 See Dow 2737 Foar? 'Y r/ : ! , onmt*mvu - 72, -
?LT : FRACTIONS & ——— | 1. BREAK ALL SHARP EDGES 1/64 MAX. " ,
NO. REVISIONS DATE [APPO| APPD - ‘ . |é MSRE FREEZE VALVE
DRAWN T DATE ?ar e DECMALS AR . : o
CHURTE Inerraa](y 11916 —_— 3. ROUGHNESS HEIGHT OF MACHINED, 5 | : _ BREACIQ 17RO
APPAOAD _ ANGLES  * SURFACES BHALL NOT EXCEED  \/ D
m DATE 2&3 . B inats . - : (mmmmmm A ) Liomiles, m
= X it Fig. 26. Fabrication drawings for freeze valve 105, drawing D-GG-C-55509. _— ARE IN ACCORDANCE WATH ASA ) —0 =5C | . [
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Fig. 27. Modifications for freeze valve 105, drawing R-GG-C-56395.

 

 

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Fig. 28. Diagram showing possible cause of freeze-valve failure after modification.

 
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Fig. 29. Freeze valve and thermocouple locations, drawing D-HH-B-4054 3.

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Fig. 31. Closeup of the bottom of freeze valve 105 showing INOR-8 gas inlet and outlet line of type 304 stainless steel
that was commoded completely. The crusty substance is the salt. Note that there is no salt in the air annulus.

R-54212

 

Fig. 32. Weld at one end of cooling shroud. The crack begins at the round salt stain and ’proceed's for about 1 in, along
the pipe. The arrow points to the salt stain with the crack at 90° to the arrow. ' - '

 

 

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37

 

 

 

 

 

 

Fig. 33. INOR-8 tube wall of freeze valve 105 where the failure occured. As polished, S0X. Bottom surface was
exposed to fuel salt and the upper surface to air, with some salt after the leak occurred. Reduced 15%.

 
 

38

PHOTO 4185-71

 

"y

 

 

 

Fig. 34. INOR-8 tube wall of freeze valve 105 where the faflure occurred. The weld supports the end of the cooling
shroud; the failure occurred outside the cooling shroud. Etchant: aqua regia. 50X. Reduced 15%.

 
 

 

a“

,

39

A comparison of the etched and Unetched views shows that somié attack occurred along the crack and the

-outer surface of one tubing and that etchmg completely removed material from the attacked region. The

attack was likely due to the simultaneous exposure of the INOR-8 to salt and moist air, and probably
involved the selective removal of chromium and iron, leaving metal that was heavily attacked by the
etchant. No such attack occurred on the inside, where only salt was present.

The failure that released salt was due to thermal fatigue. The cooling shroud was 1mt1ally 0.020 in.
thick, but the added cooling tubes increased the thickness to 0.083 in. on the bottom. This made the
shroud relatively rigid on the bottom. During freezing and thawing, the outer part of the shroud changed
temperature more rapidly than the wall of the salt-containing tube. Whereas the outer part of the shroud
was- originally thin enough to deflect to accommodate the differences in length of the two members, the
field modifications made the bottom portion quite rigid. The result was that differences in temperature
imposed on the cooling shroud a stress that was tranéférred by the rigid (%-in.) end plate to the tube wall.
A crack was nucleated at the surface and propagated through the pipe wall during the numerous cycles. .

Parts of three tubes are visible in Figs. 30 and 31. The large tube in .reldtively good condition is the type
304 stainless steel air inlet tube, the center tube is the original INOR-8 air outlet tube with two

‘thermocouples visible, and the hole is the remains of the type 304 stainless steel outlet tube. The original
'INOR-8 air inlet line was capped off and is hidden by the salt. Attack of the type 304 stainless steel by salt

when air was present is as expected. The relative nobility of INOR-8 in this environment is a strong
argument for the use of INOR-8 where salt may be present.

Mechanijcal Properfy Tests

Three rings % ¢ in. wide were cut from the pipe away from the flattened section and were pulled in
tension in the same manner as previously described for the control rod thimble rings. One rectangular piece
was cut and bend-tested with the inside surface of the pipe in tension. Table 6 lists the observed mechanical

. properties. The yield stress was essentially unchanged, and the ultimate stress was reduced about 15% from

the vendor’s certified properties. The elongation was reduced considerably but was still greater than 25%. A
gage section is difficult to define in a ring test, so crosshead travel and reduction in area are reported for the
rings from freeze valve 105 rather than percent elongation. The bend test was discontinued because of
strain limitations of the bend fixture after 0.41 in. crosshead travel, which corresponds to 33% strain in the
outer fibers of the specimen and to a 90° bend angle. The yield stress calculated from the forces on the
bend specimen is too high because elastic formulas were used in the calculation and the specimen deformed
plastically, but this calculated quantity is useful for comparison with other bend tests that will be reported
later for the mist shield. '

Table 6. Results of mechanical property tests on specimens from freeze
valve 105 (heat 5094) at 25°C and a deformation rate of 0.05 in./min

 

Yield Ultimate Crosshead Reduction

 

Type of test stress tensile stress travel in area
(psi) (psi) (in.) (%)
~ Vendot’s, tensile - 45,800 106,800 52.6
Ring, tensile 45,800 89,700 0.72 25
Ring, tensile " 48,900 - 90,100 0.59 29
Ring, tensile 41,900 90,300. 0.73 37
Wall segment, bend 71,300 0.41 334

 

2Maximum strain in outer fibers.

 
 

 

 

 

40

*

 

Fig. 35. INOR-8 surfaces exposed to salt in freeze valve 105 after deformation at 25°C. (z) Fracture of ring specimen
pulled in tension. Note surface cracks near fracture. 4X. (b) Surface of bend specimen. Note some cracks on surface and
edge cracks. 7X. Reduced 18.5%.

Figure 35 shows the tension side of the bend specimen. Some very fine shallow cracks are visible on the
surface in tension, and cracking is visible at the edges in the burrs remaining from the remote cutting
operation. . _ | _

One of the three tensile-tested rings was examined metallographically. Figure 36 is a composite of
photomicrographs of a section through the specimen showing the inside surface, which was exposed to fuel
salt (top), and the outside, which was exposed to the cell environment of 2 to 5% O, in nitrogen. The
reason for the unevenness of the oxide on the outside is not known. Possibly it was due to corrosion after
the leak, although the rings were cut approximately 4 in. away from the nearest visible residue from the salt
leak. Figure 37 shows at 500x the oxide in one of the worst areas. The crack ends tend to be blunted and
did not penetrate the metal beyond the oxidized surface. Figure 38 shows at 500x the inside of the pipe
exposed to fuel salt. About 1 crack per grain or 240 cracks per inch show, but the cracks are shallow and
blunt, having an average depth of 0.75 mil and a maximum depth of 1.5 mils. Figure 39 shows that a large
amount of strain occurred before fracture.
-—— O —

Fig. 36. Composite of photomicrographs of INOR-8 ring from freeze valve 105 that was pulled in tension. Upper
surface was exposed to salt and the lower surface to thg‘ cell environment of Ny plus 2 to 5% O;. Top portion is the
tension side, and the lower part is the compression side.

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Fig. 39. Fracture of an INOR-8 ring specimen from freeze valve 105 that was deformed at 25°C. {a) As pollshed )
etched with lactlc acid, HNO3, HCL. 40X . Reduced 29.5 %.

Summary of Observations

The failure in freeze valve 105 was due to thermal fatigue as a result of the cooling shroud being too
rigid. The mechanical properties of the INOR-8 in the freeze valve were not degraded seriously by the long
exposure to fuel salt containing some fission products. Numerous intergranular cracks were formed on the
surfaces exposed to the salt during postoperation deformation. These cracks were similar to those on the
surfaces from the primary circuit but were shallower.

EXAMINATION OF THE SAMPLER ASSEMBLY

Physical Description

A schematic view of the MSRE fuel pump is shown in Fig. 40; more details are reported elsewhere.??
The components examined were the mist shield and the sampler cage. The fabrication drawings for these
parts are shown in Fig. 41, Salt-sample capsules or additions of uranium or beryllium were lowered by a
windlass arrangement into the sampler cage. The mist shield was provided to minimize the amount of salt
spray that would reach the sampler. The vertical sampler cage rods were Y, in. in diameter and were made
of INOR-8 heat 5059, and the mist shield was made of Y;-in. sheet of INOR-8 heat 5075 (see Table 4 for

 

29. R. C. Robertson, MSRE Design and Operatiohs Report, Part I, Description of Reactor Design, ORNL-TM-728
(January 1965).
 

 

45 3

A A R e

. ORNL-LR-OWG-36043-8R2

 

 

 
 
 
 
 
 
 
  
 
  
   
   
 
   
  
   
  
 
 
   
  
  

 
  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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| © SAMPLER ENRICHER GAS FILLED EXPANSION
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Fig. 40. Section of MSRE fuel 'pump with details of several areas. The mist shield and the sampler cage examined are
shown in the lower left insert.
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47

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Fig. 42. Sampler cage. The assembly is 8.5 in. high. The salt-vapor interface corresponds with the region of highest
material deposition.

compositions). The mist shield was a 1% -turn spiral about 2 in. ID X 3 in, OD. The outside was exposed up
to the normal liquid level to salt that was agitated by the stripper jets and to salt spray and the purge gas
above that level. There was also organic vapor and soot in this region from the decomposition of lubricating
oil that leaked into the pump bowl! at a rate of a few grams per day. The inside was exposed to more nearly
quiescent salt up to the normal salt level and primarily to the purge gas above that level. The pump
components were above 500°C for 30,807 hr and in contact with fuel salt for 21,040 hr.

The general appearance of the sampler cage is shown in Fig. 42. The salt normally stayed at a level near
the center of the cage, but the level fluctuated slightly. The amount of material deposited on the surface is
much greater below the liquid interface than above. A gross gamma scan had a similar profile with a
maximum near the liquid interface. A carbonaceous deposit was present at the top of the sampler. The mist
shield had some deposits, which were heavier below the liquid level and on the outer surface of the shield.

Sampler Cage

One of the sampler cage rods was examined metallographically. Photomicrographs of samples from the
vapor and liquid regions are shown in Figs. 43 and 44 respectively. No intergranular penetration is visible at
this magnification. A heavy surface deposit was on the sample from the liquid region. This deposit was
examined by the electron microprobe analyzer. It contained some salt but had regions that were high in the
elements of INOR-8: Ni, Cr, Fe, and Mo. This is illustrated by the x-ray displays in Figs. 45 and 46, which
also show that the deposits were very inhomogeneous. |

One of the sampler cage rods was pulled in tension at 25°C. The certified properties of this material
were a yield stress of 51,200 psi, an ultimate tensile stress of 115,300, and an elongation of 51%. The
measured properties of the sampler cage rod were a yield stress of 42,500 psi, an ultimate tensile stress of
93,400 psi, and an elongation of 35.7%. The property changes are not large and are likely due to some
stress relief, which decreased the yield and ultimate stress values, and carbide precipitation, which reduced
the ductility. A composite of the tested sample in Fig. 47 shows that the fracture occurred below the center
near the average liquid-gas interface. One metallographic sample was taken on the liquid side of the
fracture. A composite of the badly cracked edges ivs-‘shown in Fig. 48. Note that the cracks extended toa
depth of 12 mils. A higher magnification view of an area near the fracture shows that the cracks extend
deeper than three grains and that several grains have fallen out (Fig. 49). Another metallographic sample
was taken % in. above the fracture. A composite of photographs along the edge shows that the depth of
cracking decreases with increasing distance into the vapor region (Fig. 50). The severity of cracking is
different on the two edges of the rod. Unfortunately, we do not know the orientation of the rod relative to
the sampler and the mist sheild.
 

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Fig. 46. X-tay displays from the surface deposit on a sampler cage rod immersed in fuel salt in the pump bowl. 400X.

 

 
  
  
 
 
 

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Fig. 47. Tensile-tested INOR-8 sampler cage rod from pump bowl. Rupture occurred near the average liquid-vapor
interface. (@) Rupture area showing extensive surface cracking. (b) Composite of entire rod, with top at right. Rod
: diameter is 1/4 in.

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Fig. 48. Section of INOR-8 sample cage rod that was deformed at 25°C. 9.2X. The fracture (right) occurred at the
salt-vapor interface. The rod is immersed further in the liquid from right to left. The length of the sample is 0.63 in.
 

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54

stt Shield

Figure 51 shows the mist shield after removal from the MSRE. The sh1eld has been split to reveal the
inside surface. Four specimens approximately 1 in. (vertical) X Y in. (circumferential) were cut from the

" mist shield. Their locations were (1) outside the spiral and immersed in salt, (2) outside the'spiral and

exposed to salt spray, (3) inside the spiral and immersed in salt, and (4) inside the spiral and exposed
primarily to gas. Bend tests were performed on these specimens at 25°C using a three-point bend fixture.3°
They were bent about a line parallel to the %-in. dimension so that the outer surface was in tension. The
information obtained from such a test is a load deflection curve. The equations normally used to convert
this to a stress-strain curve do not take into account the plastic deformation of the part, and the stresses
obtained by this method become progressively in error (too high) as the deformation progresses.

Table 7 shows the results of bend tests on the specimens from the mist shield. Note that the yield and
ultimate stresses are about double those reported for uniaxial tension. The fracture strain is the parameter
of primary interest. Unexposed INOR-8 (heat N3-5106) did not fail after 40.5% strain in the outer fibers.
(No material was available of heat 5075, the material used in fabricating the mist shield;) Although all of
the samples strained more than 10% before failure, the two samples from the outside of the spiral were less
ductile than the other samples.

- The bend samples of the mist shield were examined metallographlcally Photomicrographs of the sample
from the inside vapor region are shown in Fig. 52, and a composite of several photomicrographs is shown in
Fig. 53. The edge cracks were intergranular and about 1 mil deep. The photomicrographs of the sample
from the liquid region (Fig. 54) show that the cracks extended to a depth of about 8 mils. The composite
photograph in Fig. 55 also shows the increased frequency and depth of cracking in the liquid region.

The sample from the outside liquid region was quite similar metallographically to that from the inside
liquid region. Photomicrographs of the fracture and a typical region of the tension side near the fracture are
shown in Figs. 56 and 57 respectively. A composite of several photomicrographs is shown in Fig. 58.

 

30. H. E. McCoy and J. R. Weir, The Effect of Irradiation on the Bend Transition Temperatures of Molybdenum- and
Niobium-Base Alloys, ORNL-TM-880, pp. 7-10 (July 1964).

Table 7. Bend tests at 25°C on parts of the MSRE mist shield?

 

 

) Maximum®
, ) Yield \ \
Specimen stress? tensile Strain Environment
(psD) stre-ss (%)
(psi)
| x 10° x 10°
S-52 Mist shield top inside 97 269 46.99 Vapor region, shielded
S-62 ‘Mist shield top outside 155 224 10.74 Vapor region, salt spray
$-60 Mist shield bottom inside 161 292 31.69 Liquid region, shielded,
7 . _ salt flow
S-68 Mist shield bottom outside 60 187 1114 Liquid region, rapid
' ' salt flow
N3-5106 Unirradiated control 128 238 40.5

test, % in. thick

 

%The mist shield was fabricated of heat 5075 with certified room-temperature properties of 53,000 p31 yield stress,
116,000 psi ultimate stress, and 49% elongation.
bBased on 0.002 in. offset of crosshead travel.
"Maxzmum tensile stress was controlled by fracture of sample or by strain limitation of test fixture.
Spccunen broke.
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62

 

Fig. 58. Composite of several photomicrographs of bend specimen from outside liquid region of INOR-8 mist shield.
Top portion shows the tension side, and the lower portion shows the compression side. 24X. Reduced 44%,

Photomicrographs of the outside vapor (salt spray) bend specimen at the fracture and on the tension side
near the fracture are shown in Figs. 59 and 60 respectively. The composite of several photomicrographs in
Fig. 61 shows that the cracks are not much deeper nor more frequent than in the liquid regions {(compare
Figs. 55, 58, and 61), but the tendency for grains to fall out is much greater. This is even moreisignificaht
when one notes that the strain was the least in the sample from the outside vapor region (Table 7).

The reduced ductility of these samples remains unexplained. The maximum crack depth in the liquid
region was only 8 mils, and it is quite unlikely that an 8-mil reduction in a sample thickness of 125 mils
would cause much embrittlement, The embrittlement may have been partially due to the extensive carbide
precipitation shown in Figs. 53, 55, 56, 58, and 59. However, the outside and inside of the shield received
identical thermal treatments, but the samples from the outside had lower ductilities. Thus, attributing the
embrittlement solely to carbide formation does not seem appropriate. ' |

Summary of Observations

The only free surface in the primary circuit was in the pump bowl, and there appears to have been a
collection of material at this surface. Visual observation, gamma scanning, and the electron microprobe
show an accumulation of salt, fission products, and corrosion products at this surface. Besides having a free
surface, the region around the sampler was often exposed to a reducing environment because of the
addition of beryllium metal at this location. Fluorides of the structural metals would be reduced to the
metallic form and could accumulate at the free surface or deposit on nearby metal surfaces. Some of the
less stable fission product fluorides would have likely reacted in a similar way. Thus, it is not surprising that
the intergranular cracks are most severe near the free surface. The most important observation relative to
the cracking is that its severity was much less in material exposed primarily to gas. This allows the
conclusion that exposure to liquid salt is necessary for the cracking to occur.
 

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Fig. 61; Composite of several photomicrographs of bend specimen from outside vapor region of INOR-8 mist shield.
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' EXAMINATION OF A COPPER SAMPLE CAPSULE

Physical Description

Salt samples were routinely taken from the pump bowl for chemical analysis. About 10 g of salt was
required for an analysis, and the oxygen-free high-conductivity (OFHC) copper capsule shown in Fig. 62
was used to isolate this size of sample. The capsule was % in. in diameter by 1.6 in. long, with
hemispherical top and bottom. Salt entered through the two windows. The solid top was made of
-nickel-plated iron and was sufficiently heavy to ensure that the capsule would be submerged in the salt. A
nickel-plated steel key was attached to the top of the capsule by a loop of /;u-m.-dlam stranded stainless
steel rope, which provided the flexrblhty necessary for the sample assembly to pass through the two
15-in.-radius bends in the transfer tube The key locked the assembly to the latch on the drive unit cable.
The capsule was lowere_d into the pump bowl by a windlass arrangement that has been discussed in detail.3!

The sampler facility worked well during the operation of the MSRE; 593 samples were taken and 152
fuel additions were made. However, two small capsules were lost, and one of these (which we examined)
was retrieved when the sample cage was removed from the fuel ‘pump. The sample capsule was likely lost in
August 1967 and flattened during retrieval attempts with heavy magnets in April 1968. (The heavily
corroded steel cap was recovered at that time .) When actually recovered in January: 1971 the capsule was
discolored, deformed, and cracked ‘While in the MSRE, the capsule was almost certamly lying on the
bottom of the pump bowI where it would have been exposed to flowmg fuel salt. The capsule would have
been above 500°C for 16 400 hr and exposed to fuel salt for 13 025 hr. ‘

Exammatlon

Fxgure 63 shows a cracked area of the sample capsule w1th some deposued matenal on the outside of
the capsule. Flgure 64 is a photomicrograph of t.he cracked area polished below the sun_'face, showing the
intergranular nature and extent of the craéks;'-Figure-G_S is a photomicrograph of a cross section of the
copper capsule, showing the thick depdsit of material on both surfaces of the capsule.

 

31. R. B. Gallaher, Operation of the Sanipler-Enn'cher in the Molten-Salt Reactor Experiment, ORNL-TM-3524
(October 1971).

 
 

 

66

 

 

MSRE.

Sample capsule used in

Fig. 62.

 

 

 

 

 

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Fig. 65. Photomicrographs of copper MSRE fuel

(right).

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FeKa X-Ray Scan CrKa X-Ray Scan Mcla X-Ray Scan

Fig. 66. Views of the surface of the copper sampler in the electron-probe microanalyzer. The upper left picture is an
optical photomicrograph showing deposited metal crystals. The other pictures show the intensity of x-ray fluorescence for
various elements. These displays are a mirror image of the first optical picture. Note that certain areas are enriched in Ni,
Fe, Cr, and Mo. Some copper from the capsule is present in the deposited material. Approx 200X.

Table 8. Analysis of the copper capsule

 

 

Element Concentration Element Concentration
{ppm) (ppm)
As 1 Si 70
Be 1000 Sn ~20
Ca ~5 Sr ~1
Cr 30 Te <5
C M 233y 5
Fe - 300 235y . 500
K ~5 38y 1000
Mg .10 v 0.3
Mn 1 Zn 20
Mo 30 Zr 10,000
Nb 1 F ~1000
Ni 1500
P 2
Pb 5

 

 

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69

An electron-probe .MCromélfiis of the material in the gram boundaries and x-ray scans for the
distribution of Ni, Fe, Cr, and Cu in the surface layer in the area around the large metallic particle on the
outer surface were made. Figure 66 shows the distribution of these elements, and it appears that the surface
layers were made up mostly of copper and nickel, with some areas containing high concentrations of
chromium and molybdenum. The inner and outer surfaces of the capsule appear similar. The
grain-boundary ‘precipitate was only 28% Cu, and no other element could be detected. This observation
indicates that the remamder is a light element, perhaps berylhum or llthlum that could not be detected by
the instrument. : ) . :

The surface depos1ts were ground away from two small pleces about 16 in. square by ¥, in. long. These
pieces were used as electrodes for the spark source mass spectrograph. We do not know the exact depth of
sparking, but we suspect that it was near the surface. The analysis of the copper is shown in Table 8. Several
important points can be made from this analysis. The spectrograph was not adjusted to detect lithium, but
the presence of U, Zr, Be, and F indicates thaf the fuel salt was present. Also, the isotopic concentrations of
uranium correspond to those of the first fuel charge rather than the later fuel charge in which the uranium
was predominantly 233 U. The weight percentages of various elements present in the initial fuel charge were
approximately 10.9% Zr, 6.3% Be, 67% F, 5.1% U, and 10.8% Li. These data indicate that relative to
uranium the material in the copper capsule was enriched in Zr'and depleted in F and Be. There were also
significant quantities of Fe, Ni, Mo, and Cr, the main elements in INOR-8, as well as detectable quantities
of several other elements many of which may have been preeent in the copper initially. -

The copper is very brittle, and the present analytical tesults do not shed much light on what element is
responsible for the embnttlement -

- Summary of Observations

Although copper is generally accepted as being compatible with fuel salt, this capsule was severely
embrittled in its 13,000-hr exposure to MSRE fuel salt. Heavy deposits quite sim_ilar to those noted on the
sample cage and shield were present on the surfaces of the copper capsule. Some of the constituents of
these deposits may have caused the embrittlement, but we were unable to identify the responsible element.

; EXAMINATION OF THE PRIMARY HEAT EXCHANGER |
Physxcal Descnptlon

The MSRE heat exchanger was the tube-and shell type, wnth the fuel salt on the shell side and the
coolant salt in the U-tubes. The shell was 16% in. OD X Y% in. wall X about 8 ft long and was fabricated
from INOR-S heat 5068. The heat exchanger tubmg was % in. OD X 0.042 in. wall from INOR-8 heat
N2-5101 (see Table 4 for chemical compositions). The structural details of the heat exchanger 2 and the
special method of joining the tubes to the header, which involved welding and back brazing,®3 have been
described. The completed tube bundle is shown in Flg 67 '

 

. 32 R. C Roberstson, MSRE Deszgn and Operations Report, Part I, Descnption of Reactor Design, 0RNL-TM-728
(January 1965).
33. R. G. Donnelly and G. M. Slaughter, “Fabrication of the Molten-Sait Reactor Experiment Heat Exchanger Core,”
Welding J. (N.Y.) 43(2), 11824 (February 1964).
 

 

 

70

PHOTO 39431

 

Fig. 67. Completed tube bundle in fixture.

Examination

The heat exchanger was above 500°C for 30,807 hr and was exposed to fuel salt for 21,040 hr.
An oval piece of the shell 10 X 13 in. was cut by plasma torch near the outlet end of the heat
exchanger. This same cut severed a piece of one tube, and pieces of five others were cut by an
abrasive wheel. These parts are shown in Figs. 68 and 69. The outside of the shell had a dark
adherent oxide; the inside surfaces were discolored slightly and had a thin bluish-gray coating,
probably caused by the plasma cutting operation. Some of this powder was brushed off and found
by gamma scanning to contain '°¢Ru and '25Sb.

Photomicrographs of a cross section through the outer surface of the shell are shown in Fig. 70.
The oxide on the outside surface is typical of that observed on INOR-8 at this temperature and
extends to a depth of about 5 mils. Etching attacks the metal in the oxidized layer and etches the
grain boundaries near the surface more rapidly than those in the interior. The inside surface of the
shell (Fig. 71) shows some structural modifications to a depth of about 1 mil. No samples of the
shell were deformed.

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Fig. 68. Section 10 X 13 in. cut from the pnmaxy heat exchanger sheli The | plece was cut with a plasma torch and the
center stud was used for gu:dmg the torch

 

 

 

 

 

_ ~ Fig. 69 'l’he Y,-in.-OD INOR-S tubes from the primary heat exchanger. The different shades arise from a dark film
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structure to a depth of about 1 mil is apparent. (2) As polished; (b) etched with aqua regia.

 
 

74

Photonucrographs of a typical cross section of the heat exchanger tublng are shown in Fig. 72
The overall view shows that both sides etch more rapldly than the center to a depth of ‘about 7
mils. This is not unusual for a tubular product and is often attributed to. impurities (pnmanly
lubricants) that are worked into the metal surface during repeated steps of deformation and
annealing, The higher magnification views in the as-polished condition show that grain boundary
attack (or cracking) occurred on the surface exposed to fuel salt (outer) but not on the coolant side
(inner). The temperatures under normal operating conditions were such that the fuel salt side would
have been in compression.

Similar photomicrographs of a longitudinal section of tubing are shown in Fig. 73. The features-

are quite similar to those discussed for Fig. 72. Both figures show some metallic deposits on the
outside surface, which was exposed to fuel salt. These deposits are extremely small and could not be
analyzed with much accuracy with the in-cell microprobe. However, they seemed to be predominately
iron. Scanning across the tubmg detected no concentration gradients of the major alloying elements
in INOR-8.

‘Tensile tests were run on three of the tubes, and the results are compared with those for
as-received tubing in Table 9. The tubes were pulled in tension, and we assumed that the entire
section between the grips was deforming. The grips offer some end restraint, sO our assumption is
obviously in error, and the errors are such that our yield stresses are high and our fracture strains are
low; however, the ultimate stresses and reductions in area are unaffected by this assumption. The
as-received and postoperation tests were run by the same technique, so the results should be
comparable. The largest changes during service were reductions in the yield stress, fracture strain, and
reduction in area. The tubing was bought in the “cold drawn and annealed” condi_tidn, but such
tubing normally is cold worked some during a final straightening operation. The reduction in yield
strength during service may have been due to annealing out the effects of this working. The
reductions in the ductility were likely due to the precipitation of carbides along the grain boundaries.

Photomicrographs of one of the tubes deformed at 25°C are shown in Fig. 74. Profuse

intergranular cracks were formed to a dépth of about 5 mils on the side exposed to fuel salt, but

none were formed on the coolant salt side. Etching again revealed the abnormal etching charac-
teristics near the surface, but whatever caused this abnormality dld not result in grain-boundary
embrittlement. A

Unstressed and stressed sections of tubing were photographed over relatively 'large distances to get
more accurate statistics on crack numbers and depths. Composite photographs of the two samples are
shown in Fig. 75. The cracks are readily visible in the stressed sample but ‘may not be apparent in

Table 9. Tensile data on heat exchanger tubes (heat N2-5101)

 

 

- Crosshead Temperature Yield Ultimate Fract}u'e l_iedyction
Specimen Condition -Speefl cO Straiss Tensile - Strain in
- (in./min) (psi) Stress (psi) (%) Area (%)

7 From heat exchanger 0.05 650 44,300 - 67,400 21.3 224

6 From heat exchanger 1.0 25 66,300 118,000 37.0 205
5 From heat exchanger 0.05 25 64,800 122,000 39.0 29.0
4 As received 0.05 650 53,900 74,600 40.0 14.5

2 As received 0.05 25 73,000 127400 511 42.6

3 As received 0.05 25 70,900 126,200 50.1 40.0

1 Vendor certification 25 58,900 120,700 47.0

 

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Fig. 72. Cross section of INOR-8 heat exchanger tube. (1) Etched with glyceria regia. (b) Inside surface in the
as-polished condition after exposure to coolant salt. (c) Outside surface in the as-polished condition after exposure to fuel
salt. Reduced 34%.

 

 

 
 

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i

 

 

 

 

Fig. 76. INOR-8 heat exchanger tubing in the unstressed condition. The side with the small cracks was exposed to fuel
salt, and the other was exposed to coolant salt.

 
 

E}]

79

the unstressed sample. A higher magnification print used in making the composite is shown in Fig.
76; the cracks can be more easily seen. The stressed sample had 262 cracks per inch with an average
depth of 5.0 mils. Two counts were made on the unstressed sample. In the first count only those
features that unequivocally were cracks were counted. This gave a crack count of 228 per inch with
an average depth of 2.5 mils. In another count, we included some features that were possibly cracks,
and this gave a crack frequency of 308 per inch. Thus the number of cracks present before straining
was about equivalent to that noted after straining. Straining did increase the visible depth. Some
indirect evidence that the cracks were present initially and only spread opén further during the
tensile test was obtained from examination of the sample in Fig. 75. The sample length shown
deformed 39% (Table 9), and the combined widths of the cracks account for 35% strain. Thus, the
crack widths very closely account for the total strain and indicate that the cracks formed with little
or no deformation and spread open as the sample was deformed.

~ Summary of Observations

The primary heat exchanger opera'ted in contact with fuel salt for 21,040 hr without any leaks or other
problems developing. The outside of the shell was oxidized to a depth of a few mils. The surfaces exposed
to coolant salt showed no evidence of intergranular cracking or corrosion. The tube surfaces exposed to fuel
salt had shallow intergranular ¢racks that opened further when samples were strained but did not propagate
much deeper into the tube wall. Since the neutron fluence received by this component was quite small,
neutron irradiation evidently was not a significant factor in causing the intergranular cracking. However, the
need for contact with the fuel salt was clearly demonstrated, since cracks were formed only on the outside

- surface of the tubing.

EXAMINATION OF THE COOLANT RADIATOR
Physical Description

The energy produced in the MSRE was dissipated through a radiator cooled by air. The coolant fluid
was LiF—-34 mole % BeF, and flowed through the tube side of the primary heat exchanger, the coolant
pump, and the radiator. The coolant circuit was at service conditions for about 26,000 hr. About half this
time the system was isothermal at 650°C, and dunng the other half the salt temperature vaned from 593°C
entenng the radiator to 538°C leaving. A

The examination of the ‘tubes exposed to coolant in the pnmary heat exchanger was descnbed in the
previous chapter. We also exammed some tubes from the intet and outlet ends of the radiator and two
thermocouple wells located at the mlet and outlet ends of the radiator. The results of this examination will
be discussed in this section.
~ The detalls of the construction and operation of the radxator have been reported 3f and an assembly
drawing is shown in Fig. 77. The radiator tubmg was 3% in. OD X 0.072 in. wall, of INOR-8 heat 5097 (see

~ Table 4 for chemical composmon) The tubing was fabricated by Wall Tube and Metal Products Company,

Newport, Tennessee to specifications MET-RM-B-163, MET- NDT-3, and MET-NDT-165. After fabrication
the matenal was annealed at some unspecified temperature, likely in the range 1000 to 1200°C.

 

34. R. C. Robertson, MSRE Design and Operations Report, Part I, Descnptton of Reactor Design, ORNL-TM-728
(January 1965). '

 
 

 

80

The details of the thermocouple wells are shown in Fig. 78. The well itself was fabricated from INOR-8
heat 2477 and welded to the 5-in. schedule-40 header pipe of INOR-8 heat Y-8699 (see Table 4 for
compositions). As shown in Fig. 78, the well was machined with a lip that had to be hand fimshed in the
field to fit the contour of the 5-in. pipe.

Observations

Visual examination of the tubing revealed an adherent oxide on the outside surface, which was exposed
to air. The inside surface of the tubing was exposed to coolant salt and was very clear and free of deposits
of metal or salt.

Typical photomlcrographs of the as-received tubing are shown in Flg. 79. The relatively fine gram size
of the material and the carbide stringers in the primary working direction are evident in Fig. 79(a). Figure
79(b) shows a typical high-magnification view of the carbide stringers in the as-polished condition. The
carbide is quite brittle, and the particles fracture during fabrication to produce cracks a few tenths of a mil
long. An etched view of the inside edge is shown in Fig. 79(c). The etchant attacked the carbide readily,

-causing particles to fall out and to leave holes. A typlca] etched cross-sectional v1ew is shown in Flg. 79(d).
The stringers are not apparent since they are being viewed from the ends. |

The microstructure of the tubing was altered two ways during service. The outside of the tubing was
oxidized, and the inside of the tubing etched more rapidly to a depth of about 5 mils. These changes are
shown in Fig. 80 for tubing at the inlet end and in Fig. 81 for the outlet end. The oxidation is not
excessive, and the penetration of the oxide front is typical for nickel alloys with chromium content as low
as that in INOR-8. | - -

We investigated further the difference in etching characteristics near the inside surface of ‘the tubing. A
less aggressive electrolytic etchant was used to delineate the carbide structure better. The as-received
material is shown in Fig. 82. There is still some attack of the primary carbide stringers, but very few fine
carbide particles are present. The tubing from the inlet end of the heat exchanger is shown in Fig. 83. Fine
carbide is present throughout the material, concentrated along the grain boundaries and even more
concentrated near the surface. The tubing from the outlet end looked very similar. Thus the long time in
service caused carbide to precipitate throughout the material, particularly near the inside surface.

In further pursuit of the different etélfing characteristics near the surface, we examined the samples
with the electron microprobe analyzer. No composition variations in Cr, Mo, Ni, or Fe were observable to
within 2 um of the surface (the allowable working range on these samples) The lubricants used in drawmg
tubing are often high in silicon or carbon, but the concentrations of these elements were too low for the
electron microprobe analyzer. We machined turnings from the tube for analysis for these elements, and the
results are given in Table 10. The analysis shows a higher carbon concentration at the inside surface, a fact
that should lead to increased carbide precipitation. The carbon would be dissolved by the postfabrication
anneal and reprecipitate during service. '

We tensile tested a piece of pipe from the outlet end and a piece of as-received tubmg from the same lot
(heat 5101). The propertles are given in Table 11. The on]y significant property changes were the fracture
strain and the reduction in area. However, these changes are rather modest and do not indicate a serious
embrittlement of the material. :

The inlet thermocouple well after it was cut off with a coated weldmg electrode is shown in Fxg. 84.
The oxidized appearance and the metal fragments on the part arose from the cutting operation. The circular
piece of material is part of the 5-in. pipe wall. The thermocouple well was machined from bar stock and
extended about 3 in. into the flowing salt stream. The cross-sectional view of part of the inlet well i in Flg
85 makes the geometry more evident. The pipe wall, the welds, and the thermocouple well are evident.
 

81

 

 

 

 

 

 

 

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TUBE NOS. 21 TARU 242

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REFERENCE DRAWINGS NO.
OAK RIDGE NATIONAL LABORATORY
OPERATED v -
UNioN CARBIDE NUCLEAR COMPANY
DIVISION OF UNION CARBIDE CORPORATION
‘ OAK RIDGE, TENNESSEE .
UMITS ON DIMENSIONS UNLESS
: : , OTHERWISE SPECIFIED: GENERAL SPECIFICATIONS  |AZ5 €. 2 BOG ~sy 2
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83

 

 

 

 

 

 

 

 

 

 

Fig. 79. Typical -photomicrographs of as-teceived INOR-8 (heat 5097) tubing used in the MSRE radiator. (a)
Longitudinal view, etched with glyceria regia, 100X ; (b) as-polished longitudinal view, 500X; (c) inside edge of tublng
etched with glyceria regia, 500X ; (d) cross-sectional view, etched with glyceria regia, 100X.
 

 

 

 

 

 

 

  
 

 

 

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Kig. 82. Asreceived INOR-8 tubing used in the MSRE radiator. Etched electrolytically in aqueous solution of 5%
sodium citrate, 5% sodium acetate, 1% citric acid, 1% potassium thiocyanate. 600 mV, 1.5 mA for 15 sec plus 700 mV, 2.5
mA for 10 sec. (@) 250X ; (b) 1000X.
87

     

 

 

 

 

 

 

 

   

an. 83. Tubing from the inlet end of the MSRE INOR-8 radlator. Etched electrolytxcally in an agueous solution of 5%
sodium citrate, §% sodium acetate, 1% citric acid, 1% potassmm thiocyanate. 700 mV for 10 sec, started at 8 mA and
dropped to 4 mA. (a) 250X ; (p) 1000X.

1

 

 

 
 

= : = N T YA101190

 

 

 

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Fig. 84. INOR-8 thermocouple well removed from the inlet line to the radiator of the MSRE. Much of the
discoloration and the fragments of metal resulted from cutting the part out with a coated welding electrode.

g
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Fig. 85. Cross section of INOR-8 thermocouple well from the inlet line to the MSRE radiator. From left to right the
components are the 5-in. coolant line, the weld, and the thermocouple well. The side having the most weld metal was
exposed to air, and the other side was exposed to coolant salt.

o
 

-t

-89

Table 10. Chemical analyses of sections Qf as-received
: i INOR-8 radiator tubing -

 

o

U S . Silicon (%) ; -+ Carbon (%)

 

" Inner 5 mils of wall 0.53 0.095

~ Center 62 mils of wall 0.79 0.073
Outer § mils of wall - 079 0.036
Bulk composition 0.62°% 0.067

 

4Vendor analysis; others made at ORNL.

Table 11. Tensile properties at 25°C of INOR-8
tubing from the outlet end of the MSRE
heat exchanger and as-received material

 

Yield Ultimate Fracture Reduction
stress tensile strain, in
(psi) stress(psi) %in2in. area (%)

 

As received 63,500 123,100 52.0 44.1
Heat exchanger 64,600 123,800 38.8 29.5

 

Metallographic examination of the mounted cross section shown in Fig. 85 revealed that the weld had
cracked on the salt side of the pipe. The cracks on both of the polished surfaces in Fig. 85 are shown in Fig.
86. In the worst case the cracks penetrated to a depth of about 3 mils. The remaining half of the well was
cleaned by acid etching to remove the metal left from the cutting operation, and the weld was checked with
dye penetrant. A photograph of the weld with dye still present is shown in Fig. 87. Note that the crack, as

indicated by the dark color, extends almost completely around the weld. There is also an area where

penetration of the root. pass was incomplete. We propose that the cracks formed as the weld was made,
because of the poor fit-ip of the parts to be welded. The welds were not stress relieved, and the fact that
the cracks did not penetrate further attests to the lack of stress and crevice COI‘l’OSlOIl by lithium-beryllium
fluoride salts. _ ' ;

We did not section the outlet well but we did clean the weld and examine it with dye penetrant. No
cracks were observed.

The outside surface of the S-in. ‘pipe was exposed to air and was ox1dlzed Figure 88 shows the spotty
nature of the oxide and its maximum thickness of about 3 mils.

A cross section from the bottom of the well was examined. A macroscoplc view of this section is shown
m Fxg 89 The inside of the well was prepared by drlllmg, and the pointed shape of the drill is still apparent
at the bottom The weld metal deposit on the bottom was made to ensure that salt did not leak along the
carbide stringers. The amount of oxidation was greater at the bottom of the well than further up. The
transition to the thinner oxide about % in. from the bottom is apparent in Fig. 89. Figure 90(a) shows the

oxide layer of about 5 mils at the bottom of the well, and Fig. 90(b) shows the abrupt transition about %4

in. from the bottom. Since lubricant would have been used during drilling, we suggest that this difference
may have been due to the bottom of the well not being as clean as the sides. '

A further significant observation was that the surfaces of the well exposed to the salt looked qu1te
similar to those of survelllance samples removed from the MSRE (Fig. 91) We have previously attributed
this modified surface structure to cold workmg from machining and have shown that it can be produced in
the absence of salt. The structure likely results from carbide formmg on the slip bands produced by
machmmg Thus there is no ev1dence of corrosion of this part.
 

 

 

20

Y-101 78

 

in 100X

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Fig. 86. Cracks in the root pass of the inlet weld. The cracks occurred at the fusion line and likely resulted from the
poor fit-up of the parts. As polished.

M v-101474

 

Fig. 87. Underside (salt side) of half of the INOR-8 thermocouple well from the inlet line to the MSRE radiator. The
part was acid etched and coated with dye penetrant. Note the indication of a crack around most of the weld and an
incomplete root pass region.

 

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Fig. 88. Oxide formed on the outside of the 5-in. INOR-8 coolant line. As polished. (a) 100X ; (b) 500X.

 

 

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Fig. 89. Cross section of the infet INOR-8 thermocouple well. The outside was exposed to coolant salt, and the inside
contained the thermocouple in air. 8X; as polished.

Summary of Observations

The surfaces of the radiator (tubes and thermocouple wells) that were exposed to air formed a shallow
adherent oxide, while the surfaces exposed to salt were clean and showed no evidence of corrosion, Two
slight modifications of microstructure were noted. A shallow layer on the inside surface of the tubing
etched more readily, and we attribute this to contamination of the tubing by fabrication lubricants. A
structural modification near the surface of the machined thermocouple well was similar to that noted on
surveillance specimens and is attributed to the effects of cold working on carbide precipitation. '

The cracked weld where the inlet thermocouple well was attached to the 5-in. pipe header likely
resulted from poor fit of the parts during welding. The crack probably formed when the weld was being
made, and it is encouraging that it did not propagate further during service.

SUMMARY

The Molten-Salt Reactor Experiment operated above 500°C for 30,807 hr and was filled with fuel salt
for 21,040 hr. Operation of the system was never detained by materials problems, but the examination of
INOR-8 and graphite surveillance samples during operation and several cornponents after operation resulted
in several important observations.

1. Graphite exhibited excellent compatibility with the fuel salt. Machining marks and numbers were
clearly visible, with no evidence of chemical interaction with the fuel salt.

2. The INOR-8 surveillance specimens clearly demonstrated a progressive decrease in the creep ductility
with increasing fluence. The MSRE vessel did not become brittle enough to cause termination of operation,
but an alloy with better resistance to embrittlement by neutron irradiation will be necessary for power
reactors with a 30-year design lifetime. Considerable progress has been made in developing a modlficatlon
of INOR-8 that contains 2% Ti and has improved resistance to embrittlement. 3s

 

_35. M. W. Rosenthal et al., “Recent Progress in Molten-Salt Reactor Development,” Atomic Energy Review 9(3),
601-50 (September 1971).
 

 

 

 

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Flg. 90. Oxide inside the inlet INOR-8 thermocouple well As pohshed (@) At the bottom; (b) transition in depth
about /3 in. from bottom of well.
 

 

 

94

Y-101573

 

 

 

 

 

 

 

 

Fig. 91. The outside of the inlet INOR-8 thermocouple well that was exposed to coolant salt. Note the modified layer
near the outer surface. Etchant: glyceria regia.

3. The INOR-8 surveillance samples and components removed from the fuel salt were slightly
discolored, but no evidence of corrosion could be detected by standard metallographic examination. The
selective removal of chromium and iron from metal from the core was detectable by the electron
microprobe. No composition gradients were detectable in metal from the coolant circuit. (The coolant salt
did not contain uranium.) Thus the corrosion rate was very low in the fuel circuit, and this observation
agrees well with the chemical changes observed in the fuel salt.®®

4. Some of the INOR-8 surfaces showed shallow microstructural modlficatlons One type involved
increased carbide precipitation in a lamellar pattern and was likely due to cold working from machining.
The second type was noted only on tubing and was likely due to carbon contamination from fabrication.
Neither modification is thought to be of practical significance.

5. Grain-boundary embrittlement was noted on all INOR-8 surfaces exposed to fuel salt. The shallow
intergranular cracks were often visible in polished sections of INOR-8 as removed from the MSRE. In most
instances the material had to be strained to make the cracks visible. Cracking similar to that noted in the
MSRE was produced in laboratory experiments by exposure of INOR-8 to the fission product tellurium.®?
This experimental work has shown that the extent of cracking is very dependent on the alloy composition.

6. Freeze valve 105 failed from thermal fatigue due to nnproper construction and not from a basic
materials problem. '

 

36. R. E. Thoma, Chemical Aspects of MSRE Operation, ORNL-4658 (December 1971).
37. H. E. McCoy and B. McNabb, Intergranular Cracking of INOR-8 in the MSRE, ORNL-4829 (November 1972).
 

 

95

7. A crack was noted in the weld that attached the thermocouple well to the radiator coolant salt
header. This crack was restricted to the root pass and was likely a result of poor fit-up of the parts. The
crack probably formed when the weld was made and did not propagate during service.

8. Oxide films were formed on all the INOR-8 surfaces éprsed to air or the cell environment of
nitrogen containing 2 to 5% Q,. The oxide generally consisted of a uniform surface layer about 1 mil thick
and a selective oxidation front extending to a depth of 4 or § mils. This depth and type of oxidation are
what would be expected for an alloy that contains only 7% Cr, such as INOR-8.

9. A copper sample capsule that had been in the fuel salt pump bowl for several thousand hours was
very brittle. Chemical analysis showed that constituents of the fuel salt and INOR-8 had penetrated the
copper, but the element responsible for the embrittlement was not identified.

" ACKNOWLEDGMENT

The observations in this report cover several years and include contributions from many individuals.
W.H. Cook and A. Taboada designed the surveillance fixture, and W. H. Cook was responsible for its
assembly and disassembly. The MSRE operations staff, headed by P. N. Haubenreich, exercised extreme
care in handling the surveillance fixture and in removing the various components for examination. The
Hot-Cell Operation Staff, headed by E. M. King, developed several special tools and techniques for various
examinations and tests of materials from the MSRE. The metallography was performed by H. R. Tinch,
E.H. Lee, N. M. Atchley, and E. R. Boyd. The microprobe scans were made by T. J. Henson and R. S.
Crouse. The mechanical property tests were performed by B. C. Williams, H. W. Kline, J. W. Chumley, L. G.
Rardon, and J. C. Feltner. The chemical analyses were performed under the supervision of W. R. Laing,
E.I. Wyatt, and J. Carter. Assistance was received from several members of the Reactor Chemistry Division
in several phases of this work. J. W. Koger, J. R. DiStefano, P. N. Haubenreich, and J. R. Weir reviewed the
manuscript of this report and made many helpful suggestions. Kathy Gardner made the original drafts of
this report, and the drawings were prepared by the Graphic Arts Department.

 
 

 

 
 

 
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