FFR_chap21.txt

CHAPTER 21
MATERIALS OF CONSTRUCTION—METALLURGY*

21-1. LMFR MATERIALS

21-1.1 Metals. Alloy steel. For maximum power production, it is de-
sirable to operate an LMFR at the highest possible temperature consistent
with the mechanical properties and corrosion resistance of the materials
of construction. A maximum temperature of 500°C or higher is deemed
desirable for economically attractive operation of the reactor. No ma-
terials have yet been found that are mechanically strong at these tempera-
tures, readily fabricable, and also completely resistant to corrosion by the
{-Bi fuel.

This does not mean that there is no hope for obtaining a good material
for holding bismuth fuel. On the contrary, very significant advances have
been made in the past few years. It must be realized that before work was
started on liquid metal fuel reactors, very little was known about the
solubility and corrosion characteristics of liquid bismuth with reference
to containing materials. There is general optimism that continuing research
and development will lead to suitable materials for containing the U-Bi
fuel system. ,

The low-alloy steels offer a good compromise for use in the heat ex-
changer, piping, and reactor vessel, particularly sinee their corrosion re-
sistance ean be greatly improved by the addition to the fuel of Zr + Mg
as corrosion inhibitors. Nickel-containing stainless steels cannot be used,
despite their good high-temperature mechanical properties, because of
the high solubility of Niin Bi, and the greatly lowered U solubility in the
presence of this dissolved Ni.  Extensive engineering and fundamental
studies have been made on the corrosion of the low alloy steels by inhibited
U-DBi, as well as the mechanism of corrosion inhibition. Radiation effects
are currently being investigated.

Of course, besides steels, there are other materials, notably the rarer
metals, which have characteristics making them suitable for certain uses
in a liquid-metal system. However, unless the cost and ability to fabricate
these materials can be improved significantly, heavy dependence will have
to be placed upon alloy steels for the main containment problem.

*Based on contributions by D. H. Gurinsky, D. G. Schweitzer, J. R. Weeks,
J. =0 Bryner, M. B. Brodsky, C. J. Klamut, J. G. Y. Chow, R. A. Meyver, R. Bour-
deau, O. I'. Kammerer, all of Brookhaven National Laboratory; L. Green, United
IEngineers & Constructors, Ine., Philadelphia, Pa.; and W. P. Eatherly, M. Janes,
and R. L. Mansfield, National Carbon Company, Cleveland, Ohio.

AN

 
7H MATERIALS OF CONSTRUCTION—METALLURGY [cHap. 21

21-1.2 Graphite. In the LMFR, graphite is considered as the principal
choice for the moderating material because of its availability, cost, and
knowledge of its characteristics under radiation. However, there are addi-
tional special requirements for the graphite in the LMFR system. It not
only 1s the moderator, but is also the container material for the U-Bi
solution in the reactor. Hence it should be impervious to the liquid metal
and mechanically strong.

Experimental work at BNL has shown that graphite can be used directly
in contact with the fuel stream without danger of corrosion. By preferen-
tially reacting to form ZrC at the fuel-graphite interface, the Zr corrosion
inhibitor also prevents reaction of the U and fission products with the
graphite. Special grades of graphite are being developed that appear to
have the desired mechanical strength and low porosity required for use as
moderator and reflector in the reactor. Reactions of graphite with the
fuel, and the possible effects of pile radiation on these reactions, are de-
scribed in the following sections.

21-2. STEELS

21-2.1 Static tests. In order to attack the steel corrosion problem in a
basic manner, solubilities of the various components and combinations
have been determined. Most of these solubilities are given in Chapter 20.
However, more solubility work, important from a corrosion point of view,
is discussed here.

Solubility of steel components and inhibiting additives in liqued Bi. [ron.
The solubilities of iron in Bi, Bi+0.19%:Mg, Bi+40.29%U + 0.1 Mg, and
Bi+ 0.19 Mg+saturation Zr are given in Fig. 21-1. Uranium and Mg,
in the quantities added, have no effect on the iron solubility over the
temperature range 400 to 700°C. Zirconium increases iron solubility
slightly at temperatures above 500°C. Titanium (which might be present
as a corrosion inhibitor) has been found to decrease the iron solubility at
temperatures above 450°C, the extent of this decrease being proportional
(but not linearly) to the amount of Ti in the liquid. Below 400°C, there
appears to be a considerable increase in the iron solubility. For example,
Bi containing 1600 ppm Ti dissolved only 309 as much iron as pure Bi at
690°C, while Bi containing 300 ppm Ti (saturation) at 350°C dissolved
more than ten times as much iron ag pure Bi.

Zirconium. The solubility of Zr in Bi is given in Fig. 20-5. This appears
to be unaffected by the presence of Mg, Cr, or Fe in the liquid metal.

Chromium. The solubility of Cr in Bi is given in Fig. 20-6. This also
appears to be unaffected by the presence of Mg, Zr or Fe in the liquid
bismuth. However, the presence of Cr in Bi causes 2 marked reduetion in
the iron solubihity.
21-2] STEELS 715

 

   
   
  

 

 

 

T°C
727 637 561 497 442 395 353
500 i | | l I
— —
fe in Bi+Mg+U
100 |- —
50— —
— Fe in Bi+Mg+Zr —]
£ — ]
a
a —
.f ]
0= —
S5+ :
” &
g
- —
1 1 |
1.0 1.1 1.2 1.3 1.4 1.5 1.6

103/T°K
Fig. 21-1. Solubility of Fe in Bi alloys.

Miscellaneous data. The Fe—Zr intermetallic compound ZrFes appears
:0 decompose when added to Bi, Zr dissolving approximately to its normal
saturation and Fe somewhat in excess of its normal solubility in the presence
of Zr. The amount of excess Fe present in the liquid metal ean possibly
be attributed to a finite solubility of the undissociated intermetallic com-
pound ZrFes;.

The solubility of Ta in Bi is estimated to be less than 0.01 ppm (detec-
tion limit) at 500°C.

The solubility of Ni in Bi is close to 5% at 500°C and probably greater
than 19, at 400°C.

The =olubility of Mg in Bi is close to 4% at 500°C and 2% at 400°C.

Surface reactions. Experimental evidence has shown that the corrosion
resistance of steels in Bi is in part due to the formation of insoluble films
on the steel surfaces. The effect of these films on the corrosion behavior of
different steels 1s not readily determined by thermal convection loop experi-
ments because of the relatively low temperatures (400 to 550°C) and long
times assoclated with such tests. The comparative behavior of different
746 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

stecls and different films is more easily obtained from high-temperature
(600 to 850°C), short-time, static contact tests.

Steel specimens approximately 1/2 in. wide, 2 in. long, and 1/8 in. thick
are cleaned and given various surface treatments, such as sandblasting,
chemical etches, polishes, ete. Six to ten different materials are then placed
in a vacuum furnace, heat-treated as desired, and immersed in a Bi alloy
containing the desired additives. The crucible used to contain the liquid
metal is either a material inert to Bi, such as Mo or graphite, or the same
material as the specimen. After contacting, the samples are removed from
the solution at temperature and allowed to cool in He or in vacuum. The
adherent Bi is removed from the steel by immersing in Hg at 200°C in a
vacuum or inert atmosphere. After rinsing, the residual adherent Hg is
completely removed by vacuum distillation at 100 to 200°C. The cleaned
surfaces are examined by x-ray reflection techniques, utilizing a North
American Phillips High Angle Diffractometer.

Surface reaction of zirconium, titanium, and magnesium. When pure iron
was contacted with bismuth containing radioactive zirconium tracer for
1 hr at 450°C, a Langmuir type adsorption of the zirconium on the iron
crucible surface was obtained. Increasing the temperature to 520°C and
the contact time as much as 24 hr showed an increased amount of reaction.
The structure of this deposit is not known. On the other hand, when pure
iron 18 contacted in saturated solutions of zirconium in bismuth for times
ranging from 100 to 300 hours at 500 to 750°C neither corrosion nor x-ray
detectable surface deposits occur. At concentrations of zirconium below
saturation value, pure iron is extensively attacked.

A tightly adherent, thick, uniform, metallic deposit was found on the
surfaces of pure I'e dipsticks contacted with liquid Bi saturated with Ti
at 650 to 790°C. In all cases the x-ray patterns were the same but could
not be identified. The 15- to 25-micron layers were carefully scraped off
and chemically analyzed. The results corresponded to a compound having
the composition FeTi4Bis.

Pure Fe and 239, Cr-19, Mo steel samples contacted with 2.5 w/0 Mg
in Bi at 700°C for 250 hr showed no deposit detectable by x-ray diffraction.
Slight uniform intergranular attack was observed on all the samples.
Pure I'e samples contacted with Bi solutions containing 0.569, Mg
+ 170 ppm Zr, and 0.239, Mg + 325 ppm Zr at 700°C were not attacked
and did not have detectable surface films. These solutions acted similarly
to those saturated with Zr.

Reactions of steels with UB7 solutions. Uranium nitride (UN) deposits
have been identified on the surfaces of 59, Cr-1,/29, Mo, 219, Cr-19, Mo,
Bessemer, and mild steels, after these samples were contacted with Bi
solutions containing U or U 4 Mg. Extensive attack always accompanied
UN formation, indicating that this film is not protective. Nitrogen analyses
21-2] STEELS 747

made on these contacted specimens show that depletion of the N in the
steel is much more rapid than it is when the same steels are contacted with
solutions containing Zr.

Reactions of steels with Bv solutions conlaining combinations of Zr, Mg,
U, Th, and Ti. Deposits of ZrN, ZrC, and mixtures of the two have been
identified on many different steels contacted with Bi solutions containing
Zr with or without combination of Mg, U, and Th. No corrosion has ever
becn observed on such samples contacted at 600 to 850°C for 20 to 550 hr,
nor have films other than ZrN or ZrC been found. When a mild steel was
contacted with Bl containing 1000 ppm Zr and 200 ppm Ti at 650°C, x-ray
examination showed strong lines for TiN and a less intense pattern of TiC.

Considerable difficulty was experienced 1n establishing the correct unit
cell dimension for the nitrides and carbides of Zr and Ti. Many different
values may be found i the literature. The inconsistency in the data
probably can be attributed to the existence of varying amounts of C, O,
or N 1n the samples. Table 21-1 gives the parameters determined by a
nunber of mvestigators. The values of ap used in this research were those
given by Duwez and Odell [1]. These compared favorahly with the values
found on test specimens, powdered compact samples, and ZrN prepared
by heating Zr in purified N2 at 1000°C for 20 hr.

A nondestructive x-ray method of measuring film thickness has been de-
veloped for this research [2]. The x-rays pass through the film and are
diffracted by the substrate back to a counter. The intensity is reduced by
the absorption of the film. Unknown conditions of the substrate are
eliminated by measuring the intensity of two orders of reflection or by
nmeasuring the intensity of a reflection using two different radiations. The
method 1= accurate to about 209.

TaBLE 21-1

PuBLisHED X-rAY PARAMETERS FOR THE UNIT CELLS OF
ZrC, TiC, ZrN, axp TIN
(CuBic, NACl-TYrE)

 

 

 

Becker and | Van Arkel | Kovalski and | Dawihl and | Duwez and !
Ehert [20] [21] Umanskii [22] | Rix [23] Odell [24]
; |
 IaC 4.76 4.73 46734 4685
‘ TiC 4.60 4.26 4,4442 4.31 4.32
N 4 63 4.61 4 567
- TiX 440 4.23 4,234 4936 4.237 |

 

 

 

 

 

 
748 MATERIALS OF CONSTRUCTION—METALLURGY [caAp. 21

TaBLE 21-2

ORIGINAL ANALYSES AND Frovms FORMED ON SPECIAL STEELS
UseEp 1N StaTic TESTS

 

 

 

‘ , o7, Al N % N as Film
Material Sol) | (Toty | EENT L UUBHEN | formed

5Cr-+Mo 0.016 (0.023 0.0002 1.0 ZrN
2:Cr-1Mo 0.003 0.042 0.0001 0 »
21Cr-1Mo 0.055 | 0.050 — — ”
21Cr-1Mo 0.003 0.01 — — ”
24Cr-1Mo 0.06 0.047 0.0003 1.0 ?
2+Cr-1Mo (.009 0.013 0.0001 1.0 "
Bessemer 0.003 0.009 0.0002 2.0 ”
Carbon 0.007 0.005 0.0001 2.0 ”
2+Cr-1Mo 0.015 0.015 100 ZrC
23Cr-1Mo 0.44 0.054 0.025 50 ”
22Cr-1Mo ¢.014 0.013 0.009 70 ?
24Cr-1Mo 0.022 0.015 0.010 70 ”
24Cr-1Mo 0.02 0.015 0.011 75 ”
11Cr-1Mo 0.02 0.014 | 0.010. 70 &
RH 1081 (0.3 Ti) "

 

 

 

 

 

 

 

 

*EHN: Ister-halogen insoluble nitrogen. This is believed to be an indication
of the nitrogen combined as AIN or TiN in steels [26].

Effect of steel composition and heat treatment. It has been found experi-
mentally that some steels with very similar over-all compositions behave
quite differently in the same static corrosion tests. Films that form on
these materials range from pure ZrN to pure ZrC. Table 21-2 gives typical
analyses selected from the more than 100 steels run in static corrosion fests,
and identifies the surface films.  After contacting, the only changes in
analyses were found in the total nitrogen remaining and the amount of
ester halogen insoluble nitrogen (ISIIN) present in the steels. The only
significant  difference 1n analyses between nitride-formers and carbide-
formers in Table 21-2 is found in the relative amounts of IX'HN. The
carbide-formers have more than 509 of the total nitrogen combined as
EHXN, while the nitride-formers have only a few percent of the total nitro-
gen combined. At present, the relationship between the N, Al Cr, and the
Mo contents of the steels and their film-forming properties is not obvious.
Some excellent nitride-formers have very low nitrogen content, while some
carbide-formers have high nitrogen content. The same holds true for the
21-2] STELLS 710

Al Cr, and Mo contents of the steels. The TN content of a steel can be
readily changed by short-time heat treatment @t 700°C and higher [3], so
that this variable is controllable within limits.

To a first approximation, the corrosion resistance of a particular steel is
enhanced by high “inhibitor’” concentrations and/or the presence of in-
soluble adherent films formed on the steel surface. The first of these con-
ditions 1s neither desirable nor practical in a solution-type fuel reactor be-
cause of the adverse effect of Zr on the U solubility. At present, work is
being done to measure quantitatively the etfects of different alloying con-
stituents on the activities of N and C in steels. Consider the following
reactions:

ZI‘(Bi) + N(stccl) O~ ZrN (Alm), (2]*1)
Zr(Bi) + C(Hteel) - Zr(-\j(fllm)- (21—2)
Assuming that the films are insoluble in Bi, then at equilibrium

1

v and Koy = (@ (a0

. 1
Ky = Tam)(an) (21-3)

azy (aN)
If the products of the Zr activity in the Bi with the activities of the N and
(' in the steel are not sufficient to satisfy the respective equilibrium con-
stants, the reactions will not occur, and the steel will not form ZrC or
ZrN films. If the activity products are greater than the constants, Ko
or Az, the reactions will proceed until the activities are lowered to these
values. Thus, for a fixed Zr activity, the activities of N and C in the steel
determine whether the carbide and nitride film-producing reactions should
occur. The excess of N or C above these equilibrium values should be a
meazure of the driving force of reactions (21-1) and (21-2) to the right.
Solution rate tests. The solution rates of Fe into Bi, and Bi+ Zr and Mg,
were measured in erueibles of a carbon steel, a 239% Cr-1% Mo, a 5%
Cr-1 29 Mo, and an AIST type—410 steel. The crucible, Bi, and additives
were equilibrated at 400 to 425°C, the temperature rapidly raised to 600°C,
and the concentration of Fe in solution measured as a function of time.
Results are shown in Fig. 21-2. In the presence of Zr-+ Mg, the 5%
Cr—1 27 Mo and the AISI type—410 steels dissolved at approximately
the =ame rate, while the 219 Cr-1% Mo steel dissolved more slowly. No
detectable dissolution of Fe from the carbon steel was measured in 44 hr
at 610°C. These results are parallel to the thermal convection loop results,
and consistent with the film-formation studies in that the measured solu-
tion rates are inversely proportional to the ability and rate at which the
steels form ZrN films. At present no data are available on rates of solution
for ZrC-forming steels.
 

750 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

 

] |
70 r ] | ] T I
/Sofubflify {Temp Cycle}

60 — /5% Cr Steel into Pyre 8i |
|

l
I
|

12% Cr Steel into / —
Bi + 0.01% Mg . 5% Cr Steefl into
................ graneer BI + 0‘10/0 Zr + 0.]0/0 Mg
L /‘?\__ o . .
12% Cr Steel into Bi + 0.05% Zr
- / -

+ 0.04% Mg "

_______ |-
_________ T——2.1/4% Cr Steel into B + 0.05% Zr

—

 

 

 

 

 

e D=7 7 21020 Steel into Bi + 0.04% Z + 0.12% Mg
J','/ / eel Into Bi r i }_
i E l ! ! 1 i I} I | | |
0 1 2 3 4 3 6 7 g 20 40 60 80

Time in Hours

F1a. 21-2. Dissolution of Fe into Bi (plus additives) at 600°C from steel crucible.

Rates of precipitation. The rate of precipitation of iron from bismuth in
a pure iron steel crucible is very rapid. Iron precipitated from bismuth,
saturated at 615°C, as rapidly as the temperature could be lowered to
425°C. The addition of Zr plus Mg to liquid metal did not change the rapid
precipitation of most of the iron from the bismuth under these same condi-
tions, but produced a marked delay in the precipitation of the last amount
of iron in excess of equilibrium solubility. An apparently stable super-
saturation ratio of 2.0 was observed for more than 7 hr at 425°C in a
pure iron crucible containing Bi 4 1000 ppm Mg <+ 500 ppm Zr, and 1.7
for more than 48 hr at 450°C. In a 59 Cr steel crucible, a supersatura-
tion ratio of iron in Bi+ Mg+ Zr of 2.9 was observed after 24 hr at 425°C.
This phenomenon may be due to the ability of the formed surface deposits
to poison the effectiveness of the iron surface as a nucleation promotor or
catalyst, the different supersaturations observed being due to the relative
abilities of a Zr-Fe intermetallic compound or of ZrN to promote nuclea-
tion of iron. This observed supersaturation suggests that mass transfer
should be nearly eliminated in a circulating system in which the solu-
bility ratio due to the temperature gradient does not exceed the meas-
ured “‘stable supersaturation’” at the cold-leg temperature.

Precipitation rate experiments made in AISI type-410 steel crucibles
show that Zr 4 Mg stabilize Cr supersaturations of 2.0 to 3.0 for more
than 24 hr. However, no Cr supersaturation was found during precipita-
tion rate experiments made in pure Cr crucibles when Zr+4 Mg were
present in the melt [4]. The measured supersaturations should therefore
be due to the films present on the steel surfaces.
21-21 STEELS 751

21-2.2 Corrosion testing on steels. The research effort on materials for
containment of the LMIER has been concerned mainly with low-alloy
steels having constituents which have low solubilities in 131, such as C, Cr,
and Mo.  Although the solubilities of Fe and Cr are only 28 and 80 ppm
respectively at the intended maximum temperature of operation, severe
corrosion and mass transfer are encountered when pure Bi or a U-Ii solu-
tion 1s circulated through a temperature differential in a steel loop. This
results {rom the continuous solution of the pipe material in the hot portion
of the system and subsequent precipitation from the supersaturated solu-
tion in the colder portions. Zirconium additions to U-Bi greatly reduce
this corrosion and mass transfer.

The behavior of steels in U-Bi is studied in three types of tests. Thermal
convection loops are used to test materials under dynamic conditions. In
these, the fuel solution is continuously circulated through a temperature
differential in a closed loop of pipe. Variables such as material composi-
tion, maximum temperature, temperature differential, and additive con-
centrations are studied in this test. More than sixty such loops have now
been ran at BNL. The prineipal imitation in these tests is that the veloc-
ities obtained hy thermal pumping are extremely low when compared with
the LAIFR design conditions.

Foreed circulation loops are used to study materials under environments
more closely approximating LMER conditions. Three such loops are now
i operation at BNL and two more are under construction. A very large
loop (+ in. ID) which will circulate U-Bi at 360 U.S. gpm and transfer
about 2% X 10% watts of heat, is now under eonstruction and is expected to
g0 nto operation late this year.

Statie tests, as discussed previously, in which steels are isothermally im-
mersed in high-temperature U-Bi containing various additives, are used
to ~tudy their corrosion resistance and the inhibition process as a function
of additive concentration and steel composition.  Most of the tests have
been performed on a 239 Cr-19 Mo steel (Table 21-3). However, some
testx have also been made with higher Cr steels, 139 Cr-1/2% Mo,
1200 Cr=1 29 Mo, and carbon steels.

21-2.3 Thermal convection loop tests at BNL. A typical thermal con-
veetion loop that has been used at BNL is shown in Fig. 21-3. The loop
1= provided with a double-valve air lock at the top of the vertical section
which permits taking liquid metal samples while the loop is running without
contaminating the protective atmosphere. The hot leg is insulated and
heat 1= <upplied to that section of the loop while the cold leg is exposed and
two =mall blowers are utilized to extract heat. The hottest point in the
loop 1= at the “tee”” at the upper end of the insulated section, and the coldest
in the bottom of the exposed section. The total height of the loop proper is
TaBLE 21-3

CoOMPOSITION OF STEELS T ESTED

 

 

 

 

 

 

Steel C Mn Si P{max) | S(iax) Cr Mo Others
Carbon Steel 0.08 0.85 0.01 0.09 0.27 — — —
Bessemer 0.07 0.42 0.009 0.056 0.022 — — —
RH 1081 0.31 0.12 0.14 0.018 0.020 — — 0 30 Ti
1/2Cr-1/27Mo* 0.10-0.20 | 0.3-0.61 | 0.1-0.3 0.045 0.045 0.5-0.81 | 0.45-0.66 —
12Cr-1/2Mo* 0.15 0.3-0.6 0.50.1 0.045 0.045 1.0-1.5 0.45-0.66 —
2:Cr-1Mo* 0.15 0.3-0.6 0.50 0.045 ¢.030 1.9-2.6 | 0.87-1.13 —
5Cr-1/2Mo* 0.15 0.3-0.6 0.50 0.045 0.030 4-6 0.45-0.65 —
5Cr-S1* 0.15 0.3-0.6 1.0-20 0.045 0.03 4-6 0.45-0.65 —
9Cr-1)Mo* 0.15 0.3-0.6 0.25-1.0 | 0.045 0.03 8-10 0.9-1.1 e
AISI Type 410% | 0.15max | 1.00 0.75 0.030 0.030 | 11.5-13.5 - N1 0.50 max
18Cr—8Ni 0.08max | 2.00max | 0.75 0.030 0.030 18-20 — Ni &-11
AISI Type 304%*

ATST 4130* 0.28-0.33 { 0.4-0.6 0.2-0.35 1 0.04 0.04 0.8-1.1 | 0.15-0.25
Rex AA* 0.73 4.0 V 1.15; W 18 Bal Fe
Stellite 90* 2.75 27.0 Fe balance

 

 

 

 

 

 

 

 

 

*Nominal composition.

NOLLIOHLSNOD 40 SIVIHUHLVIX

ADUATIVIIIN

13 dvHD]
21-2] STEELS 753

 

 

 

Fic. 21-3. Thermal convection loop. A. Air lock. B. Hot leg. C. Cold leg.
D. Fans. E. “Tee” connection. F. Melt tank with AISI type—410 steel filter

bottom.

approximately 15 in. and the total length of the loop is approximately
10 in.  With this configuration, the flow rate is approximately 0.05 fps
when Bi is circulated with a 100°C temperature differential. Temperature
differentials ranging from 40 to 150°C can be conveniently applied to the
loop. Radiographic inspection of the loop while in operation is periodically
made to monitor it for corrosion at the hottest section and deposition at
the coldest section. The inside of the steel pipe for the loop is either acid-
cleaned or grit-blasted. The pipe is then cold-bent to the desired shape,
and welded at the “'tee’” by the inert-gas shielded-arc process.

The general procedure for running the loop is as follows: (1) Solid Bi
is charged into the melt tank. (2) The entire system is leak-checked with
a mass spectrometer. (3) The Bi is melted and introduced into the uni-
formly heated (550°C), fully insulated loop through a 35-micron AISI
SUMMARY OF THERMAL ConvEcTiON Loor DaTa

TasLE 214

All loops were fabricated from 1,/2 IPS Sch 40 pipe of the steel indicated

 

Additives, nominal

Liquid metal

 

Duration

 

 

 

 

Test Steel Welding composition, ppm temperature, °C of test, Resulte
no. rod < hr
U | Mg | Zr | Others | Max | Min | Diff,
1 2:Cr-1Mo 5Cr—+Mo 1000 | — | — — 550 | 510 40 405 Plugged
2 noo» no 1000 | — | — | — | 550 | 475 | 75 310 | Plugged
3 ” 7 ” ” 1000 | — | — — 5500 | 432 | 118 260 Plugged
4 » " ” ” 1000 | 350 | 250 — 550 | 460 90 13,550 No corr; no deposition
5 M 7 ” " 1000 | 350 | 250 — 530 450 100 11,673* | Weldcorr. (5Cr-1,/2Mo);
mocderate deposition
6 " ” ” » 1000 | 350 | 250 — 550 | 450 | 100 10,928 Weld (5Cr-1,/2Mo) and
pipe corr.; moderate
deposition
7 7 ” 23Cr-1Mo 1000 | 350 | 250 — 525 | 425 | 100 9,834* | Pipe corr.; shight deposi-
tion
8 ” ” 5Cr—$Mo 1000 | 350 | 250 — 500 | 400 | 100 10,869*  Nocorr.;slight deposition
9 ” " " 7 1000 | 350 | 250 — 6500 550 50 5,643
600 | 525 75 2,686
600 | 500 100 4,152* 1 Weld corr. (5Cr-1/20No)
moderate deposition
10 ” " 2:Cr-1Mo 1000 | 350 | 325 — 500 | 400 100 5,205% | Weld (21Cr-10Mo) and

 

Normalized
and tempered

 

 

 

 

 

 

 

 

 

 

pipe corr.; slight dep-
osition

 

=1

w2 |

STVIHALLVIA

ADUATIVILIAN—NOILIQYLENOD A0

13 'dVHD]
19
20
21
22
23

24

 

 

2UCr Mo
Be msert
200 T o
raphite tnsert
10T 1Mo

o
=
2

2:Cr-1Mo

23Cr=1Mo
nitrided after
welding
2;7Cr—1Mo

25Cr-13 o
Bessemer
("arbon steel
RH1081
1Cr-$Mo
1:Cr-5M Mo
14Cr—523lo

5Cr—Si1

9(Cr-1M7o

 

D1 Mo

KO SN o

1 )

Carbon steel
RH1081
14Cr-33Mo
11Cr-3\o
13Cr—33 o
5Cr-5Mo

9Cr-1Mo

 

100

1000

10300

1000

1000

1000

1000
1000

1000
1000
1000
1000
1000

1000

 

aol) 1Bl

300

300

300

350

350

300
350

350
350
350
300
350

350

 

250

250
250
250
325
400
325
250

250

 

 

250

650
250

450
500
n25
500
550
550

250

 

400

440

440

415
405
425
400
440
420

400

 

100

110

110

130

150
135

135

95
100
100
100
130

 

1,631*
15,086

16,906

10,649*

10,425*

5,323

1,180
12,356

8,231*
8,538*
9,194*
963
6,240*
3,340

6,025

 

No corr.; slight deposi-
tion

Severe corr.; moderate
deposition

Corroded  through at
weld  (HbCr-1/2Mo);
moderate deposition

Weld corr.(5Cr-1/2Mo);
moderate deposition

No corr,; slight deposi-
tion

Heavy pipe corr.; heavy
deposition

Plugged; severe corrosion

No corr.; no deposition

No corr.; no deposition
No corr.; no deposition
No corr.; slight deposition

Nocorr.;slight deposition

Slight corr.; some depo-
sition

Severe corr.; very heavy
deposition

 

 

*Test in progress as of 3/15/58. Time indicated is duration at temperature differential,

based on radiographic inspection.

Results indicated for these loops are

continued

[z-12

STHHLS

-3
o
n
TasLE 21-4 (continued)

 

Additives, nominal

Liquid metal

 

 

 

 

 

 

’Ir‘l(;s't Steel W i}iflmg composition, ppm temperature, °C I?)lfll tif:sl’:,n Results
U | Mg | Zr | Others | Max. | Min. | Diff. hr
25 18Cr—8Ni 18Cr-8Ni1 1000 | 350 | 250 — 550 | 400 150 630 Plugged; severe corr.
26 2LCr-1Mo 5Cr—4\o — — i 450Th ! 550 | 500-| 50— 1,204 Plugged; severe corr.
480 70
27 23Cr-1Mo 5Cr—$Mo 1000 | — | 400 | 400Th{ 550 | 435 | 115 8,567 Severe corr,; very heavy
precipitation
28 21Cr-1Mo 5Cr—$Mo 1000 | 350 | — | I000T1 | 550 | 445 105 5,413 Plugged; severe corr.
29 23Cr-1Mo 5Cr-iMo 1000 | — | 250 | 500Ca | 530 | 450 | 100 2,374 Plugged

 

 

 

 

 

 

 

 

 

 

 

 

 

1Y)

STYIUILVYIN

Jd0

NOILOAHLENOD

ADHATIVLAKN

17 "dVHO]
21-2] STEELS 757

type—410 stainless-steel filter. (4) Zr and Mg are introduced into the loop
through the air lock. (5) The Bi in the loop is sampled, using a graphite
sample extractor, to check for additive concentration. (6) Uranium 1Is
added if additive concentrations are as desired; if necessary, additive
concentrations are adjusted prior to U addition. (7) The temperature dif-
ferential is obtained by removing the insulation from the cold leg and
starting the fans. The temperature differential is usually applied in two
steps, first 40°C and then the differential at which the loop is to operate.
(8) The entire loop is radiographed every 750 hr. (9) The liquid metal is
sampled at regular intervals. (10) After completion of the test the entire
loop is sectioned longitudinally and transversely for metallographic ex-
amination.

In Table 21-4, data from 29 thermal convection loop experiments at
BNL are summarized. The first three loops were fabricated from 219
Cr-19, Mo steel pipe and the U-Bi solution was not inhibited. Loops No. 4
to 17 inclusive were made with 219, Cr-19,, Mo steel and inhibited with
Mg and Zr. Loops No. 18 to 25 inclusive were fabricated from various
tvpes of steels, ranging from Bessemer to 187, Cr-89; NI austenitic steels.
Loops No. 26 to 29 were made from 219, Cr-1%, Mo steel pipe. The pur-
pose of these tests was to study the effectiveness of Ca, Th, and Ti as in-
hibitors.

[t can be seen from the first three tests that deposition in the cold legs of
uninhibited loops after a few hundred hours is sufficient to stop flow. These
tests show conclusively that uninhibited U—-B1 solution causes serious mass
transfer of this steel even at a temperature differential as low as 40°C.
Metallographic examination of the hot legs of these loops showed a gen-
cralized intergranular attack.

The data from loops inhibited with 350 ppm of Mg and 250 to 350 ppm
Zr (loops No. 4 to 17) show that mass-transfer rate can be decreased con-
siderubly by the introduction of these additives. This effect is attributed
to the formation of a ZrN film on the steel surface [4,5]. The data demon-
strate, however, that this film is not completely protective in 23% Cr-1%
Mo steel svstem. Test No. 4 indicates that for a differential in the order of
490°C. the film was sufficiently protective to prevent corrosion or deposition
in 13.550 hr of test. For temperature differentials of 100°C or higher, in-
cipient corrosion can be expected in about 5000 to 6000 hr. Some of the
21 ('r—1 Mo loop sections were joined with 5 Cr-1/2 Mo welding rod. This
higher chromium material has lower resistance to inhibited U-Bi than the
pipe. =0 that corrosion generally starts at these welds. Increasing the
temperature of the hot leg seems to increage the rate of corrosion, as il-
lustrated by the results of loops No. 5 to 9 inclusive, which were all tested
at 100°C temperature differential.
758 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

Loop No. 10, which was normalized from 954°C and tempered at 732°C
after welding, stood up poorly when compared with other tests. The heat-
treating was done in an argon atmosphere, and no subsequent alteration
was made to the surface left by the heat treatment. This heat treatment
was thought, from some observations in the pumped loops, to improve cor-
rosion resistance. It is believed that the poor results of test No. 10 are due
to alteration of the surface during the heat treatment and not to the
metallurgical structure of the steel. Loops No. 11 and 12 had Be and graph-
ite inserts in the hot leg to study their effect on mass transfer and also the
stability of U, Mg, and Zr concentrations. No detrimental effects on either
have heen observed.

Loop No. 15, made of a 2% Cr-1 Mo steel that was internally nitrided to
a depth of 0.015 inch, appears to be standing up much better than other
2% Cr—1 Mo loops tested at a 125°C temperature differential, The added
nitrogen in the steel probably promoted the formation of the ZrN film. A
slight loss in Zr hag been ohserved in this test, but other additive concen-
trations have remained constant.

Results of tests No. 18 to 25 show that higher-alloyed steels are inferior
to the carbon steels in inhibited U-Bi. Loop No. 18, fabricated from
Bessemer carbon steel, showed exceptional resistance to U-Bi. Metal-
lographic examination of this loop indicated no evidence of corrosion or
deposition after 12,356 hr of operation at 135°C temperature differential.
X-ray diffraction studies of a polished insert in this loop indicated that a
ZrN film was present; however, no film was detected on the pipe surface.
Considerable evidence of structural instability in the form of grain coarsen-
ing and graphitization of the Bessemer steel was found in the metal-
lographic examination of the loop. A medium carbon steel containing Tj,
Rh-1081, also exhibited good resistance to U-Bi corrosion. The lower
chromium steels, such as 1/29 Cr-1/29, Mo and 119, Cr-1/29, Mo
(tests No. 20, 21, and 22), also seem to be standing up well to inhibited
U-Bi. The testing of these stecls will be increased.

Loops No. 26 to 29 were run to study the effectiveness of Th, Th - Zr,
Ti-+ Mg, and Ca 4 Zr as inhibitors. It can be seen from these tests that
these inhibitors were much inferior to Mg and Zr combinations. In tests
No. 27 and 29 it was found that a slow but continuous loss of Zr, Th, and
Ca oceurred. Horsley [6] also ran Bi loops containing Ca and Zr as in-
hibitors. He reported similarly that the loops plugged, and that Ca and Zr
were lost from the melt. The results of loop No. 28 indicate that Mg and
Ti provide some inhibition, but are not nearly as effective as Mg and Zr.
Metallographic examination of this loop shows that the corrosion was uni-
form and that most of the attack took place 4 to 6 in. downstream from the
“tee,”” in an area which is normally somewhat lower in temperature than
the “tee.”
21-2] STEELS 759

Metallographie examinations of 239, Cr-19, Mo loops inhibited by Mg
and Zr show that corrosion starts in the form of a pit. After the pit has
penetrated about 0.020 or 0.025 1n. into the pipe, the progress of corrosion
generally proceeds laterally on the pipe, widening the pit rather than
deepening it. A typieal pitted area formed by U-Zr-Mg-Bi in 219, Cr-19,
Mo steel s shown in Fig. 21-4. The attack is transgranular throughout
the pitted area. Horsley [6] reported intergranular attack at the bottom
of pit= formed in a similar steel by Zr-Ca-Bi. Metallographic examination
of plugged thermal convection loops shows the deposit to be very flaky and
not tightly adherent to the loop walls. Chemical analysis of the deposition
n a 240, Cr—19, Mo steel loop indicated it to be about 959, Fe and 29, Cr.
ZrN films have been positively 1dentified by x-ray reflection in only two
thermal convection loops: Bessemer steel loop, and one 239, Cr-19%, Mo
stecel loop. The protective film is possibly so thin that it can be identified
only under 1deal conditions.

21-2.4 High-velocity tests. Although thermal convection loops are
convenient for extensive corrosion testing under dynamic conditions, the
velocity is not large enough to give design data for actual operating condi-
tion=. These data must be obtained by operating loops in which the bis-
muth =olution is pumped at considerably higher linear velocities through
<uitably designed test sections.

Attainment of higher flow velocities complicates corrosion testing meth-
od~.  lLarge heat inputs are necessary to obtain temperature differentials
comparable to those readily attained in thermal convection loops. Special
equipment 1s required to measure flow. Pumps must be designed leaktight
to maintain absolute system purity. The U-Bi must be prevented from
freezing in the piping.

~ome important features of the BNL loops are (1) the systems are com-
pletely sealed and operate under a purified inert-gas blanket, (2) samples
ot the liquid metal can be taken at any time during operation without
contamination, (3) radiography permits nondestructive examination of
test =aniples so that long runs are possible, and (4) the safety control system
ix designed to prevent the freezing of the U-Bi (and subsequent bursting)
m the piping.

In corrosion loops (HVL T and IT) at BNL [7], no valves are used to
hold the fluid up in the system, and flow is measured with a submerged
orifice located in the sample tank. Piping in these loops is 3/4-in. schedule—
10 except in the test sections. A GE G-6 electromagnetic pump is used in
HVIL I. while Callery 25-20 electromagnetic pump 1s used in HVL 1L,
Flows in the order of 1 to 2 gpm are achieved with both pumps. An inter-
mediate heat exchanger does about 50% of the heating and cooling. Re-
sistance furnaces provide the heat. The head developed is sufficient to
760 MATERIALS OF CONSTRUCTION—METALLURGY {crAP. 21

(b)

(a)

 

Fic. 21-4. Pitted area at "'tee” in Loop #12. (a) Macrograph. (b) Micrograph
(original 250X) of pitted areas.
21-2] STEELS 761

i Sampling
4177 Vessel

 

Cooler

    
  

. //4— Heat Exchanger

 
 

-7
. )
# -<—— Fyrnace

g

<" 17 1/2" Hot Test Section

  

Charge & Dump Tank

Dump Valve

F1c. 21-5. High-velocity pump test loop.

allow the use of short small-diameter test sections in which velocities up to
8 fp= are attained.

A third corrosion loop (loop G) uses a canned centrifugal pump to cir-
culate the U-Bi, permitting flows up to 4.8 gpm and velocities up to 14 fps
to be attained. Engineering devices such as bellows-sealed valves and
pressure transmitters are being tested in this loop, as well as materials for
high-velocity corrosion resistance. Heating and cooling 1s the same as in
HVL I and II. The loop is fabricated for the most part of 249% Cr-1% Mo
steel, I-in. schedule—40 piping.

Two more loops (HVL III and IV), similar to loop G, are now under
construction. These are shown in Fig. 21-5. Flow rates up to 12 gpm,
velocities up to 25 fps, and temperature differentials of 150°C will be
attainable in these loops. HVL III will be fabricated of 239 Cr-19% Mo
steel. while HVL IV will be of 1% Cr-1/29% Mo steel.

While the basic material of construction of these loops is a Cr—Mo steel,
test sections are usually made up of a variety of steels and weldments.
All welds are made by the inert arc process. Cleaning, for the most part,
1s done by grit-blasting.
TasLE 21-5

Resurrs FroMm Higa Vevocity Loors

 

 

 

 

 

Temp béBUIk)’ Tem%.cFllm, Additive cone. Th}le
Loop no. Material* © Flow, Remarks
test, gpm
Max | Min | Max { Min | Mg | Zr | U | hr
HVLI:
Run 1 | 25Cr-1Mo H20 414 525 1 400 | 350 | 300 1 920 | 1026 1.20 | No corrosion
Run 2 | 24Cr-1Mo 520 414 525 | 400 | 350 [ 350 | 1000 | 1026 1.20 | Cavitation pits in high veloc-
ity test section
Run 3 | 23Cr-1Mo 520 414 525 | 400 | 350 | 300 | 1000 | 1006 1.25 | No cavitation
Run 4 | 23Cr-1Mo 544 417 550 | 400 | 350 | 250 | 1000 | 2591 1.1 Severe pits and mass transfer;
loop sectioned
Run 5 | 11Cr-3Mo 520 445 925 | 428 | 350 | 250 1 1000 | 40007 No corrosion; some transfer
(loop refabri- after 4000 hr
cated)
HVL II: | 22Cr-1Mo 520} 445 922 | 427 | 350 | 350 | 1000 | 7400t Slight pit corr. in welds; trans-
Run 1 fer detected after 2500 hr
at AT loop still in opera-
tion 7000 hr
Loop G: | 23Cr-1Mo 525 473 D29 | 458 | 350 | 350 | 1000 | 938 4.0 Pt. corr. of 4-6 Cr-1Mo welds;
Run 1 corr, of AISI 410 SS.
Run 2 | 23Cr-1Mo 525 450 526 | 435 | 350 | 250 | 900 | 25001 | 4.8 No corr. after 2500 hr

 

 

 

 

 

 

 

 

 

 

 

 

 

*This is the major material of construction. The actual test section is a composite of many materials.

TStill in operation. Test time as of 3/15/58.

G9L

ADHATIVIAN-—NOILLINQYULSNOD J0 STVIHALVIN

12 "dVHO]
21-2] STEELS 763

 

F1a. 21-6. Prempltatlon of Fe-Cr alloy in hlgh-velomty cold-leg test section
(HVL I-Run 1).

Results obtained thus far with the pumped loops are shown in Table
21-5. In most cases a run consists of a test on a particular test section rather
than on the entire loop. The exception is Run 4 of HVL I, which was
continued until the loop plugged, at which time the entire loop was dis-
mantled and refabricated of 12% Cr-1/29% Mo steel. The film temperature
reported is the calculated liquid-steel interface temperature. In all tests
Zr and Mg were used as inhibitors.

The purpose of Run 1 on HVL I was to study the effect of velocity on the
material deposited in the cold portions of the system. It was felt that a
high flow rate might dislodge the loose precipitated crystals and circulate
them to the hot sections, where they would redissolve in preference to the
pipe wall. The high-velocity section was placed at the region of the coldest
bulk liquid. All test specimens were fabricated of 239% Cr-19% Mo steel
and welds were performed with a 5% Cr-1/2% Mo bare filler rod.

Examination of the test sections after 1026 hr of test showed that no
corrosion had taken place in the hot sections. A one-grain, adherent layer
of precipitated Fe—Cr alloy was found in the high-velocity cold section
(Fig. 21-6).

To test the effect of impact on the inhibiting film, a right-angle, high-
velocity section was Inserted in the hot leg of HVL I-Run 2. The section
was about 5 in. long, 0.355-in. ID and contained a graphite insert specimen
and an annealed and hardened 21% Cr-19% Mo steel specimen. All welds
were made with 5% Cr-1/2% Mo bare filler rod. The flow through the
764 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

  
 

Bi Channel
Cavitation
Pits !
Cavitation
e Pits

Fig. 21-7. Cavitation-erosion on downstream side of right-angle bend in HVL
I[-Run 2.

 

F1g. 21-8. Preferential attack on 59, Cr-19, Mo Weld in HVL I-Run 4.
21--2] STEELS 705

small diameter was 5 fps. Physically, the right-angle section was located
just outside the furnace. Polished bushings and tab samples were also
placed in the furnace leg. Temperature conditions were identical to Run 1.

This run was terminated after 1026 hr of AT operation, and samples
were removed for metallographic examination. No corrosion was detected
on the polished samples located in the furnace. No erosion or corrosion
wis observed on any of the graphite samples. Again a very small amount of
deposition was found in the cold section, this time between the pipe wall and
the cold sample bushing inserts. Large pits, some about 1/2-in. diameter
and 1/8-in. deep, were found on the stecl samples located in the exit side of a
right-angle bend (Fig. 21-7). If a vortexing of the fluid (therefore a locally
increased velocity) is assumed to have occurred after the change of direction
in the right-angle bend, then a condition for cavitation may have existed,
i.e.. the static head at this point (7 psi gauge) may have been exceeded by
the dynamie head.

Run 3 of HVL I was a duplicate of Run 2 except that the static head at
the right-angle bend was doubled. Examination of the specimen after the
1000-hr run showed that the cavitation pitting was eliminated.

tun 4 of HVL I was intended to test the inhibiting film under an in-
creased temperature gradient (150°C) with the maximum temperature
riseid 1o 530°C. The test section consisted of samples of 23% Cr-1% Mo
<tee] thoth annealed, and normalized and tempered), mild steel, and several
erades of graphite. All welds were made with 5% Cr-1/29% Mo bare filler
roel. This run was terminated after 2591 hr of operation because of ex-
ten<ive pitting in the hot section and plugging in the finned cooler section,
1~ revealed by radiographs.  Attempts to drain the loop completely were
wr=eces<ful. Upon sectioning, localized masses of intermetallic compounds
were tound, which probably developed when the system was cooled. These
formations did not redissolve on heating because of the lack of good mixing,
s were visecous enough to prevent the bismuth from draining.

The entire loop was sectioned and examined. A severe pit-type corrosion
wi~ found in the hot sections; welds (Fig. 21-8) as well as parent material
Tie. 21-9) were grossly attacked; 22% Cr-1% Mo steel in the normalized
«uid tempered condition was less corroded than the same material in the
simcaled condition: carbon steel samples showed little or no attack.
Corrosion oceurred in erevices between the samples and pipe walls. The
mas=-transfer plugs occurred in the region of the coldest film temperature
rather than the coldest bulk temperature. Veloeity did not sweep away
the depoxited material, but rather a plug of high density was produced

Fro 21-100,

The minimum film temperature for this run may possibly have been as
el s 23°C Tower than the 400°C reported because of an error involved in
~electing the wetual cooling area.
766 MATERIALS OF CONSTRUCTION-—METALLURGY [crAP. 21

 

Fia. 21-9. Attack on annealed 239, Cr-19;, Mo hot test specimen in HVL
I-Run 4.

 

F1a. 21-10. Cross section of deposited Fe-Cr alloy in finned-cooler section of
HVL I-Run 4.
21-2] STEELS 707

Loop G-Run 1, had a hot-leg test section consisting of specimens of
2107 Cr-1¢7 Mo, 139, Cr-1/20; Mo, AISI type-410, and Bessemer steels,
joined with welds made with 239, Cr-19; Mo, 17% Cr-1/29, Mo, 5%
Cr-1,2¢;, Mo, AIST type-410, and mild steel bare filler rods. This run was
terminated after 938 hr of operation at & 75°C {ilm gradient and a flow of 4
fps in the test seetion. Lxamination of the test section showed that welds
made with a 539 Cr-1/29, Mo rod were severely attacked, while those
made with a 239, Cr-19;, Mo rod and a 139, Cr-1,/29% Mo rod were not
attacked. The only other corrosion observed in this run was some slight
attack on welds and base material of AISI type-410 steel. Cavitation pits
were also observed on the pump impeller.

Loop G-Run 2, HVIL I-Run 5, and HVL II-Run 1 are still under test.
In these loops are specimens of 239, Cr-19; Mo and 139, Cr-1/29, Mo
steels having various heat treatments, AISI type—410 and Bessemer steels,
and welds made with 119}, Cr-1/29, Mo, 239 Cr-19, Mo, 5% Cr-1/2%,
Mo, AISI tvype—410 and mild steel bare filler rods. No corrosion has been
detected radiographically in the loop G test section after 2500 hr of opera-
tion. No corrosion has been detected in the HVL I-Run 5 test section;
however, slight deposition has been detected in the finned cooler. This
precipitate was first observed after 1400 hr of operation but is still not seri-
ous after 4000 hr.

Pitting of welds made with 59, Cr-1/29, Mo rod and possible corrosion
of a 21¢7 Cr-19, Mo weld have been detected in Run 1 of HVL II. This
loop first operated 2500 hr with a film AT of 75°C (300 to 425°C film) with
no radiographically detectable corrosion and little or no mass transfer.
The temperature differential was then increased to 75°C bulk (522 to
427°C film). After 100 hr at this new differential, deposition in the cooler
was ob=erved. Pitting of the 59, Cr-1/29; Mo weld was detected after
2000 hr at the new differential. However, after a total of 7400 hr of tem-
perature gradient operation, the amount of pitting and transferred ma-
terial was not serious enough to stop loop operation.

The effect of velocity on corrosion and mass transfer is not apparent at
this time. This is mainly due to a lack of tests in which velocity is the only
vuriable.  Correlation between the pump loops and thermal conveetion
loops suggests that velocity has little effect on mass transfer other than on
the tvpe of plug formed, provided conditions for cavitation do not exist.
However, results from tests now under way should definitely evaluate this
variable.

21-2.5 Rapid oxidation of 219, Cr-19; Mo steel. In a pumped Bi loop
containing Mg+Zr, the 239, Cr-19;, Mo steel adjacent to a pinhole leak was
found to be severely oxidized. The appearance of the oxide scale was very
<imilar to that reported by Leslie and FFontana [8]. These investigators
768 MATERIALS OF CONSTRUCTION—METALLURGY [cHaP. 21

found that rapid oxidation of a 169}, Cr-259; Ni—69, Mo steel will occur in a
stagnant atmosphere, and that MoQ3 catalyzed the rapid oxidation of this
steel. They also reported that Bi2Oj3 could produce a similar effect.

A test simulating a leak in a loop was made to study this phenomenon.
A 219, Cr-19, Mo steel pipe was filled with Bi containing 1000 ppm U,
350 ppm Mg, and 250 ppm Zr. A 1/32-in. hole was drilled in the pipe below
the Bi level. A patch of asbestos tape 2 in. in diameter was put over the
hole. The tube was then heated to 594°C and pressurized to 5 psi to force
out a small amount of Bi. The pressure was dropped as soon as some Bi had
leaked out and the tube was then heated at 594°C for 1025 hr. The appear-
ance of the pipe underneath the ashestos tape patch and a cross section of
the oxidized area are shown in Fig. 21-11. A complete oxidation through
the pipe wall has occurred in the area adjacent to the leak.

Tests run at 738 and 816°C in covered crucibles show that chemically
pure Bi2O3 can promote rapid oxidation in 239, Cr-19; Mo, 119, Cr~1/29%,
Mo, and carbon steels. Tube tests and crucible tests are being continued to
determine the minimum temperature at which rapid oxidation is a problem.

21-2.6 Radiation effects on steels. Since steel will be in direct contact
with the liquid U-Bi fuel, it is important to determine the effects of fission
recoils and fast neutrons on the rate of corrosion or erosion. If corrosion
inhibition is achieved by a layer of ZrN between the Bi and the steel, there
1s concern that fission recoil particles might destroy this film. Loecal heating,
resulting from the stopping of the particles, may cause a differential
expansion between the layer and the steel. Consequently, the layer may
break away from the steel, leaving the surface exposed to Bi attack. On
the other hand, neutrons should not he detrimental to a thin ZrN layer,
per se.

Effects of neutrons must also be determined on the mechanical properties
of the steels at reactor temperatures. Radiation-induced increases in tensile
strength and elastic modulus may not anneal out at LMIFR operating
temperatures. A decrease in the impact strength is not considered too
probable at these temperatures, although the possibility must be
mvestigated.

To study the radiation effects on materials, a capsule test has been de-
veloped. The capsules, in which samples are inserted in a highly enriched
U-Bi1 solution containing Mg and Zr inhibitors, are exposed in the BNL
reactor and irradiated to the desired level. The test has the advantage of
attaining high temperatures (700°CT) and a high fission recoil density.

Samples of 1467 Cr—1.267 Mo and 2197, Cr—-19 Mo have been placed in
one of these capsules with Bi containing 1000 ppm U, 2500 ppm Zr, and
3500 ppm Mg. The exposure was approximately 2.2 X 10" nvt. Sectioning
of this capsule has shown no corrosion of the samples. A second capsule
21-2)] STEELS 769

 

Fic. 21-11. Rapid oxidation of 239, G-19, Mo steel pipe after 1025 hr at 594°C
at pinhole leak.

has been put in the BN L pile containing samples of 119, Cr-1/2%, Mo and
mild steels. Results are not yet available.

Dynamic tests of the reaction between Bi and steel in the presence of a
radiation field must be completed before a final selection can be made of
materials for the LMFR. The effect of velocity on corrosion is not certain
from the out-of-pile studies, so that no exact analogy can be made between
out-of-pile forced circulation loops and in-pile capsules. There has been
limited work done at Harwell [6] with thermal convection loops in and out
of a radiation field. These loops had no U but did contain Ca and Zr
inhibitors. The data suggest that pile radiation may have induced some
acceleration of mass transfer.
770 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

An in-pile, forced-circulation loop has been built at BNL and two others
at Babeock & Wileox Research Laboratory to test the corrosion stability
of LMFR materials under conditions to be expected in the reactor experi-
ment. In this loop, Bi containing approximately 1500 ppm of U235 180 ppm
Zr, and 350 ppm Mg will be pumped at o rate of 5 to 7 gpm. The bulk
AT will be approximately 75°C, with a maximum temperature of 500°C.
There are three sample sections in the loop: one, containing samples of
119, Cr—1/29, Mo steel, 259, Cr—19; Mo steel, Be, and graphite, will be at
the center of the reactor and will be in a flux of approximately 3 x 10'3
thermal neutrons; one within the shield will see delayed neutrons at a
temperature of 500°C; and the third section will be outside the reactor at a
temperature of 425°C. This test is presently being assembled, and will be
operating late in 1958.

21-3. NONFERROUS METALS

Several nonferrous metals are receiving attention as container materials,
principally because of their low solubility in bismuth. At this time only a
small amount of work has been done on testing these metals. The following
is a brief discussion of some of the most important of them.

21--3.1 Beryllium. In the Liquid Metals Handbook, beryllium has been
reported to have a good resistance to attack by liquid bismuth at 500°C
and probably also at 1000°C. Recent work at Harwell has shown that
beryllium has good resistance to mass transfer by liquid bismuth circulating
through a temperature gradient of 500°C, with a base temperature of
300°C. Recent advances in the production technology of beryllium metal
have improved its mechanical properties sufficiently so that it is now pos-
sible to consider this metal for reactor core vessel or moderator. However,
no data on reaction between beryllium and uranium additives and fission
products dissolved in a liquid metal fuel are available. Since uranium is
known to form a stable intermetallic compound with beryllium, the possi-
bility of such a reaction must be investigated before beryllium can be
recommended as a moderator in contact with a liquid-metal fuel. Static
tests on the resistance of Be to attack by U-Bi at 550 and 650°C showed
some (but not conclusive) evidence of a slight attack (less than 1 mil/yr
average penetration). A slight attack on Be was noted by Brasunas by a
297, U-Bi alloy after 4 hr at 1000°C.

21-3.2 Tantalum. Recent work at the Ames Laboratory indicates that
tantalum 1s an excellent material to contain U-Bi. A5 w/o U in Bi solution
was circulated through a 3/4-in. OD by 0.030-1n. wall tantalum loop at a
rate of 800 lb/min. The U-Bi was circulated with an electromagnetic pump
and the temperature differential was 100°C (950 to 850°C).
21-4] BEARING MATERIALS 771

Liquid metal samples taken during operation showed that the Ta con-
centration never exceeded 6 ppm. After 5250 hr of operation, the loop was
shut down and examined metallographically. No transferred material was
detected in the cold leg and the corrosion wus less than 1 mil. The tantalum
remained shiny and ductile throughout the experiment.

I'ven though the above results are hopeful, the use of tantalum uas a
container material for a U-Bi reactor system is limited by (1) its poor air
oxidation resistance, (2) difficulty of fabrication, (3) cost.

21-3.3 Molybdenum. Molybdenum is also known to be highly resistant
to U-Bi. However, its use as a container material is hampered by its poor
oxidation resistance and difficult fabrication.

Some success has been obtained, however, with Mo applied as a coating
on low-chrome steel. The process was developed by the Vitro Corporation
and is deseribed in detail in KLX-10009. Essentially, the coating is applied
by first electrophoretically depositing 809, Mo 4 209, MoO3 on the steel.
The coating is then pressed, reduced in a Hs atmosphere, repressed and
<intered in an Hy 4+ HCl atmosphere,

These specimens have been subjected to static Bi which was temperature
cveled at 550 to 400°C. Tiltered liquid metal samples of the Bi were taken
and analyzed for the presence of Fe and Cr. The Bi was allowed to stay at
the 530°C temperature overnight before a liquid metal sample was taken.
After 4300 hr at temperature and some 17,000 cycles, the concentration
of Fe was 6.9 ppm and Cr 2 ppm. By comparison, the solubilities of I'e
and Cr at 550°C were 30 ppm and 80 ppm respectively.

On the basis of their low solubility in U—Bi such materials as tungsten,
tantalum, molybdenum, and beryllium could be classified as container
nuiterinls.  Their practical use, however, is governed by their poor air
oxidation resistance, fabrication difficulties, availability, and cost.

21-4. BrearincG MATERIALS

The relative bearing properties of materials for use as valve seats, disks,
and guides are being determined in Bi containing Mg and Zr. Testing is
done under a boundary lubrication condition.

The test apparatus consists of a main test chamber and a sump tank to
facilitate easy handling of the liquid Bi and to permit periodic sampling of
the liquid metal. The main test chamber, containing the samples and the
sample loading mechanism, is shown in the cutaway of Iig. 21-12. The
method of transmitting the load with air pistons through flexible beilows
can be clearly seen. A second chamber mounted on top of the main test
chamber coutaing a pc Thymotrol motor which rotates the cylindrical
specimen. The power input versus torque characteristics of the motor when
772 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

 

 

 

 

 

 

 

 

 

 

 

 

Motor Shaft Bellows Seal
Crank
Spfia rk Air
! \ Cylinder
K
e
Splash ]
Baffles
bt————Test Vessel
f N
Liquid Actuator
tevel — Shaft
Probe
Fixed
L Specimen
Crank 1 N
Rotating
\L Specimen
- O
\ )

 

 

 

 

Fig. 21-12, Bearing materials test apparatus.

operated in a helium atmosphere using ‘‘high-altitude’ aircraft brushes
has been determined.

A test consists of contacting the rotating cylindrical specimen with a
flat specimen under a constant force for a set period of time under set condi-
tions. After the contact run, the Bi is removed from the specimen. The
degree and kind of scoring, galling, material transfer, and depth of wear on
the surface were noted. Surface roughness measurements are made with a
profilometer. Hardness measurements are made. Coefficients of friction
are calculated from the measured torque data by the equation

T
f=5 (21-4)

where f=the coefficient of friction, P> = the applied load (Ib), r = the
radius of sleeve (ft), and T = the torque (ft-lb).

In general, the hard-to-hard material combinations have shown good
wear resistance except for some scoring. The best hard material tested
thus far is AloO3 flame-coated on AISI 4130 steel. When this material was
contacted against itself, no wear or scoring could be detected. This ma-
21-3] SALT CORROSION 773

terial will be thermally cyeled and exposed to inhibited U-Bi for long times
to evaluate its utility. Stellite 90 and Rex AA also behaved well. Contacts
made with common die steels and low-alloy steels have exhibited severe
scoring and wear. Corrosion has also been detected on these samples. Of
the cemented carbides, only TiC with either a mild steel or 239, Cr-19, Mo
binder has heen tested. This material did not show good wear properties
and also exhibited some pitting corrosion.

Graphitar versus tool steel and Mo versus Rex AA or Stellite 90 have
shown the best results of the hard versus soft combinations tested. The
results have been good in that the wear has been very smooth; however,
the wear has heen excessive. The use of these combinations would be limited
to very low-load applications.

21-5. SaLT CORROSION

In earlier chapters, it was pointed out that one of the chief advantages
of the LMFER lies in the possibility of easy chemical processing. Several
processing techniques have been studied, most of which are based on
pyrometallurgical processes. The two chief pyrometallurgical methods
under consideration are the chloride process, in which the bismuth fuel is
contacted with a ternary mixture of molten chloride salts, and the fluoride
proces=s, in which the bismuth fuel is contacted with molten fluoride salts
contaming hydrogen fluoride. As may be imagined, the construction
material problem for these plants is very difficult.

A corrosion test program is actively under way at BNL and Argonne
National Laboratory on the chloride and fluoride processes respectively.
At BNL, these tests have consisted principally of rocking furnace and tab
exposure tests.

In the rocking furnace test, a piece of tubing approximately 12 in. long
and 1 2 1in. ID, containing a charge of either salt or a mixture of salt and
bhi=muth, is placed on a rocking rack in a furnace. This rack alternately
tilt< to one end for a period of 1 min and then to the other end for a like
amount of time. The two ends of the furnace are kept at 450 and 500°C
in order to give a temperature differential and thus induce mass-transter
corro=ion. The standard test period has been 1000 hr. These tests are part
of the initial sereening program. When they are completed, the metals
which have given the best performance will be further evaluated in test
loop=s and pilot-plant equipment.

At present, only molybdenum has been satisfactorily tested against a
mixture of salt and bismuth fuel. However, the results are definitely en-
couraging, It has been found that the ternary salt, MgCle-NaCl-KCl,
with or without zirconium and uranium chlorides, can be contained fairly
well 11 austenitic stainless steels, particularly 347 stainless steel. When a
774 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP, 21

mixture of bismuth fuel and the ternary salt containing less than 1% BiCls
was tested, the ferritic stainless steels were the best materials. These
include 410, 430, and 446 stainless steels. Probably the best of the ferritics
is the 229 Cr-19% Mo stainless steel.

During one step in the chloride chemical process, it is necessary to have
the ternary salt, containing more than 1% BiCls, in contact with bismuth
fuel. For this mixture only molybdenum has been satisfactory. However,
considerably more testing is required before this can be considered a
satisfactory material.

The experience in handling salt with larger-sized equipment is quite
limited. A small loop built of 347 stainless steel has been operated satis-
factorily for a fairly short time. A much larger loop, loop “N,” is now
being constructed at BNL. This will contact the chloride salt and the
bismuth fuel. The salt part of the loop is constructed of 347 stainless steel.
The bismuth fuel section of the unit is constructed of 21% Cr-19% Mo
steel. The actual eontacting units are constructed of both 347 and the
low-chrome steels. This pilot plant, when placed in operation, should
furnish considerable information on the corrosion characteristics of the
molten chloride salt.

The fluoride process also presents difficulties with materials of construc-
tion. The mixture of the molten fluoride salts, containing HF, with the
bismuth fuel is extremely corrosive. Pure nickel has been found to stand
up fairly well to the molten fluoride salts alone. However, the combination
of the three materials has proved to be very corrosive even to nickel. The
extensive development program to investigate the materials of construction
for the fluoride process is continuing.

21-6. (GRAPHITE

21-6.1 Mechanical properties. In the proposed LMFR system, the
moderator, graphite, is also employed as the container material. Therefore,
the graphite should have good physical properties such as strength, hard-
ness, and resistance to shock. Since graphite is to be the container material
for the bismuth solution, it should theoretically be completely impervious
to the solution. For this reason, special graphites, more impervious than
the usual reactor grades, have been developed and are under development.
Physical properties of typical examples of these graphites are given in
Table 21-6. In comparison with the usual reactor grade, AGOT, having
a compressive strength of 6000 psi, these impervious grades have a strength
of 6500 to 9700 psi.

Another special requirement for the graphite is that it withstand erosion
or pitting by the flowing fuel. Test sections of accurately bored graphite
were placed in test loops where the flow velocity of bismuth was 6 to 8 fps.
No observable effect was noted after 1000 hr of test at 550°C.
21 6] GRAPHITI 775

Although tests so far have been on rather small samples, the mechanical
properties of these improved graphites appear sufficiently good for use in
LEMFR systems. These new graphites must be manufactured in large sizes
in order to conveniently make up the core of an LMFR. "The graphite
industry in the United States is now developing manufacturing techniques
for making such large sizes.

21-6.2 Graphite-to-metal seals. I.caktight joints of steel to graphite are
required at several places in the core of an LMFR. These seals must with-
stand an average of 125 psi at approximately 5530°C. This is done by joining
finely machined steel and graphite surfaces under sufficient spring loading
to prevent bismuth leaking across the seal.

Tests were run by Markert at the Babeock & Wileox Research Center
to evaluate =uch pressure secals. Three-inch and six-inch steel pipes
(21, Cr-19. Mo) with machined ends were pressed against a flat surface
of a block of MH4LM graphite (Great Lakes Carbon Co., density 1.9 g/ce).
The graphite surface had been prepared by sanding and polishing with
No. 000 emery paper. A seal was effected against Biat 438° with a pressure
differential across the seal of 100 psi, and with 1500 psi stress between the
egraphite and the steel. The minimum stress that may be used without
vizible Bi leakage at this pressure differential was found to be as low as
600 psi. It was not necessary to resort to complicated interface configura-
tions to obtain a seal. Thesc Initial results are very encouraging, and
turther development work is being directed toward more complicated seals.

21-6.3 Graphite reactions. If graphite is to be in direct contact with the
1'=B31 fuel, it should be inert to the various fuel constituents and also to
fi=sion products and corrosion products. Work has heen done at various
locations on these reactions. Thermodynamic data on chemical equilib-
rium, when available, have proved to be extremely valuable in guiding
the experiments.

U raniim-graphite reactions. The reaction between uranium and graph-
ite i~ probably the most important one to consider in the LMEIR. Mallett,
Grerds, and Nelson [11] reported that uranium forms three stable carbides:
UC, UCs, and UaCy. Further work on this subject [7,12] indicates that
when less than 197, U 1s present in bismuth, it does not react with graphite
to form carbides at temperatures below 1200°C.

However, the nitride of uranium, UN, has been identified on graphite
contacted with 0.059, U n Bi at 850°C for 28 hr. This nitrogen was un-
doubtedly adsorbed on the surface of the graphite and had not been dis-
lodged by outgassing at high temperatures and vacuum.

When zirconium and magnesium are present with uranium in the bis-
muth, zirconium reacts preferentiaily with the graphite to form ZrC. This
TaBLE 21-6

GENERAL PHYSICAL ProOPERTIES OoF GraruiTe AT 20°C

 

Grade

Base grades
R-0015 R-0018

ATJ

R 0025 R-0020

Tmpregnated grades

ATJ-82

MI4LLA-90

 

Manufacturer

Max. production size™

National Carbon Company

40" dia. 407 dia. 207 x 247 x 8”7

 

National Carbon Company
407 dia. 40" dia. 207 X 24" x 8”7

(rreat Lakes
("arbon Co.
48 (lia.

 

 

 

Dengity
Electrical resistivity

Thermal conductivity
Flexural strength
Compressive strength

Coeflicient of thermal
expansion

Helium flow at
< p> =27 atm
Ap=1atm

W

a

W

&

W

L

W

a

W

W

d

 

.85
16
.40
0.28
0.23
3250
3000
7500
7500

—

1.85
1.53
1.77
0. 21
0.17
3400
3200
8700
8700

3
U

Lo b
it

1.73
I.16
1.43
(.30
0.22
3300
3300
8400
8500

<t
Do

o
o0

160

 

1.90
1.21
1.48
0.27
0.23
4000
SN
S600
3500

b LS
e

9

-1 o

1.90
1.49
1.64
0.22
0.21
4100
3900
9100
9000

4
6

b 1D

ot
=1 =I

L.88
.12
1.48
0.31
(.22
4600
4600
9700
9700

B DO
T

0.16
.12

 

1.90
0.64
0.69
0,45
0.46
2750
2900
65650
H250

st

b
<

~1

0.88
0.43

 

1
L

J

Units

g
mil-cm

mé2-em

cal (see)(em)(°C)
cal (see)em)(°C)
psi

Pl

psi

psi

X 10-6,°C
xX107¢/°C

ml min through
I em cube

 

 

continued

9LL

ADHNTIVLIKN—NOLLODUESNOD) A0 STVINIILVIA

1% "dVHD]
TasLe 21 6 (continued)

 

('C'N

 

 

 

 

Grade AT1--82 Graphite-G  Graphite-A R 4 CEY AGOT Units
Manufacturer N até&mal Cau:bon Graphit-e S-pe.(tia.lties NCC NCC
Max. production size® ompany . Coipm?ttlon . 2" dia. | 16" x 16"
40" dia. 53" dia. | 77 dia. 35" dia. 40" dia.
Density 1.92 1.88 1. 88 1.93 1.98 1.90 1.70 g/ee
Electrical resistivity % 1.14 1.20 0.89 1.07 1.12 1.51 0.73 mil-cm
a 1.20 1.25 1.04 1.17 1.14 0.94 mi2-cm
Thermal conductivity w 0.30 0.29 0.38 0.34 0.35 0.13 0.53 cal/(sec) {(em) (°C)
a 0.27 0.26 0.31 0.30 0.30 0.33 cal/(see) (em)(°CH
Flexural strength W 2400 2800 4400 4800 3850 2400 psi
a 2050 2400 3900 4000 3550 2000 psi
Clompressive strength  w 7500 (500 7400 8500 6900 6000 psi
a 6500 6200 8500 9000 7200 6000 psi
Coeflicient of thermal
expansion w 2.2 2.3 3.1 3.3 3.2 2.5 2.2 x106,°C
a 2.2 2.7 3.7 3.8 3.8 3.8 X 1076/°C
Helium flow at
< p>=2.7atm w 5.7 1.7 0.21 300 ml/min through
Ap=1atm 0.0427 0.0557 0.0217 1 em cube
a 0. 00040

 

 

 

 

 

 

 

 

*Ag gpecificd at present time by the individual manufacturers.

TProcessed and measured as small samples.

[9-12

HALIHAVHD

=1

=1
778 MATERIALS OF CONSTRUCTION—METALLURGY [cHaPp. 21

is predictable from the chemical thermodynamic data. An experiment in
which graphite was contacted with 1000 ppm U, 50 ppm Zr, and 300 ppm
Mg in Bi at 1000°C for 8 hr showed only a single intense x-ray diffraction
line corresponding to the most intense line for ZrC. The x-ray analysis
was carried out after the adherent bismuth was removed from the samples
by mercury rinsing.

These experiments indicate that the reaction of uranium with graphite
is not likely to' occur under the LM IR operational conditions and can be
prevented by the addition of zirconium to the bismuth.

Zircontum and titanium reactions with graphite. Since zirconium and pos-
sibly some titanium will be present in the bismuth, reactions of these
materials with graphite have been investigated. As described above, zir-
conium reacts to form the carbide with a strong negative free energy. At
temperatures around 550°C, ZrC and solid solutions of ZrC and ZrN have
been 1dentified on graphite surfaces contacted with bismuth solutions con-
taming 130 ppm Zr.

On the other hand, no reaction between graphite and 1600 ppm Ti in
Bi solutions has been observed up to 800°C for contact times up to 170 hr.
A strong TiC x-ray pattern and a less intense ZrC—ZrN solution pattern
were observed on graphite contacted with approximately 0.29;, Ti and
0.29, Zr in Bi at 1250°C for 44 hr.

When steel samples are reacted with U-Bi fuels containing zirconium
and magnesium, the x-ray patterns of the surface are those for pure or
very nearly pure nitrides or carbides. When graphite is contacted with the
fuel, however, solid solutions of the carbide and nitride are often found.
The unit cells vary from 4.567 Kx* to 4.685 Kx for the zirconium com-
pounds and from 4.237 Kx to 4.320 Kx for the titanium compounds. These
parameters are low for complete carbon carbide structures.

Parameters for carbon-deficient carbide structures have been reported in
the literature [13]. Yor ZrC the reported ao varied from 4.376 Kx at 20
atomic percent C to 4.67 Kx at 50 atomic percent C. However, up to the
present time no evidence of carbon deficient structures has been observed
in studying the graphite-fuel experiments. The low parameters are instead
believed due to nitrogen replacing the carbon atoms in the carbide lattice
(NaCl-type). Parameters for such solid carbide-nitride solutions are de-
scribed by Duwez and Odell [1].

Fission product-graphite reactions. The products of uranium fission may
also react chemically with graphite to form carbides, A series of experi-
ments have shown that materials such as cerium will definitely react with
graphite. When 25 ppm Ce in bismuth was placed in contaet with graph-
ite at 700°C for 110 hr, CeCs was identified as a film on the graphite.
Graphite contacted with 140 ppm Sm in bismuth at 800°C for 140 hr, on

*Kx = 1000x units = 1.00202 4 0.00003A..
21-6] GRAPHITE 779

the other hand, gave an x-ray diffraction pattern of the graphite surface
which could not be identified or indexed. Under similar reaction conditions,
neodymium, barium, or beryllium in bismuth solutions have no reaction
product. However, Miller [12] has shown by an autoradiograph technique
that 180 ppm irradiated Nd in bismuth reacts with graphite at 1100°C in
100 hr, concentrating the radioactive Nd at the graphite-liquid metal in-
terface. When the same experiment was repeated with the addition of
100 ppm Zr to the solution, no Nd was identified at the graphite surface.
This experiment indicates that zirconium will probably form the car-
bide and nitride preferentially to most fission products. However, further
research is required to determine whether a zirconium carbide-nitride layer
on the graphite can be depended upon to prevent the adhesion of the fission
products to the graphite surface. This point is not only important from
the chemical and graphite surface point of view, but is also important from
the neutron economy point of view. To obtain the highest breeding ratio,
the fission products, which are all fairly good neutron adsorbers, must be
removed from the graphite core soon after their formation. This is espe-
ciallv true of samarium, which has a very high neutron absorption cross
section. Therefore, the experiments reported above on zirconium are quite
encouraging in that there is no trace of a samarium film on the graphite.

21-6.4 Radiation effects on graphite. The graphite core, in order to
serve as moderator and container for the flowing fuel, must be stable to
radiation. Iission recoils may cause spalling and reduction in the thermal
conductivity that might increase the thermal stresses within the core struc-
ture. The graphite must not adsorb large quantities of U or fission products.

A capsule test has been developed in which samples are irradiated in a
highly enriched U-Bi solution containing Mg and Zr inhibitors for study
of radiation effects on materials, The test has the advantage of attaining
high temperatures (700°C) and a high fission recoil density.

Ciraphite samples have been exposed in these capsules under conditions
given in Table 21-7. Metallographic examination of these graphite sam-
ples indicates that there is no excessive spalling or corrosion.

However, some samples were treated with Bi containing Zr at 1300°C
to obtain 10- to 30-micron ZrN-ZrC layers on the graphite prior to irradia-
tion. and postirradiation examination indicated some change of this layer.
The cause of these effects i1s still being determined.

The effect of neutrons on the growth and thermal conductivity proper-
ties of special low-permeability grades of graphite has been measured.
Three sets of samples have been irradiated to exposures as great as 5 X 10%°
thermal neutrons/em? in the temperature range 400 to 475°C. Results of
these tests are listed in Table 21-8. The change in physical length of the
graphite is primarily contraction and should not present a major engineer-
g problem.
TaBLE 21-7

UranNiuM SoLuTioN CapsurLE CoNDITIONS ON GRAPHITE

 

Sol. concentration, ppm

 

 

 

 

 

Capsule Za}rii)l(:a f Exp;):ture, R::;;rréd. Observations
U-235 Zr Mg Y
6 Graphite (AGHT) 4000 200 350 12.8 X 108 392 Zrn layer not evident
after irradiation
8 » ” 3250 200 350 6.36 320 No spalling
10 7 ” 3750 180 700 12.3 320 No spalling
15 ” ” 4450 1500 1900 13.1 420 No spalling

 

 

 

 

 

 

 

 

 

STVISHLVIN 082

NOILLDAYISNOD 40

 

ADHNTIVILHW

1Z "dVHD]
 

[9-12

TapLe 21-8

Puysicar ProreErTYy CHANGES IN Low PrrMmeapiLiTy GrapHITES INDUCED BY
NEUTRON IRBRADIATION AT ELEvATED TEMPERATURES*

 

 

 

 

 

Thermal conductivity at Electrical resistivity,
Graphite Kxposure Irrad., 50°C, cal/{em)(°C)(sec) Gross ohm-em X 1075
type nut 7°C growth, 9
Preirrad. Postirrad. Preirrad. Postirrad.
At 1.8% 1029 475 0.290 (0.198 0.01 9.51 18.21 %
Gt 1.8 475 0.393 0.286 0.002 7.18 16.31 o
ATI-R82% 1.8 475 0.253 0.163 0.02 11.9 24 43 S
R-0025% 1.8 475 0.217 0.127 0.02 13.3 25.84
-3¢ 10 475 0.308 0.137 — 11.2 19.4
G--77 10 475 (.329 0.131 — 8.2 18.4
G-121 10 475 0.347 0.141 — 7.4 17.8

 

 

 

 

 

 

 

 

 

*Irradiations and measurements were made at the Hanford Atomic Products Operation of the General Electric
Company.

FGraphite Specialties Company.

iNational Carbon Company.

8L
 

782 MATERIALS OF CONSTRUCTION——METALLURGY [cHaP. 21

On the other hand, the neutrons reduced the thermal conductivity by
25 to 359, of the preirradiation value. Moreover, some of these graphites
had a preirradiation conductivity only 659, that of the usual reactor-grade
graphite. The combination of low permeability and neutron irradiation
therefore reduces the thermal conductivity to 5097 that of the usual reactor-
grade graphites.

Bismuth penetration into graphite permits the diffusion of some uranium
into the graphite. The resulting fission recoil particles may {urther reduce
the thermal conductivity of graphite. Tests are now under way to deter-
mine the degree of damage produced.

21-6.5 Bismuth permeation and diffusion into graphite. Iarly in the
work on the Liquid Metal Fuel Reactor it was recognized that a special
type of graphite was required if it were to be both a moderator and a con-
tainer for the reactor fluid. Such an impermeable graphite would have not
only the usual advantages of being both structural material and moderator,
but would also not hold up quantities of coolant or fuel, with resulting
decreased neutron efficiency and control. Besides these characteristics,
because the design fluxes are of the order of 10! n/(cm?)(sec) it is necessary
that the graphite have a high degree of resistance to radiation damage,
specifically to physical growth and reduction in thermal conductivity.

In considering impermeable graphite, two characteristics of the graphite
are concerned: liquid pickup and permeability. Liquid pickup refers to
the amount of fluid which is held in the interior pore volume of the graphite
in the manner of a sponge. Permeability is the rate at which a fluid can be
made to flow through the graphite. Both these properties depend primarily
on the accessible void volume and the pore spectrum.

In research work on graphite, it is customary to divide the size of pores
into two categories: macropores larger than 1 micron and averaging 2.5
microns in radius, and micropores with radii less than 1 micron and pre-
dominantly below 0.5 micron.

The development of new types of “impermeable” graphite has necessi-
tated an examination and improvement of manufacturing processes con-
current with experiments on bismuth uptake [14].

The classic process of graphite manufacture is based on cokes and pitch
binders, baking and pitch reimpregnations, and finally graphitization.
Around this scheme has evolved a complex technology involving careful
particle and flour sizing to obtain optimum compaction, elaborate baking
and graphitizing schedules, and extremes of pressure vacuum treatments.
The production of the new relatively impermeable grades was made pos-
sible by three significant advances over the older technology: (1) the ability
to use raw materials of more advanced form, including graphites and blacks
of various types, (2) impregnation techniques now span a variety of resins
21-6] GRAPHITE 783

with various viscosity and wetting properties, and (3) new forming tech-
niques permit much more uniform and finer-grained artificial graphites.
This last includes, in particular, the development of pressure baking, where
heat is applied by passing electric current through the carbon in the mold
and under pressure.

The conventional pitch-type impregnation has the effect of increasing
density without markedly reducing permeability. This is apparently due
to the tendency of the pitch to coke out only in the voids of large effective
pore radius. Conversely, the newer impregnating materials, with their
lower viscosities and increased wetting, tend to block the pores themselves
as well as the larger open volumes. Consequently, there is no general rela-
tionship between density and permeability. Several conclusions may be
drawn: (1) The newer impregnates primarily attack the macropore distri-
bution and shift it from the 1- to 5-micron range into the submicron range.
(2) Optimum particle packing in the original base material is, in general,
not an advantage upon reimpregnation. (3) It is essential to use base ma-
terials in which the long tail at high pore radii is missing. The relatively
minor effect of the present impregnates upon the micropore distribution
demonstrates that the present materials, markedly improved as they are,
do not as yet represent the achievable ultimate.

The amount of bismuth uptake in graphite is probably the most im-
portant property concerned in evaluating the graphite, and was one of the
first investigated. For this purpose, a simple pot arrangement is used to
hold samples of graphite in molten bismuth at pressures from vacuum to
530 psi and at temperatures from 550°C. The graphite samples (0.5-in.
OD and 1.75-in. long) are outgassed in a vacuum at 550°C and then sub-
merged in the bismuth. Helium pressure is then applied to the molten bis-
muth. The amount of bismuth uptake into the graphite is determined by
the difference in the density of the sample before and after submersion.
The accuracy of this measurement is within 0.01 g bismuth per ce graphite.

The degree of bismuth uptake by graphite as a function of time, de-
termined at a pressure of 250 psi at 550°C, is shown in Fig, 21-13. As was
mentioned above, there is no correlation between uptake and graphite
denzity. The densities of these graphites range from 1.73 to 1.92 g/cc. The
total percent of void volumes in the impregnated grades varies from 16.0
to 19.0¢¢ of the bulk volume. Of these totals, the inaccessible volumes
range from 6 to 109%. Although samples EY-9 and ATL-82 have nearly
the xame density, the bismuth absorption differs by as much as a factor of
3 because of the difference in pore spectrum.

As can be seen from the figure, the amount of bismuth uptake varies as
a function of time. This behavior was obtained using separate samples for
each point on a curve and also by measuring the same sample at various
time intervals. In making determinations, considerable oscillation about
784 MATERIALS OF CONSTRUCTION—METALLURGY [crAP. 21

 

1.3 T ] l [ ! |

Right Hand
Scale

Bismuth Uptake, grams/cc graphite

 

 

 

 

Immersion, Hundreds of Hours

Fia. 21-13. Bi penetration; successive immersion of same specimen. Tempera-
ture = 550°C, pressure = 250 psi, outgassed 550°C for 20 hr.

a mean value is found for investigations extended to as much as 3500 hr.
Evidently these variances are caused by outgassing by the graphite over
the time interval of the experiment. Outgassing the graphite at 900°C in-
stead of at 550°C reduced the amplitude of the excursions, but the mean
value remained from 0.425 to 0.525 g bismuth per cc graphite for most of
the types investigated. When the outgassing temperature was increased
to 900°C, the saturation or maximum value of uptake was reached in some
cases within 2.5 hr.

The rate of bismuth penetration into graphite was determined in order
to estimate the effect of an unexpected pressure excursion in the reactor.
Samples were subjected to 250 psi for times varying from 5 see to 5 min,
as shown in Fig. 21-14. This time span far exceeds that expected for a
reactor pressure surge. The test conditions were 250 psi at 550°C after an
outgassing period of 20 hr at 550°C. The data indicate that the practical
maximum uptake is reached in about 10 sec for all graphites except types
A and G. These graphites, which are essentially coatings instead of bulk
impregnations, have their uptake increased continuously with time. In
a long-term test their equilibrium values were not reached until after some
800 hr of submersion.

Since the core of the reactor will be subjected to various pressures, a
study was made of the effect of pressure on the absorption of bismuth by
graphite. Long-term tests covering hundreds of hours were conducted at

Bismuth Uptake, grams/cc grophite
21-6] GRAPHITE 785

 

IIEIII T T lll|[

level |

 

 

Bismuth Penetration, in Grams Bi/cc Graphite

 

 

 

0 | L 1 Pt | o e ey
! 10 100 500
Time of Immersion, in Seconds

Fig. 21-14. Short-time Bi penetration. Temperature = 550°C, He pressure = 250
psi, outgassed 550°C for 20 hr.

 

125 ps1 to duplicate the test discussed above. It was found that the bismuth
uptake is approximately the same as for the 250-psi pressure and the rela-
tive absorption remain the same between the different grades of graphite.

In another series of tests, samples were immersed for 20 hr at 550°C
at varying pressures, as shown in Fig. 21-15. The samples for each curve
were first evacuated for 20 hr at 550°C before being immersed in bismuth.
With each type of graphite, the bismuth uptake at 450 psi corresponds ap-
proximately to the values attained at 250 psi for longer periods of sub-
mersion. Type R, CCN, HLM, and ATI~-82 are insensitive to pressure
imereases beyond 200 psi. The remaining three types, A, G, EY-9, do in-
crease continuously In bismuth uptake and furthermore show a threshold
pressure below which no bismuth penetrates the graphite for the 20-hr
duration of the test.

However, results of long term tests at 125 psi showed that graphites
hiving a threshold pressure at 20 hr do absorb bismuth after several
hundred hours.

After a pressure surge in the reactor core, the operating pressure will
return to approximately 120 psi, and the amount of bismuth in the graphite
nmight decrease. To investigate this, samples impregnated at 450 psi were
resubmerged in bismuth at 25 and at 100 psi to determine what quantity
of bismuth might leave the graphite. The dotted lines in Fig. 21-15 con-
nect these points. It can be seen that there is no significant reduction of the
bismuth contained in each type of sample.
786 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

 

81— e
EYQ 50 Add. Hrs.

T ¢ e 20
- Ach.
o ,/////////////// Hfi_
G - 140

R Hrs.

J . s
4 a-" ——— v— N
nlllllfin\

 

. ,,rl//////

Bismuth Penetration, groms Bi/cc graphite

 

 

| | l [
0 100 200 300 400 500
Helium Pressure in psi

 

Fic. 21-15. Effect of pressure on Bi penetration; successive immersions of 20
hr. Temperature = 550°C, outgassed 550°C for 20 hr. Dashed lines for reduced
pressure after 450-psi impregnation.

Caleulations of the percent of voids filled with bismuth were made for
the maximum bismuth uptake obtained in the experiments shown in
Fig. 21-15. Table 21-9 gives these calculated values for the approximate
saturation level reached. In this table, the last graphite, AGOT, is the
conventional reactor graphite. The 1009, filling of the voids is obviously a
good check of the assumption that it is quite permeable. All the other
graphites are impermeable grades under development by various comi-
panies. The percent voids filled for these graphites do not represent total
saturation of the accessible voids. Rather, these values show that about
1/3 to 1/2 of the accessible void volumes have been filled in these experi-
ments.

In studying threshold penetration effect, surface tensions of bismuth on
various surfaces of graphite were measured (Table 21-10). Although dif-
fering from the accepted values, these determinations probably represent
more closely the actual circumstances in a reactor core. In none of the four
cases was wetting of the graphite obtained by the bismuth or bismuth
solution.

Uranium diffusion into bismuth in graphite pores. Since a certain amount
of fuel absorption will have to be tolerated with the graphites now avail-
able, it is essential to measure the diffusion of uranium into graphite by
21-6G]

GRAPHITE

TasLE 21-9

Voips FiLLep at 450 psi AFTER 20 HR.

 

 

 

 

 

 

 

787

 

 

 

Graphite type Voids filled with Bi, 97
100 37
Y-9 44
A 17
G 37
HLM 32
M 23
R 32
AGOT 100
TaBLE 21-10
SURFACE TENSIONS
_ : Time af Wetti : i
Ciraphite surface Constituents 1me 4 te_r erting Surface tension,
contact, min | properties dynes/cm
Smooth Bi 1 None 276
29 257
78 241
Rough and loose | Bi D None 153
particles 60 142
Polished Bi+ 350 ppm Mg 15 None 66
90 66
Polished Bi+4 350 ppm Zr 15 None 285
: 120 275
240 282
360 283

 

 

 

 

 

 
788 MATERIALS OF CONSTRUCTION—METALLURGY [cuap. 21

means of the bismuth solution. FExperiments to measure this effect have
been made. This was done by first impregnating graphite with bismuth so-
lution containing magnesium and/or zirconium. After bismuth impregna-
tion at a given pressure, uranium was added to the solution and the graph-
ite allowed to soak in the bismuth solution for a period of time. The amount
of uranium which diffused into the graphite was measured by sectioning
the graphite and analyzing for uranium concentration as a function of
distance from the surface of the sample. These experiments were run at
550°C with a pressure of 200 psi. The graphite was first allowed to soak
in the bismuth solution for 90 hr; then the uranium was added and the
conditions were maintained for the duration of the experiment. Results of
two experiments are given in Table 21-11. In the first experiment, the
bismuth contained 390 ppm Mg and 1000 ppm U. The uranium concen-
tration in the graphite specimen was found to be less than in the melt
solution and decreased from the sample face inwards.

The second experiment was performed exactly like the first except that
no magnesium was present. The graphite specimen (Great Lakes Type
HLM) absorbs less bismuth than the EY-9 graphite used in the first ex-
periment. However, the uranium concentration near the surface of the
specimen built up to an amount considerably greater than that initially in
the solution, and the concentration gradient is much steeper than was
found when magnesium was present in the solution. This high value for
the uranium-to-bismuth ratio near the interface may be explained by
assuming that uranium reacted with impurities present on the graphite
surfaces. Apparently when magnesium is present in the solution it reacts
preferentially with these impurities.

These experiments definitely show that uranium and other solutes
present in the bismuth can be expected to diffuse into the graphite as far
as the bismuth has penetrated. Ior the graphites now at hand, this means
diffusion through the entire thickness of the graphite for the long-term
exposures contemplated in a reactor core. Of course, since the diffusion of
uranium itself takes considerable time, fission will convert it to other prod-
ucts before it has an opportunity to diffuse many inches into the graphite.
The effect of diffusion of the various solutes and fuel into graphite on
neutron economy and reactor operational characteristics is recognized, and
studies have to be made in large-scale experiments.

In general, it is believed that the graphites at hand will meet the require-
ments for the first experiment of an LMFR reactor. It i1s already possible
to produce some of these in sizes as large as 40 to 60 in. in diameter. As
this development progresses, graphites of greater impermeability will be
produced. Improvements in graphite have taken place steadily, and
markedly improved materials are anticipated in the future.
21-6] GRAPHITE 789

TaBLE 21-11

URANIUM DIFFUSION INTO (GRAPHITE

 

 

 

 

 

 

 

 

Specimen no. Distance, in. ‘ Bi, 9, 1 Mg, ppm ‘ U, ppm
A. EY-9 Graphite
1 0 0312 32.2 1150 900
2 0.0937 30.5 1050 840
3 0.1562 29.6 750 810
4 0.250 28.0 800 760
5 0.500 | 26.0 820 740
B. HLM Graphite
1 0.0312 16.35 5600
2 0.0937 16.87 2630
3 0.1562 16.38 460
4 (.250 16.78 70
. 5 0.500 17.78 30
|

 

 

 
790 MATERIALS OF CONSTRUCTION—METALLURGY [cHAP. 21

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2. D, T. Kzating and O. F. Kammerer, Film Thickness Determination from
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11. M. W. Marrerr et al., The Uranium-Carbon System, USAEC Report
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13. G. V. Samsonov and N. 8. RoziNova, Some Physicochemical Properties
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