ORNL-3253.txt

 

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ORNL-3253
UC-25 - Metals, Ceramics, and Materials

 

 

 

CORROSION OF VOLATILITY PILOT PLANT
MARK | INOR=-8 HYDROFLUORINATOR
AND MARK Il L NICKEL FLUORINATOR
AFTER FOURTEEN DISSOLUTION RUNS

A. P. Litman

 

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OAK RIDGE NATIONAL LABORATORY
operated by

UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION
 

 

Printed in USA. Price $0"—?5. Available from the

Office of Technical Services
Department of Commerce
Washington 25, D, C,

 

 

 

 

LEGAL NOTICE

This report was prépored as on account of Gevernment sponsored work. Meither the United Stotes,

nor the Commission, ner any person octing on behalf of the Commission:

A, Makes any warronty of representotion, sxpresssd or implied, with respect to the accurscy,
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or his employment with such contracror.

 

 

 
-

ORNL~ 3253

Contract No. W-7405-eng-26
METALLURGY DIVISTION
CORROSION OF VOLATILITY PILOT PLANT MARK I INOR-8 HYDROFLUORINATOR
AND MARK ITIT L NICKEL FLUORINATOR AFTER FOURTEEN DISSOLUTION RUNS

A, P, Litman

DATE ISSUED

FEB -9 1362

OAK RIDGE NATTONAL LABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U. 5. ATOMIC ENERGY COMMISSION

 

VAT
CORROSION OF VOLATILITY PILOT PLANT MARK I INOR-8 HYDROFLUORINATOR
AND MARK TTIT L NTCKEL FLUORINATOR AFTER FOURTEEN DISSOLUTION RUNS

 

 

"A. P. Litman
ABSTRACT

Process corrosion dccurring in the current Veolatility Pilot Plant
hydrofluorinator and fluorinator in operation at the Oak Ridge National
Laboratory was evaluated by visual .inspections, chemical analyses, “trans-
port studies of Ni, Mo, Cr, and Fe, gamma radiography, wax replication,
ultrasonic thickness measurements, -and metallographic studies. The modes,
mechanisms, and rates of corrosive attack seem to agree well with previous
experimental work. Significant bulk metal losses and moderate pitting
attack were noted in both vessels. Maximum attack in the hydrofluorinator,
which operated from 650 to 500°C, occurred in the middle vapor region at
a calculated corrosion rate of 20 mils/month, based on 765 hr of molten
_ fluoride salt residence time, and 0.06 mil/hr, based on 338.5 hr of
hydrogen fluoride exposure time. The fluorinator, which operated at
about 500°C, sfistained maximum bulk metal losses in the lower vapor region
at calculated rates of 28 mils/month, based on 694 hr of salt residence
time, and 0.9 mil/hr, based on 30.9 hr of fluorine exposure time.

Calculations based on losses in wall thickness indicate that both
vessels should be capable of handling approximately 100 additional
dissolution runs. These calculations include pitting corrosion in the
hydrofluorinator but ignore effects resulting from intergranular or

other forms of selective attack which may be present in both vessels.
1. INTRODUCTION
The Oak Ridge National Laboratory (ORNL) Fluoride Volatility

Process is being developed as a nonaqueous technique for reprocessing

zirconium-clad highly enriched uranium fuel elements or homogeneous
-2 -
fluoride salt mixtures (such as the NaF-ZrF,-UF, Alrcraft Reactor BExper-
iment fuel which has been processed, or the LiF-BeF,-ThF,-ZrF,-UF, fuel
from the proposed Molten Salt Reactor Experiment). In heterogeneous
fuels the zirconium and uranium are converted to their respective tetra--
fluorides in an NaF-ZrF, or NaF-LiF-ZrF, melt, with HF used as the oxi-
dant. The UF, is further oxidized, in a differént vessel, to UFg by
contact with elemental fluorine. The volatile UFg is purified by an
absorption-desorption cycle on NaF pellets and collected in cold traps.
The hydrofluorination step is not required for homogeneous fuels.

The nature and extent of the corrosion occurring in earlier hydro-
fluorination énd fluorination vessels, which must sustain the most cor-
rosive environments in the process, have been discussed in other
reports, -3

The present Volatility Pilot Plant {VPP) hydrofluorinator is an
INOR-8% vessel approximately 17 ft in height and consists of a top right
cylinder 24 in. in diameter, a bottom cylinder 5 1/2 in. in diameter,
and a conical section connecting the two cylinders. The top cylinder
was formed from 3/8—in. rolled and welded plate, the bottom cylinder
from 1/4-in, plate, and the conical sectionifrom 1/2-in. plate.

The VPP fluorination vessel was constructed entirely from 3/8-in.-
thick L (low-carbon) nickel plate., The fluorinator was made by Jjoining
two 1l6-in.-diam right cylinders with a 5.5-in.-diam neck, The combined
assembly hds a height of 7 ft. Figure 1 is a simplified schematic dia-
gram of the current VPP flowsheet showing relative positilons and configu-
rations of the Mark I INOR-8 hydrofluorinator and the-Mark ITI L nickel

fluorinator.

 

1A, P, Litman and A. E. Goldman, Corrosion Associated with Fluori-
nation in the Oak Ridge National Laboratory Fluoride Volatility Process,
ORNL-2832 (June 5, 1961).

 

 

°p, D. Miller et al., Construction Materials for the Hydrofluorinator
of the Fluoride-Volatility Process, BMI-1348 (June 3, 1959).

 

 

3A. E. Goldman and A. P. Litman, Corrosion Associated with Hydro-
fluorination in the Oak Ridge National Laboratory Fluoride Volatility
Process, ORNL-2833 (November 1961).

“Nominal INOR-8 composition (wt %): 71 Ni—16 Mo—7 Cr—5 Fe.

 

 

 
LIQUID
WASTE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UNCLASSIFIED
ORNL-LR-DWG 61615

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fa
DISPOSAL
I DESORPTION
/o IN. NaF
SELLETS OUTLET UFe COLD | F2
TRAPS
AQ. KOH
SPRAY
NaF -LiF-ZrFa ABSORPTION Fa TOWER
H2 ' OUTLET
UFe
NEUTRALIZER MOVABLE COLLECTION
{ 10% KOH) BED
ABSORBER .
400-100%C T} LIQUID WASTE
HF + Ho |
i
FUEL CHARGING |
Ho CHUTE F2
FREEZE VALVE
3
HF f— _1» . ’ NaF -LiF-2r Fyq
RECYCLE k\ /)
YSTEM .
S - NaF -LiF -ZrF4-UF 4
\ r , WASTE SALT
Zru H ( """"" “:
SPENT | = . = !
FUEL L | : L so0o
[ !
o ! =
650-500°C 1 ! L .7
| \ LN ]
i —
HF ) FREEZE ” -
[T_:J LINE FLUCRINATOR
HYDROFLUOQRINATOR
Fig. 1. Simplified Flowsheet — ORNL Fluoride Volatility Process.
- -

2., HYDROFLUORINATOR CORROSTION

Corrosion in the Volafility Pilot Plant was observed through 14
dissolution runs using both Zircaloy-2 dummy fuel elements and nonirra-
diated Zircaloy-2 fuel elements containing 0.2 to 0.4 wt % U. Hydro-
fluorination process cycling for the dissolution runs is detailed in
Table l.' Special efforts were made to keep temperatures as low as
practical by using lower melting NaF-LiF-ZrF, salt baths rather than NaPF-
ZrF, melts and by reducing the initial dissolution bath temperature of
650°C to about 500°C as rapidly as possible, Higher temperatures have a

strongly adverse effect on Volatility Process corrosion, ™3

2.1 Reaction to Environment

After run TU-7, the hydrofluorinator was carefully inspected for
determining the extent of process corrosion and for evaluating its‘future
usefulness for processing naval reactor fuel elements. The nondestructive
inspection procedures included visual examination, chemical analyses of
corrosion-dissolution products, transport studies of Ni, Mo, Cr, and Fe,
complete gamma radiography of the vessel walls, wax replication of a
portion of the interior walls of the hydrofluorinator, and ultrasonic
thickness measurements to determine bulk metal losses. There were no
surveillance corrosion specimens included in the hydrofluorinator., How-
ever, metallographic examination was done on an INOR-8 pipe support clip
which had been exposed to the same environments as the salt regibn of

the hydrofluorinator vessel proper.

2.1l.1 Visual Examination

 

Following an inspection of the interior walls of the hydrofluorinator
by means of the naked eye, the visual inspection techniques consisted in
low-magnification viewing with the Omniscope,” an interior-type periscope,

and the Questar,® a catadioptric instrument using a combined lens-mirror

 

*Manufactured by Lerma Corp., Northampton, Mass.

éManufactured by Questar, Inc., New Hope, Penna.
Table 1. VPP Mark 1 INOR+8 Hydrofluorinator Process History

 

- | Wall T erat
Salt Composition, Vesse emperature

 

 

 

Run No. of Fuel NaF-LiF-ZrF4 During Dissoclution (°C) Molten Salt HF Flow HF Exposure
Designation Elements Dissolved (mole %) Vapor Regicn Salt Region Residence Time Rate Time
- (hr) (g/min) (hr)
Initial Final Moximum Minimum Maximum Minimum _
Salt transfer 27-27-46 27-27-46 435 425 565 535 44.5 0 0
studies. ‘ :
T-14 1 43-22-35b 31-17-52 410 N.A, 560 520 87 104 38
T-2 1 38-30-32b 30-27-43 380 N.A. 530 525 . 95 40 41.5
T-3 1 39-39-22b 31-26-43 575 N.A, 625 550 . 57 . 90 25
T-4 1 38-37-25b 30-27-43 590 500 630 495 51.5 90 22,5
T-5 1 ' 38-37-25b 30-27-43 570 460 625 530 62 90 27
T-6 1 38-37-25b 31-30-39 550 390 650 500 59.5 118, 150 26
T-7 2 37-37-26b 27-27-46 390 440 655 525 53 135 23
TU-1€ 2  40-35-257  34.26.40 560 400 670 490 44 92 23
( + 0.3 wt% U)
TU-2 2 37.38-25%  30-27-43 620 450 650 500 35 125 24
(+0.2 wt% U)
TU-3 2 33-41-26d 30-31-39 570 390 655 495 32.5 150 19.5
(+0.3 wt % U) _ , )
TU-4 2 42-34-24d 37-29-34 575 440 650 535 36 146 11
TU-5 1 42-35-23d 29-29-42 525 400 650 500 : 36 150 22
TU-6 1 33-37-25d 34-28-38 525 385 650 500 35 121 19
( + 0-3 wt % U) "
TU-7 1 : 39-38-23d 30-30-40 515 405 650 500 37 150 _ 17

 

(+0.2wt% U) 765 338.5

 

%Simulated fuel element; made of Zircaloy-2.
bBarren salt charge; 60 to 115 kg,
©Zircaloy-2 cladding; Zr-U matrix.

Barren salt charge; 95 to 110 kg.
-6 -

system. In order to examine the vertical walls. of the hydrofluorinator

with the Questar, highly polished mirrors and a light source, both held

by thin structural members, were lowered into the vessel and the Questar
was positioned above the fuel-charging chute.’

The regions of the hydrofluorinator that had been in contact with
process vapors were covered with a beige-colored deposit, while the salt-
containing area appeared to be black with tints of green and brown. Dark-
colored coarse flakes of material were noted on top of the hydrogen
fluoride distributor plate (Fig. 2). The flakes had the appearance of
dried salts discolored by corrosion-dissolution products. (The LiF-NaF-
Zr¥, salts used during dissolution are white in color and develop a
greenish cast as UF, is added to the mixture.) The deposits on the dis-
tributor plate also appeared to contain metallic particles., Some of the
’defiosits adhering to the vertical walls and interior piping in the salt
-region were of appreciable thickness and were loosely adherent, Typical
of thése'fegions'is the deposit noted on the thermocouple well and shown
in Fig. 3.

In order to study the actual walls of the hydrofluorinator, several
| attempts were made to remove the corrosion-dissolution deposits. The
cleaning schedule used and subsequent observations are given in Table 2.
Despite the ambitious cleaning program, about half of the wall deposits
remained tenaciously attached to the walls of the hydrofluorinator. Addi-
tional low-magnification studies of the interior of the vessel gave indi-
cations of pitting attack, especially in the conical section joining the
top and bottom right cylinders and in the lower third of the top cylinder.
(More information on this pitting attack was acquired later by gamma,
radiography and wax replication of the hydrofluorinator and is given in

Sec. 2.1.3.)

2.1.2 Chemistry of Corrosion — Transport of Ni, Mo, Cr, and Fe

 

Chromium and iron, constituents of the INOR-8 hydrofluorinator, read-

ily react at elevated temperatures with HF gas to form fluorides, probably

 

'7J. B. Ruch designed and operated the Questar mirror-light extension
apparatus and was instrumental in producing the photographs in Figs. 2
and 3. R. E. McDonald served as consultant in this work.
UNCLASSIFIED
PHOTO 540%0

L " 4
. = = y
- - .r,'._’. i.* * fan 4 - 5

¥ b s ‘v“ d‘-:ur
.' Sk 2
E - v -

| v »

 

Fig. 2. Portion of the HF Distributor Plate Inside the VPP Mark I
INOR-8 Hydrofluorinator After Run TU-7 Showing Typical Deposits Occurring
from Dissolution and Corrosion. Photograph taken through Questar optical
system. Approx 2.5X.

 

 
UNCLASSIFIED

PHOTO 53644

 

Fig. 3. Portion of the Thermocouple Well in the Lower Salt Region
of the VPP Mark I INOR-8 Hydrofluorinstor After Run TU-7 Showing Typical
Loosely Adherent Deposits. Photograph taken through the Omniscope optical
system, Approx 4X.

 
-9 -

Table 2. Cleaning Schedule of VPP Mark I INOR-8
Hydrofluorinator After Run TU-7

 

Treatment

Observations

 

0.35 M ammonium oxalate,
at 60°C for 4 hr

4 high-agitation (by
nitrogen sparging)
water rinses at 25°C
for 8 hr ‘

Nitric acid (5 wt % in
water) at 25°C in lower
3 ft of vessel for 3 hr

0.35 M ammonium oxalate
at 95 to 100°C for 4 hr

Several fiater rinses at
25°C for 6 hr

Aluminum nitrate (5 wt %
in water) (adjusted to
3.5 pH by potassium
hydroxide) at 25°C for
7 hr; high agitation by
nitrogen sparging

Vapor and salt regions had a dull-
gray appearance; heavy deposits
still present in bottom of vessel

Same as above except some lcoose
deposits had been removed from
bottom '

Bottom of hydrofluorinator seemed

brighter and less congested with
loose deposits; crystals similar
to metal whiskers noted on the
pipe support clips

None

No significant change“in vessel
appearance -

About half the wall deposits still
present, mostly in the lower
portions of the vessel; most of
the loose material had fallen
to the bottom of the vessel

 
- 10 -

CrF, and FeF,. Also, NiF, can be produced during hydrofluorination, but
only because of the continuous removal of hydrogen since the free energy
of formation (above 490°C) of nickel fluoride by Ni + HF reaction is not

8 Similarly, molybdenum, the other major constituent in INOR-E,

favorable.
can be forced to react with IF even though a positive free-energy change
is involved. Evidence of small but finite dissolution rates of molybdenum
metal during hydrofluorination conditions has been reported.® All the
oxidation reactions indicated above are part of the initial stages of
corrosion in the VPP hydrofluorinator.

The corrosion resistance of the hydrofluorinator would be consider-
ably enhanced if the corrosion products (fluorides of Ni, Mo, Cr, and Fe)
were adherent to the hydrofluorinator wa}ls and impervious to the further
passage of HF, ofVlOW'volatility, unaffected by ercsion, and not soluble
in nor complexible with the NaF-LiF-ZrF, dissclution bath. However, most
of these conditions do not exist and therefore the fluoride films are not
protective. Morecver, all or some of the fluorides formed from the
INOR-8 + HF reaction can be reduced by the zirconium metal present in the
system, by more electropositive elements present (for example, Cr metal
should reduce the fluorides of Mo, Ni, and Fe) or by hydrogen gas from
dissclution and corrosion reactions. At 600°C, the free energies of
formation (relative to HF gas as zero) of MoF,, NiF,, FeF,, CrF,, and
ZrF, are +14, +2, -5, -12, and -31 kcal per gram-atom of fluorine, re-
_spectively.8 Finely divided particles of Ni, Mo, Fe, and Cr, often

10 are evidence of

found in the off-gas stream from the hydroflucrinator,
the postulated reduction reactions.
The oxidation-reduction cycles described are believed to account for

the major source of corrosion during the hydrofluorination stage of the

 

8A. P. Litwan and R. P. Milford, "Corrosion Associated with the Oak
Ridge National Laboratory Fused Salt Fluoride Volatility Process,”
presented at Symposium on Fused Salt Corrosion, Fall Meeting of the
Electrochemical Society, Detroit, Michigan, October 1-5, 196l.

°A. E. Goldman and A. P. Litman, Corrosion Associated with Hydro-
flucrination in the Oak Ridge National Laboratory Fluoride Volatility
Process, ORNL-2833, pp 45, 49-50 (November 1961).

108, c. Moncrief, Results of Volatility Pilot Plant Dissolution
Run T-7, ORNL CF-60-12~-57 (Dec. 22, 1960),

 

 

 
i

- 11 -
Fluoride Volatility Process. Since laboratory experiments are complicated
by the numerous possible interactions, most of the knowledge gained to
date has been through small-scale process studies and subsequent exami-
nation of process development vessels,

Chemical analyses of the off-gas stream from the hydrofluorinator,
of the waste salt from the VPP, and of the movable-bed absorber have been
studied191? as a means of estimating past and future corrosion cf the
dissolver vessel. Figure 4 shows the transport paths of Ni, Mo, Cr, and
Fe during the 14 dissolution runs, and approximate mean percentages of
the total of each element found either as a metal or as a fluoride;
Quantitatively, during each run considerably more Ni, Cr, and Fe were
found in the waste salt, movable-bed absorber, or off-gas system than
had been introduced into the system from the feed salts or the fuel ele-
ments. This waé the first positive indication of hydrofluorinator cor-
rosion. On the other hand, only about half the molybdenum introduced
from the feed salt was found in other regions of the process system.n"'l2
Other work® indicates that the molybdenum may be plating out in the
hydrofluorinator and/or possiblyl4 combining as an intermetallic compound
with nickel in the INOR-8. The transport and final disposition(s) of

molybdenum are being investigated.

2.1.3 Gamma Radiography

 

After run TU-7 the entire shell of the hydrofluorinator was radio-

graphed to help confirm or deny the pitting corrosion first noted by

 

, 1lg, ¢. Moncrief, Results of Volatility Pilot Plant Cold Uranium
Flowsheet Demonstration Run TU-1, ORNL CF-61-6-62 (June 22, 1961).

 

12p, . Moncrief, Results of Volatility Pilot Plant Dissolution

Run TU~2, ORNL CF-61-6-79 (June 23, 1961).

138, C. Moncrief, Results of Volatility Pilot Plant Dissolfition

 

Run TU-3, ORNL CF-61-7-26 (July 13, 1961).

'147, H, DeVan, private communication, Aug. 29, 1961,
- 12 -

UNCLASSIFIED
ORNL-LR-DWG 61591

   
  
   
   
 

—= UF, PRODUCT.

   
   
   
   
  
  
 

MOVABLE
BED
ABSORBER

 

 

 

 

 

    
   

o * ¥ [20% Cr

o NI

20% Mo > %Mo % PERCENTAGES .
5% Fe (AS FLUORIDES) OF TOTAL RECOVERED
~1%Cr \

OFF-GAS | 32 :

 

 

 

(AS METAL AND
- FLUORIDES)

WASTE SALT PLUS

Ni,Cr,Mo, Fe fi

 

BARREN SALT

 

 

WASTE

 
 

 

 

 
   
 

 

 

 

 

 

 

 

 

SALT
CONTAINER
* v
| ‘ FLUORINATOR 80% Ni
L NICKEL
FUEL ELEMENT gy
(Ze-Sn-U NaF-LiF-ZrF, 80 Cr .
+ + °
Ni, Fe, Cr) Ni, Mo, Cr, Fe (AS FLUORIDES)
HF

 

HYDROFLUORINATOR
INOR-8 (Ni-Mo-Cr-Fe)

Fig. 4. Transport Paths of Ni, Mo, Cr, and Fe in the VPP Fluoride
Volatility Process During l4 Dissolution Runs. Percentages are approximate
means of the totals of each element found in the off-gas system, movable-

bed absorber,and waste salt.
.

- 13 -
visual examination and to reveal the condition of the weld joints and
possible cracking in the vessel walls. Cassettes containing film were
wrapped around the exterior walls of the vessel, and an Iri®? gamma
source was located inside the hydroflubrinator along the vertical center
line. The source strength at the time of use was 17 curies. A 5-mil
lead foil directly in front of the film and a 10-mil lead shield behind
the film were used to prevent scatter and to intensify the image.

- The radiographic film revealed only three regions in the hydro-
fluorinator where pitting seemed to be present. One area, which had
been exposed to process vapors, was within a 2~-in.-diam circle about
36 in. down from the top of the upper cylinder in the north quadrant.
The deepest pit here, based on a 2% sensitivity factor for the film and
radicgraphic technique, was about 7.5 mils. The other two areas were
in the east gquadrant of the 1/2-in.-thick conical section oflthe hydro-
fluorinator and about 6 in. down from the top of the cone. The maximum
depth of pitting here seemed to be about 10 mils. |

All weld joints appeared to be in good condition on the hydro-

fluorination vessel, and no macrocracking was obvious.

2.1l.4 Wax Replication

 

To cross-check the radiographic work and to prove'conclusively the
pitting attack, wax replications were made of the interior walls of the
vessel, The technique for cobtaining these impressions was proposed by
R. P. Milford (Chemical Technology Division) and the early experimental
work was done by Smifih;l5 important modifications and the actual wall
impressions were made by Crump.l6
Briefly, the replication technique involved lowering into the hydro-

fluorinator and horizontally positioning an air cylinder to which was

attached a small container filled with dental mclding wax, heating the

 

15M. 0. Smith, Volatility — Proposed Wax Replication Technique for
Measuring Hydrofluorinator Pit Depths, ORNL CF-61-3-59 (Mar. 15, 1961).

 

 

16, F. Crump, Jr., Wax Replication Studies of Corrosion Pits in the
Volatility Pilot Plant Hydrofluorinator, ORNL CF-61-8-20 (Aug. 9, 1961).

 

 
- 14 -

wax to a suitable temperature, and activating the air cylinder to press
the softened wax against the wall of the vessel, Figure 5 shows the
replication device in mockup position auring heating. Since the device,
as presently designed, requires largé internal clearances, only portions
of the top cylinder of the hydrofluorinator could be replicated.

Initial replication confirmed the pitting attack in the vapor-phase
region observed on the radiographic film. Duplicate impressions, at
different times, disclosed that the maximum pitting depth here was 8 mils.
Figure 6 shows portions of the replicétions illustrating the configuration
of the pit. The height of the replication was measured by a toolmaker's
microscope and was cross-checked by an optical comparator and a dial gage
with a long lever arm.

The excellent results obtained with the initial replication made it
desirable to extend the work to a survey of the entire top cylinder. These
replications were done in a spiral, clockwise pattern starting at the top
of the cylinder and continuing down to the conical section. The results
are given in Table 3. While the 8-mil pit previously found proved to be
the deepest discontinuity, many other pits were found varying from 0.5 to
5.5 mils in depth. |

Replications were also made of portions of circumferential and longi-
- tudinal welds in the hydrofluorinatof; portions of the replications are
shown in Fig. 7. The impressions indicated the welds to be in good condi-

tion with little, if any, evidence of corrosive attack.

2.1.5 Ultrasonic Thickness Measurements

 

A Vidigage,l7 which had an accuracy of approximately il%, was used
to determine wall thinning. Readings were taken every 3 surface contour
inches, generally vertical, in all four guadrants, and the results were
compared with base-line data obtained with the same instrument when
the hydrofluorinator was installed in the pilot plant, The losses in

wall thickness are plotted in Fig. 8 vs elevation, starting from the top

 

_ 17pn ultrasonic measuring device, manufactured by Branson Instruments,
Inc., Stamford, Conn. ‘

‘™
| UNCLASSIFIED
! PHOTO 55021

  
  
   
  
      

 

CLEVIS

a
; WAX CONTAINER
WA X

HEAT SOQURCE
AIR CYLINDER

* oLl

Fig. 5. Wax Replication Device Used for Obtaining Impressions of Pits
in the VPP Hydrofluorinator. In order to insert or remove device through

the fuel charging chute, the air cylinder was positioned vertically within
the clevis.

 
SR

UNCLASSIFIED
s PHOTO 54901

  

Fig. 6. Duplicate Wax Replications of an 8-mil Pit (Encircled) Found
in the VPP Hydrofluorinator North Quadrant, About 36 in. Down from the Top
of the Upper Cylinder.

 
»)

- 17 -
Table 3. Depth of Pits in the Vapor Region of the VPP Mark I
INOR-8& Hydrofluorinator as Determined by Wax Replication

 

Distance Down from
Top Cylinder Pit Depth
(in.) Quadrant (mils)

 

Initial Replication

 

32 North

W

~

I_l
MO -
W I~

H

N

o0

. -

V
East
Later Replications

North

 

5.5, 5.1,

U1 -3 W\t \in

, 3.5, 3.1, 2.8

OO O

East

N
|—J

HOoOOOOO

oyttt Wi

-
|_‘- .
. _b'- .

South

W
W
O .
Wn

36 3.5, 3.2,

-
Do

\.-l-\
N
O

West

UIUI\.(\JUILN Ut &g
W
O

I~
X
OO0OWWO ~MPOOM

North
63
66
69

*

U\ \in

 

O oo

 
 

 

 

a I8 -

UNCLASSIFIED
PHOTO 54922

 

UNCL ASSIFIED
PHOTO 54912

 

Fig. 7. Portions of Wax Replications of Welds in the VPP Hydrofluori-
nator. (a) Circumferential weld, southwest quadrant, 48 in. down from top
of vessel; (b) longitudinal weld, southeast quadrant, 21 in. down from top

of vessel. Approx 4X.

 

 
)

#

- 19 -

24-in.0D

T

I YT RREFIN T

 

TOP

72 in.

—3g-in. WALL

 

 

 

WLttt fo i n

 

   

16 in.

 

 

-———'/2-in. WALL

196 in.

e = 51/2-in. OD

 

108 in.

     
 

b 1,
/4 n.

 

 

 

WALL
BOTTOM
CYLINDER
- ::E::__—-
FV-1000

MATERIAL: INOR-8

SUMMARY OF
PROCESS CONDITIONS FOR 14 DISSOLUTION RUNS

 

CYLINDER

_ /

 

 

 

 

TEMP (°C) 380-620, VAPOR REGION
490-670, SALT REGION
TIME (hr) 765-SALT
338.5-HF
SALT COMPOSITION Ncfl'_-l_iF-Zr'E4
(27/43 -17/41-22/52 mole%)
PLUS 0.2/0.4 wt% U
Fig. 8.

o

\
CONTOUR SURFACE INCHES FROM TOP

 

UNCLASSIFIED

CRNL-LR—-DWG 61589

0.06 mils/hr
(HF TIME)

20 mils/month
(SALT TIME)

/////// -

6

 

 

 

 

 

 

 

VAPOR REGION
AVERAGE LOSS-6 mils

SALT REGION

VAPOR-SALT
INTERFACE REGION
AVERAGE LOSS-6 mils

o
£
¥
n
0
O
3
w
o
!
14
L
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a

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75in,

 

120 in.

 

 

0 S 10 15 20
WALL THICKNESS LOSS (mils)

——X INDICATES PITTING ATTACK

the VPP Mark I INOR-8 Hydrofluorinator.

Wall-Thickness Losses and a Portion of the Pitting Attack on
- 20 -

of the upper cylinder. ©Since the losses did not seem to vary particu-
larly with gquadrant geometry, they were plotted at each elevation. The
maximum and minimum loss points in the bottom and top cylinders and in
the conical connector were connected to form loss ranges in each geometric
region,

Average bulk metal losses in the top and bottom cylinders were 6 and
4 mils, respectively, but maximum losses were 18 and 12 mils. The average
wall-thickness loss in the connecting cone was 10 mils, while the maximum
loss there was 16 mils, Similarly, the average losses in the salt, salt-
vapor interface, and vapor regions were, respectively, 4, 6, and 6 mils,
'with corresponding maximum losses of 12, 16, and 18 mils. -

The depths of the pits in the upper portion of the hydrofluorinator,
as determined by wax replication, are shown in Fig. 8 as extensions of
the appropriate wall-thickness loss points, depending on the particular
quadrants where each form of corrosion was found.

The maximum attack to date occurred in the middle vapor region at a
calculated corrosion rate of 20 mils/month, based on 765 hr of molten
fluoride salt residence time, and 0.06 mil/hr, based on 338.5 hr of HF

exposure time,

2.1.6 Metallography and Chemistry of Corrosion Product Layers

 

Additional information on INOR-8 corrosion in hydrofluorination serv-
- ice was obtained by examination of a pipe support clip originally located
below the salt level ih the current VPP hydrofluorinator. Metal thickness
.losses of 5 mils were noted by micrometer measurement. These correspond
to salt-phase hydrofluorinator wall thinning found by ultrasonic thickness
measurements,

A black, semiadherent scale was present on the clip and was found to
be depleted in chromium and iron when compared with the base metal (Fig. 9).
The nickel content of the scale was comparable with the nominal INOR-8
analysis, but the molybdenum cbntent was considerably higher. Scale x-ray
diffraction patterns could not be matched with known compounds. Since
the metallic content of the scale was less than 50% of the total weight, a

good possibility exists that the scale is an Ni-Mo-F complex,
 

- 21 -

UNCLASSIFIED
ORNL-LR-DWG 61655R2

 

 

O
CORROSION PRODUCT ANALYSES
(wt %)
Ni Mo Cr Fe
0002 , 69 28 Q.l 0.4(a)
i e—66 3l 0.3 0.6(b)
< -~ 19 = 5 (c)
7 16 7 5 (d)
0.004 -
00061 -
500 X ETCH: MODIFIED CHROME REGIA

Fig. 9. Corrosion Product Layers on INOR-8 as the Result of Hydro-
fluorination Service Showing Composition of (a) Surface Scale (Not Shown
in Photomicrograph), (b) Spongy-Surface Layer, (c) Subsurface Layer, and
(d) Substrate Base Metal.

 

 
- 22 -

Metallographic examination of the clip disclosed a soft (53-63
diamond-point hardness) spongy-surface layer and a subsurface layer of
variable hardness (115-335 DPH) as compared with a base INOR-8 value of
210 to 234 DPH (Fig. 10). Millings from the two layers were subjected
to spectrochemical analyses, with the results shown in Fig. 9. The spongy-
surface layer had approximately the same metallic composition as the scale
previously described. The subsurface layer was deficient in chromium, had
a slightly higher molybdenum content, and had the same iron content when

compared with the base metal.
3. FLUORINATOR CORROSION

As indicated in Fig., 1, after the fuel elements were dissolved in the
hydrofluorinator, the process salts, containing uranium as UF,, were trans-
ferred into the L nickel fluorinator, where the UF, was fluorinated to
the process product UFg. Process history for the current VPP fluorinator
is shown in Table 4. It should be noted that prior to exposure of the
vessel to fluoride salts the unit was fluorine-conditioned at temperatures
greater than subsequent operating temperatures, in accord with previous

Studies.18

3.1 Reaction to Environment

After run TU-7, the Mark III fluorinator was subjected to an ammonium
oxalate wash and then to several water rinses to remove process salts. The
fluorinator was examined by visual inspection, chemical analysis of wall’
deposits, complete gamma radiography of the fluorinator walls, and ultra-

sonic thickness measurements to determine corrosion.

3.1.1 Visual Inspection — Wall Deposit Chemistry

 

The interior of the fluorinator was inspected by means of the unaided

eye and the Omniscope,”? All interior areas which had been exposed to

 

IBA. P, Litman and A. E. Goldman, Corrosion Associated with Fluori-
nation in the Oak Ridge National Laboratory Fluoride Volatility Process,
pp 28-30, ORNL-2832 (June 5, 1961).

 

 
UNCLASSIFIED
PHOTO 54013

 

(@)
ETCH: MODIFIED CHROME REGIA

UNCLASSIFIED UNCLASSIFIED
Y=a1182 ¥Y-angl

 

e ———— D O

(&) (¢)
AS POLISHED AS POLISHED
UNCLASSIFIED
Y=41181

PP LA v DPH
e G e "'qr';_‘-_"'- (500!} LOAD)

   

o 335
SRR s R
> 0.004

0.002
0.004

. DEFORMATION 0
’1;7 LINES

4 234

 

  

(d)
AS POLISHED

Fig. 10. Corrosion Product Layers on INOR-8 After Exposure to -
drous HF and Fused LiF-NaF-ZrF, at 500-650°C. Typical areas showing (a
uniform duplex surface layers, (b) uniform intermediate layer, discontinuous
surface layer, and hardness impressions, (c) variable thickness intermediate
layer, discontinuous surface layer, and tendency for separation of the
product layers, (d) an unusually thick intermediate layer showing brittle
nature of layer compared with base metal.

 

 
Table 4.

VPP Mark lIl L Nickel Fluorinator Process History

 

Vessel Wall Temperatures (°C)

 

 

 

Salt Molten F2 Flow
Run Composition, Without F2a With F, Salt Rate F2
Designation NaF-Li F-ZrF, Residence (liters/min E.xposure
(mole %) Vapor Salt Vapor Salt Time (hr) at STP) Time (hr)
Region Region Region Region
F, conditioning None 630-310 670-560 0 5 psig 53
. (static)
Salt transfer 27-27-46 ~375 565-550 49 0 None
studies
T-1 31-17-52 175-150 555-540 38 0 None
T-2 30-27-43 180-150" 570-545 78 0 None
T-3 31-26-43 170-160 550-525 94 0 None
T-4 30-27-43 340-325 515-485 15 0 None
T-5 30-27-43 375-360 580-550 27 0. None
T-6 31-30-39 480-475 575-560 15 0 None
T-7 27-27-46 360-340 605-580 18 0 None
F. sparge tests None 525-500 575-550 520-495 570-535 45 B8 12.0
TU-1 34-26-40 405-375 530-510 360-350 505-500 52 12 2.5
{+ 0.3 wt % U)
TU-2 30-27-43 475-450 515-490 460-.440 505-500 40 6 2.5
(+ 0.2 wt % U)
TU-3 30-31-39 325-300 575-550 330-320 510-500 38 9 1.75
(+0.3wt%U)
TU-4 37-29-34 325-300 535-510 330-315 515-500 50 4, 12 1.25 1.0
(+ 0.4 wt% )
TU-5 29-29-42 390-350 570-525 360-325 510-500 36 4, 16 1.0, 0,5
(+0.3wt%U)
TU-6 34-28-38 375-350 580-560 360-340 505-500 34 6, 16 1.0, 0.4
{+ 0.3 wt % U)
TU-7 30-30-40 360-340 575-550 360-340 510-500 65 6, 16 1.0, 0.7
{+ 0.2 wt % U) 694 30.9

 

a . . O . . . O . . .
In a few instances, excursions to $80°C maximum in salt region ond 650°C moximum in vapor region were noted.

-478—
¢

- 25 -

process vapors were covered with relatively thick, light-green deposits.
The process vapors were usually F, and UFg plus volatile molybdenum and
chromium fluoride corrosion products, probably as MoFg and CrFs. The
middle-neck region of the vessel, which joins the top and bottom right
cylinders, showed the heaviest deposits. Most of the deposits seemed to
be loosely adherent, and where the deposits had flaked off, the substrate
metal had a dark, dull appearance. PFigure 11 illustrates these findings
in the upper conical section of the fluorinator. At higher magnificatioh
evidence of pitting corrosion was found, especially in thé top cylinder
of the vessel, _

Samples collected from the deposits in the middle neck and in the
upper neck (top-flange region) were ana;yzed by wet chemistry and x-ray
diffraction. The latter indicated the deposits to be greater than 90%
NiF,. Complete analyseé of the deposits are shown in Table 5, The salt-
cortaining region of the fluorinator Wés free of deposits and had a bright,

etched appearance.

Table 5. Chemical Analyses of Vapor-Region Deposits in
the Mark III L Nickel Fluorinator After
Run TU-7 and an Ammonium Oxalate Wash

 

Analysis (wt %)

 

 

Element - Middle Neck “Top-Flange Region
Ni 57.3 51.0
Cr 1.0 1.2
Fe 0.1 2.0
Mo 0.03 0.01
Sn 1.0 0.1
Zr _ 0.6 0.9
Na ' 0.9 1.0
Li 0.08 - 0.05
U 14 ppm 22 ppm

 

3.1.2 Gamma Radiography

 

The technique for obtaining complete gamma radiography of the walls
of the VPP fluorinator was similar to that described for the hydrofluori-

nator. Evidence of pitting corrosion was noted in thg top cylinder and
- 26 -

UNCLASSIFIED |
PHOTO 55075

SALT SAMPLER LINE

 

 

0 2 4

INCHES

Fig. 11. Interior of the VPP Mark IIT L Nickel Fluorinator After
Run TU-7 Showing Upper Conical Section. Loosely adherent NiF, deposits
have flaked off in several regions, revealing darker colored substrate
metal., Approx 1/2 size.

 
—%

- 27 -

cone of the vessel, énd the attack seemed to be especially intensive in
the top and bottom third of the cylinder. The pits appeared to be slight-
ly deeper than those noted in the hydrofluorinator. With the 2% sensitiv-
ity factor for the radiographic technique taken into consideratidn, a
first approximation for the maximum pit depths was 10 mils.

Initial plans to wax replicate portions of the fluorinator for quan-
titative study of the pitting attack were abandoned to avoid the pessibility

of metal ignition by reaction of traces of dental wax with fluorine.

3.1;3 Ultrasonic Thickness Measurements

 

- Wall thinning on the VPP fluorinator was determined by Vidigage meas-

‘urements taken every contour surface inch in each quadrant, and the results

were compared fiith base~line data. Figure 12 shows the bulk metal losses
found. Quadrant geometry did not appear to be significant in analyzing
the data; and so all data are presented at each elevation and the loss
ranges outlined, |

Maximum fluorinator wall thinning of 27 mils occurred in the middle-

-neck region which Jjoins the top and bottom 16-in. right cylinders. This

‘area was exposed essentially to process vapors. Average bulk metal losses

for the salt, salt-vapor interface, and vapor regions were 5.5, 7, and
8 mils, respéctively, while maximum wall-thickness losses for these regions

were two to three times the respective averages.

3.1.4 Nature of Fluorination Corrosion

 

Metallographic and dimensicnal changes on L nickel were folléwed1dur-
ing the fluorination process runs by means of control specimens located in
the salt and interface regions and in the lower part of the Vapor region
of the VPP fluorinator. This work will be reported in detail at a later
date. However, to complete this current evaluation of fluorination corro-
sibn, a brief summary is included here.

The contrcl specimens, l/4-in.-diam rcds 36 in. long, were held in
place by use of gas-tight metal connectors attached to short lengths of
nickel pipe, which were welded to the bottom head of the fluorinator. At
- 28 -

UNCLASSIFIED
ORNL-LR-DWG 61590

 

 

—— ]

     
  
 
  
 
    
  

 

@
2 E
Qo
-
& &8
o -
T
TP | S &
V-100 CYLINDER > g

 
 
 

~ MATERIAL:
35-inTK L NICKEL PLATE

   

0.9 mils/hr
2 (F2 TIME)

7

 
 

80in.

 

BOTTOM
HEAD

 

 

   
    
   

BOTTOM
CYLINDER

 

 

-~—— CONTOUR SURFACE INCHES FROM TOP

APOR SALT
INTERFACE
REGION

b

SALT REGION
AVERAGE LOSS-5.5mils
19in

AVERAGE LOSS-
T mils

]
-1

'SUMMARY QF PRQCESS CONDITIONS

TEMP (°C) 350-520, VAPOR REGION
500-570, SALT REGION

TIME (hr) 690-SALT
30.9-F,

 

293/4 in,

BOT TOM

CONE

SALT COMPOSITION NoF-LiF-ZrFs
(27/37—-17/31—-34/52 mole %)
PLUS 0.2/0.4 wt% U

 

 

 

 

[

f 1
BOTTOM 0 5 10 15 20 25
PLATE WALL THICKNESS LOSS (mils)

 

 

 

 

Fig. 12. Wall-Thickness Losses on VPP Mark III L Nickel Fluorinator.
- A

- 29 .
convenient intervals, dimensional and metallographic changes were noted
on the rods, which were then replaced by new specimens.

Summation of the dimensional losses on those portions of the corro-
sion rods exposed to the salt and interface regions compared favorably
with losses in the same regions on the fluorinator wall found by ultra-
sonic thickness measurements. However, no such correlation was noted for
the lower vapor region.

Metallographic examination of salt-region samples from the L nickel
specimens disclosed spotty loss of grain boundary material and grain
boundary widening. Frequently, a few grains had sloughed off due to this
attack. A gray, semicontinuous film 0.1 to 0,5 mil thick was on the
surface of the samples, and the intergranular attack proceeded below this
film to a maximum depth of 1.5 fiils. Similar results were noted for the
salt-vapdr interface samples but to a much more moderate degree. Aside
from surface roughening and a thick continuous scale, no unusual effects
were noted for the vapor-region samples from the specimens. The vapor-
induced scale was considerably different in character from the films

noted on the salt and interface samples.
4, DISCUSSION AND CONCLUSIONS

Corrosion rates on the VPP INOR-8 hydrofluorinator after 14 runs
appear to be consistent with previous ORNL developmental work on the
dissolution process, with the exception that the region of meximum gttack
has shifted from the salt-vapor interface tc the vapor region. Reasons
for this shift are not clear, particularly since the vapor-region tempera-
tures have been somewhat lower than those in the salt-containing region.
One possible reason for the losses in the lower-interface and salt-vapor
regions may be the protective nature of the higher concentration molybde-
num phase, compared with the base metal, produced at the INOR-8 surface-
hydrofluorination interaction region. This higher molybdenum phase 1is
believed to have formed as the result of previous process corrosion.
Another possibility — at least for the high attack in a localized vapor

region — is that barren salt impinged, during transfér, on the upper walls
——y

- 30 -

of the hydrofluorinator. Rough calculations indicate-that the barren-salt
hydrostatic head, plus nitrogen overpressure, is sufficient to cause salt
stream impingement on the opposite wall of the hydrofluorinator only a few
inches below the elevation of the barren-salt inlet,

Based on the 125-mil design corrosion allowance for the hydrofluori-
nator, an average of 18 hr per dissolution run, and an admittedly question-
able linear extrapolation of the 0.06 mil/hr currefit corrosion rate, the
useful life of the VPP Mark I hydrofluorinator seems to be 116 runs.

Current corrosion rates for the VPP Mark ITIT L nickel fluorinator and
the nature of attack on control specimens in the current vessel compare .
closely with corrosion experienced by the Mark I and II vesselst? used in
the pilot plant. Also, the geometric location and the maximum losses for
the latest vessel, that is, the vapor region, match the Mark T major loss
area even though the vessels have different configurations.

Based on a corrosion rate of 1 mil/hr based on fluorine time and
assuming a corrosion allowance of 175 mils, a fluorine exposure time of
1.5 hr per run, and linear extrapolation, the VPP Mark ITIT fluorinator -
should have a useful life of 117 runs to match the predicted life of the
hydfofluorinator._ This calculation does not include intergranular or

other forms of selective attack.
ACKNOWLEDGMENTS

The author is indebted to Volatility Pilot Plant and other ORNL per-
sonnel for their continued cooperation on this project. Particular thanks “
are due to E. C. Moncrief for providing information on pilot planf run
conditions; Inspection Engineering Division for their careful radiography
and ultrasonic thickness measurements on the process vessels; G. I. Cathers
for assistance in clarifying process chemistry; E. E. Hoffman, R. P. Milford
D. A. Douglas, Jr., and J. H. DeVan for their critical review of this
report; Graphic Arts personnel; and Metallurgy Reportis Office for typing

of the final manuscript.

 

195, P. Litman and A. E. Goldman, Corrosion Associated with Fluori-
nation in the Oak Ridge National Laboratory Fluoride Volatility Process,
ORNL~-2832 (June 5, 1961). . »

 

 
]

32.
33.
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by,
45,
46,
47,
48.
49,
50.
51.
52.
53.
54,
55.
56,
57.
58,
59.
60.
61.
62.
63.
64,
6569,
70,

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- 31 -

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ORNL~3253

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UC}L—'HU)"UUJEE!L—'.EIbthZQL—'UJEZ

Mann

MO NOQOQOQY S g H

i

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- 32 -

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