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Printed in the United States of America. Available from
National Technical Information Service
LJ.S. Department of Commerce
5285 Port Royal Road, Springfield, Virginia 22161
Price: Printed Copy $4.00; Microfiche $3.00

 

 

 

This report was prepared as an account of work sponsored by the United States
Government. Neither the United States nor the Energy Research and Development
Administration/United States Nuclear Regulatory Commission, nor any of their
employees, nor any of their contractors, subcontractors, or their employees, makes
any warranty, express or implied, or assumes any legal liability or responsibility for the
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ORNL/TM~-6002
Distribution
Category
UC-76

Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

STATUS OF TELLURIUM-HASTELLOY N STUDIES IN MOLTEN FLUORIDE SALTS

J. R. Keiser

Date Published: October 1977

NOTICE This document contains information of a preliminary nature.
It is subject to revision or correction and therefore does not represent a
final report.

0AK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION
for the
ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

LLOCKHEED MARTIN ENERGY RESEARCH L:B

I

3 445k 0509854 1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
CONTENTS

ABSTRACT . . . & ¢ & v v ¢ o o« s o o o
INTRODUCTION . . & v v o o o o & o o o+ &
TELLURIUM EXPERIMENTAL POT 1 . . ., . . .
CHROMIUM TELLURIDE SOLUBILITY EXPERIMENT
NizTep, CAPSULE TEST . . . . . « . . . .

TELLURIUM SCREENING TEST . +o v v o + .+ .

CHROMIUM-TELLURTUM~-URANIUM INTERACTION EXPERIMENT

S]MARY - s - » » » ® . - . . - . - . - .

ACKNOWLEDGMENTS . . . . . . « + « « .

REFERENCES . + o ¢« ¢ ¢ v o v ¢ o s o & &

»

-

Page

10
13
13
22
22

23
STATUS OF TELLURIUM-HASTELLOY N STUDIES IN MOLTEN FLUORIDE SALTS

 

J. R. Keiser

ABSTRACT

Tellurium, which is a fission product in nuclear reactor
fuels, can embrittle the surface grain boundaries of
nickel~base structural materials. This report summarizes
results of an experimental investigation conducted to under-
stand the mechanism and to develop a means of controlling
this embrittlement in the alloy Hastelloy N. The addition
of a chromium telluride to salt can be used to provide
small partial pressures of tellurium simulating a reactor
environment where tellurium appears as a fission product.
The intergranular embrittlement produced in Hastelloy N
when exposed to this chromium telluride-salt mixture
can be reduced by adding niobium to the Hastelloy N or by
controlling the oxidation potential of the salt in the
reducing range.

 

INTRODUCTION

Hastelloy N composed of 7% Cxr, 16% Mo, balance nickel, was used to
contain the LiF-BeF,-ZrF,~U¥, fuel salt in the Molten Salt Reactor
Experiment (MSRE). Tensile testing of Hastelloy N surveillance specimens,
which had been exposed to fuel salt in the MSRE, produced cracks in the
grain boundaries connecting to the salt-exposed surfaces of the specimens.
Surface chemical analyses of the affected grain boundaries indicated the

1>2  Because the depth of crack-

presence of the fission product tellurium.
ing observed in the MSRE would not be acceptable when extrapolated to the
30-year design life of a Molten-Salt Breeder Reactor (MSBR), work began
to provide for the MSBR primary containment vessel a material that would
resist grain boundary attack by tellurium. In~reactor experiments,
identified as TeGen capsules, were used to evaluate the embrittlement

resistance of various alloys, but these experiments were time consuming
and expensive, and the number of alloys that could be tested at any one
time was limited.

To evaluate many alloys and modifications for their resistance to
tellurium attack, a test was sought to simulate the appearance of
tellurium as a fission preduct in reactor fuel salt. A total of five
experiments have been conducted to learn more about the behavior of various
tellurium compounds in salt and to determine the effects of these telluride-
salt mixtures on potential containment vessel materials.

The tellurium effect was measured in terms of the number and depth
of cracks produced in a tensile specimen that had been strained to fail~-
ure at room temperature after exposure in a molten salt that contained the
telluride. Straining was used to open visible cracks along embrittled
grain boundaries to facilitate counting, but it also caused a few other
cracks, which were unrelated to the presence of tellurium, to form.
These other cracks were factored into our analysis.

The results of the tellurium-Hastelloy N-molten fluoride salt studies
follow in chronological order. The last experiments were not completed

because the MSBR program was terminated.

TELLURIUM EXPERIMENTAL POT 1

Our first tellurium experiment (pot 1) was in the form of a static
pot built to evaluate the use of lithium telluride as a means for adding
tellurium to salt. This pot (Fig. 1) allows tellurium to be added
periodically in the form of salt pellets containing a measured amount of
lithium telluride. Three viewing ports permit observation of the pellets
after their addition to the salt. Electrodes for voltammetric measurements
of the salt are inserted through Teflon seals and are used to detect and
measure the concentration of certain species in the salt. About 600 ml
of the salt LiF-BeF;~ThF, (72-16-12 mol %) was used, and its temperature
was maintained at 650°C.

In the first experiment, tellurium was added as the lithium telluride,
Li;Te, which was prepared by the ORNL Chemistry Division. Initially,
two pellets, containing a total of 70 mg of Li,Te, were added toc 600 ml

of salt. Voltammetric examination of the salt by D. L. Manning of the
 

Y~-129592

 

Fig. 1. Tellurium Experimental Pot 1 Showing the Three Viewing Ports
and Accesses for Electrochemical Probes,

Analytical Chemistry Division gave no indication of the presence of
tellurium. Subsequently, three more LisTe pellets were added, and the
salt was sampled for chemical analysis. Three additions of CrF, totalling
1.82 g were then made, followed by the addition of three more Li;Te
pellets. A final addition consisting of 0.2 g of BeO was made.

Results are summarized as follows:
1. The Li>Te pellets did not melt and disappear immediately; some
evidence of the pellets remained on the salt surface for the duration
of the experiment.

2. No electrochemical evidence of a soluble tellurium species was
detected.

3. Following the LisTe additions, visibility through the view ports
was limited by a bluish-grey deposit that was subsequently identified as
predominantly tellurium.

4. Chemical analysis of the salt sample taken after the addition of
five LisTe pellets showed that the tellurium content was less than 5 ppm.
The conclusion was that use of Li,Te is not feasible for adding tellurium to
MSBR fuel salt.

A second experiment was conducted in the same static pot at 650°C
using the reportedly more soluble lithium telluride, LiTesz. After the
pot had been drained and cleaned, fresh salt was added, and three pellets
containing a total of about 0.1 g of LiTe; were inserted. During the
following three weeks, electrochemical examinations of the salt by
D. L. Manning revealed no indication of a soluble tellurium species. Since
other experiments had shown that metal specimens could be embrittled by
tellurium even when tellurium levels in the salt were below chemical detec-
tion limits, a Hastelloy N tensile specimen was inserted into the salt and
three additional LiTes salt pellets were added. After about 650 hr the
specimen was removed from the salt, tensile~tested to failure, and
examined metallographically. Intergranular cracking, probably due to
tellurium, was found, indicating that some mechanism exists for transport
of tellurium or LiTej through the salt. Because concurrent experiments
had identified simpler methods of adding tellurium to salt, no further

work was carried out with either of the lithium tellurides.

CHROMIUM TELLURIDE SOLUBILITY EXPERIMENT

The addition of a soluble chromium telluride — either CryTes,
Cr3Tey, or CrsTeg — represents another method for adding tellurium to
molten MSBR fuel salt. If an excess of chromium telluride is maintained,

the chemical activity of tellurium in solution in the salt will be
determined 1if the temperature and the salt composition are not changed,
To determine whether there is a temperature at which one of these chromium
tellurides can provide a reasonable amount of tellurium in solution, we
determined solubility of the chromium tellurides as a function of temperature.

A Hastelloy N pot was filled with about 500 ml of the salt Li¥F-Bel,-
ThF, (72-~16-12 mol %), and the temperature of the pot was controlled at
700°C. After a sample of the salt was taken, CriTey was added and a
small Hastelloy N sheet specimen was inserted into the salt. After 170 hr
the specimen was removed and after 250 hr a salt sample was taken. The
temperature was then lowered to 650°C, and a day later another salt sample
was taken.

This sequence was then repeated at 600°C, Next, the salt temperature
was raised to 700°C, Cr.Tes was added, and another Hastelloy N specimen
inserted. The specimen was removed and salt samples were taken under the
same time-temperature conditions as discussed above.

The two Hastelloy N specimens were weighed and submitted for Auger
electron spectroscopic (AES) examination. No weight changes were detected,
but evidence of tellurium in the grain boundaries was found by AES. The

results of the chemical analysis of the salt sample are shown in Table 1,

Table 1. Results of Chromium Telluride Solubility Measurements

 

Tellurium and Chromium Content of Salt Samples (ppm)

 

 

Sampling

temperature After After

°c) Background CraTe CroTes
(no Te added) Addition Addition

700 Te <5 Te <5 Te <5

Cr 44 Cr 75 Cr 30
650 Te 15.1 Te 7.5

Cr 105 Cr 120

600 Tea <5 Te <5

Cr Cr 88

 

a ..
Insufficient sample.
Tellurium concentrations at 700°C were not as high as was expected, but
some tellurium was in soclution as dewonstrated by the tellurium found on
the grain boundaries of the specimens.

Following the solubility measurements, two tensile specimens were
exposed to the salt-Cr;Tes solution. Both specimens, one of standard
Hastelloy N and one of 2.6% Nb—0.7% Ti-modified Hastelloy N, showed a
weight increase after 500 hr exposure at 700°C. After room-temperature
tensile testing, the standard Hastelloy N specimen had significantly more
and deeper cracks than did the modified Hastelloy N specimen, as can be
seen in Fig. 2 and Table 2. Also exposed to this salt-Cra2Tes; solution were
eight specimens that had been specially prepared for examination by AES.
Unfortunately, the annealing treatment used before test resulted in large
grains, and the data obtained were not interpretable.

Because specimens exposed to the CryTez-salt solution had a surface
reaction laver, we decided to use CrgTey for future experiments because
of its lower tellurium activity. Accordingly, another pot was prepared,
filled with salt, and doped with Cr3Tey. Tensile specimens of several modi-
fied Hastelloy N alloys with different combinations of niobium, titanium,
and chromium were exposed to the salt-CraTey system at 700°C. Examination
of these specimens after tensile testing gave results included in Table 2.
These results indicate that niobium as an alloying agent reduces embrittle-
ment of Hastelloy N, but neither chromium nor niobium with titanium exhibit
as strong an effect.

Standard Hastelloy N specimens intended for Auger studies were also
exposed in this salt-Cr3Tey system to provide information about the rate of
tellurium attack (Fig. 3). As part of the Auger examination of these
specimens, the depth of tellurium penetration was measured as a function

of exposure time,?

These results are shown in Fig. 4.

For grain boundary diffusion where the diffusing species is soluble
in the bulk, a 1/4 power dependence is predicted. 1If essentially no
bulk solubility is assumed, a power dependence of 1/2 is expected. As

seen from Fig. 4, these data fit a tl/z

plot reasonably well. Because
the exposure times considered were relatively short and because scatter
in the measurements of depth was fairly large, the penetration rate calcu-

lated from these data has a rather large standard deviation.
 

   

[
i
-
e ; -
i LA o
,Az"f
P
o i
B ; 5
L
..... i
(a) |

g Y —-134595

 

 

(b |
Fig. 2. Photomicrographs of (a) Standard Hastelloy N, and (b) 2.6% Nb—
0.7% Ti-modified Hastelloy N Exposed to Salt plus CrpTesz for 500 hr at

700°C, Then Strained to Failure at Room Temperature. Etched with aqua
regia. 100X,
Table 2. Intergranular Crack Behavior of Hastelloy N Specimens FExposed
in Chromium Telluride Solubility Experiment

 

 

Exposurea Salt Crackb Average Density
Alloy Time Additive Degiity Crack Depth Times Depth
(hr) (m ) (um)
Standard 504 CroTes 1.46 x 10* 42.5 0.6205
2.6% Nb, 0.7% Ti 504 CroTe; 0.71 33.1 0.2350
Standard 499 CraTey 1.38 50.4 0.6955
2.6%2 Nb, 0.7% Ti 499 CraTey 0.97 24.3 0.2357
1.0Z Nb, 1.0%Z Ti 244 CriTey, 1.02 37.4 0.3815
1.07% Nb 244 CrsTey 0.17 30.5 0.0519
7% Cr 500 Cr3Tey 0.47 35.6 0.1673
10% Cr 500 Cr3Tey 0.28 41.3 0.1156
12% Cr 503 CrsTey 0.71 43,7 0.3103
157 Cr 503 Cri3Tey 0.57 48,9 0.2787

 

A1l exposures were at 700°C,
bMeasured number of cracks per meter of length along edge of a polished

cross section of specimen.
Y-132871

 

 

 

 

 

Fig. 3. Hastelloy N Tensilé. Specimens IL2signed for Auger Studies
Mounted in Holder Used for Salt Exposures. Note the reduced section
in the lower portion of the specimens.
10

ORNL -DWG TT-11777

 

 

175 1

150 ¢+
E 125
A /
=z
Q
2 100 ¢
[v 4
'—.
1
=
al
0.

75
5
=2
Q@
-
3 J
= 507
b

o/ L E

3 @ Q a
}
0 -+ l 4— I: 4 l 4
o 5 10 15 20 25

SQUARE ROOT OF TIME (/%)

Fig. 4. Tellurium Penetration versus Time for Hastelloy N Exposed
at 700°C to LiF-BeFz~ThFy (72-16-12 mol %) Containing Cr3Teus. Data
obtained by AES.

Ni3zTe, CAPSULE TEST

Brynestadl+ reported that a Hastelloy N specimen exposed to vapor
above NigTe; + Ni for 1000 hr at 700°C did not show intergranular cracking.
This suggested that the NijTe; + Ni system has a tellurium activity at
which Hastelloy N is not attacked. To further evaluate this potentially
significant result, a test was designed to expose Hastelloy N temnsile
specimens to salt containing NisTep; + Ni. A capsule was built and filled
with about 310 ml of LiF-BeF,~Th¥, salt. A mixture of 62 at. %

Ni—38 at. 7 Te, which had been annealed at high temperature to promote
formation of NisTesz plus a small excess of nickel, was added to the
capsule, and four Hastelloy N specimens were inserted. One specimen was

removed aflter 1000 hr at 700°C, strained to failure, and examined
11

metallographically. Contrary to Brynestad's earlier results in vapor,
extensive intergranular cracking was found after exposure in salt. An
x-ray examination® of the NiiTe, + Ni mixture showed no evidence of
unreacted tellurium, indicating that even the low tellurium activity
associated with NiszTe, in this system causes intergranular attack of
Hastelloy N. All three remaining specimens, which were exposed for
longer times at 700°C, showed intergranular attack, and the results of

metallographic examinations are given in Table 3.

Table 3. Intergranular Crack Behavior of Hastelloy N
Exposed in NijTep, Capsule Test at 700°C

 

 

Exposure Crack Average Density

Alloy Time Density Crack Depth Time
(hr) (m %) (um) Depth
Standard = 1079 8700 19.7 0.1714
Standard 2297 7700 34.8 0.2680
Standard . 4377 9400 36.1 0.3393
Standard ° 4377 4700 34.7 0.1631
1.1% Nb 2080 300 48.2 0.0145
2.0% Nb 2080 1300 36.2 0.0471
Standard 2976 7900 23.2 0.1833
1.1%7 Nb 2976 400 44 .3 0.0177
2.0% Nb 2976 800 37.5 0.0300
4.47 Nb 2976 4000 29,1 0.1164

 

Because other experiments were indicating that alloying with 1 to 27 Nb
lessens the tellurifim embrittlement of Hastelioy N, four additional modified
specimens were exposed to this NisTez-salt mixture in the capsule described
above. The specimens contained additions of 0, 1.1, 2.0, énd 4.4% Wb
and all were exposed for about 3000 hr at 700°C. After exposure, metallo-
graphic examination of the tensile tested specimens confirmed the effect
of small additions of niobium on the tellurium embrittlement of Hastelloy N.
Results are included in Table 3. Photomicrographs of these specimens

are shown in Fig. 5.
Y-143473 Y-143475

-

Y-143474 Y-143476

g
JER L

 

0% Nb 1.1% Nb 2.07% Nb h.47% Nb

ig. 5. Effect of Niobium on the Tellurium Embrittlement of Hastalloy N that Had Been Exposed About
3000 hr at 700°C to LiF-BeF2-ThF, (72-16-12 mol %) Containing NizTez + Ni, 100x%,

¢t
13

TELLURIUM SCREENING TEST

To compare the resistance of various alloys to tellurium attack,
we constructed a large pot to permit simultaneous exposure of a large
number of specimens to a salt-tellurium system. This pot is equipped
with a stirring mechanism, electrochemical probes, and five accesses
for insertion of specimens. Up to 100 specimens can be exposed to salt
at one time (Figs. 6 and 7). The pot was filled with abofit 11 liters
of LiF-BeF,~ThF, salt, and a mixture of Crs;Te, and CrsTeg; was added.
Specimens that had been selected from the moie than 50 different avail-
able modified and standard Hastelloy N compositions were exposed to this
salt-~telluride mixture for timeé ranging from 250 to 5000 hr at 700°C.
Standard Hastelloy N was cracked to about the same extent when exposed
in this system as when exposed in previously discussed syétems. All

® The most

results of the various exposures are tabulated elsewhere.
notable result of these tests was the observation that the addition of
1 to 2 at. Z Nb to Hastelloy N éignificantly reduced the cracking,

as shown in Figs. 8 and 9. A more thorough discussion of these results

will be given elsewhere.®

CHROMIUM~TELLURIUM-URANIUM INTERACTION EXPERIMENT

The research éffort to solve the problem of tellurium intergranular
attack of Hastelloy N was predominantly concerned with the development
and testing of new alloys. The'possibility éf a chemical change in the
salt that could alter the extent of attack by tellurium was also consi-
dered. It has been suggested7 that since chromium ion activity can be
controlled, within a certain range, by the U(IV)/U(III) ratio, it might
be possible to control tellurium activity by "complexing"" reactions

with chromium or other salt constituents, for example,
2UF3 4+ MF> + xTe & ”MTe.X" + 2UFy ,

where M denotes chromium or another salt component.
Electrochemical studies reported by Manning and Mamantov® indicated

that in a relatively reducing LiF-BeF>-ThFy melt, tellurium could exist
14

¥=-197736

 

Fig. 6. Tellurium Screening Pot Used for Exposing Tensile Specimens
to LiF-BeF>~ThFy (72-16-12 mol 7) Containing CrTe;,266-
15

Y=137735

 

Fig. 7. Holder with 25 Tensile Specimens Prepared for Imnsertion in
Tellurium Screening Pot. The five top specimens will be in the wvapor
space above the salt, while the other 20 specimens will be in the molten
salt.
 

0% Nb

Fig. 8.
LiF-BeF;~ThF,

¥-140868 Y-140872

 

- Y-140873

-
v

 

1.1% Nb 2.0% Nb 3.37% Nb

Effect of Niobium Additions on the Embrittlement of Hastelloy N Exposed 1000 hr at 700°C to
(72-16-12 mol %) Containing CrTe;, 26g. 100X.

91
17

ORNL-DWG 76-7234

 

 

(CRACK FREQUENCY ) x { AVERAGE DEPTH)

 

 

i 1 ! |
O 1 2 3 4 S
NIOBIUM CONCENTRATION ( wt %)

 

Fig. 9. Effect of Niobium in Modified Hastelloy N on Grain Boundary
Cracking when Exposed in Salt—CriTey + CrsTeg for 250 hr at 700°C.
18

as a telluride rather than as elemental tellurium. Their calculations
showed the critical potential for the formation of a telluride corre-
sponded to a U(IV)/U(ILII) ratio of about 150 at 650°C.

To check this hypothesis, a salt pot was built, equipped with elec~-
trochemical probes, and filled with about 1.4 liters of MSBR fuel salt
(LiF-BeF;~ThF,~-UF,). With the close cooperation of D. L. Mamning,
Analytical Chemistry Division, the oxidation potential and impurity
content of the salt were monitored voltammetrically. Additions of CrFe:
and about 1 g of Cr3Tey were made, and a Hastelloy N specimen was exposed
for 500 hr to salt with a U(IV)/U(III) ratio of about 90. Metallographic
examination of the specimen after deformation showed extensive cracking
(Fig. 10).

To lower the oxidation potential of the salt, a beryllium rod was
immersed in the salt for an hour after which time the U(IV)/U(II1) ratio
had decreased to about seven. After another addition of CrFs; and Cr3Tey
to the salt, another specimen was exposed to the mixture for 500 hr.

A metallographic examination (Fig. 10) of the tensile tested specimen
showed a complete absence of grain boundary cracks. These results were
encouraging so a series of exposures was carred out over a wider range
of U(IV)/U(III) ratios. Specimens were exposed for about 260 hr at
U(IV)/U(111) ratios of approximately 10, 30, 60, 85, and 300. Photo-
micrographs of the gage section of the tensile specimens from this

. series of tests are shown in Fig. 11. As is evident in Fig. 12, a
marked change occurred in the cracking behavior of Hastelloy N exposed
in this system in going from a ratio of about 60 to about 100.

The results of the metallographic examination of the specimens exposed
in the Cr-Te-U interaction experimeni are collected in Table 4 and are given
in the order of exposure. Before each exposure both CrFez and CrsTey (or
CrTei.265) were added to the salt. According to the voltammetric measure-
ments, the Cr++ content of the salt increased with successive CrFep addi-
tions. As can be seen from Table 4, the first and eighth exposures were
at about the same oxidation potential (90 and 85), but the cracking behav-
ior of the two differed movre than would be expected from the difference in
exposure time and oxidation potential. This suggests that the difference in
Cr++ concentration between the early and later exposures is an important

factor that affects the cracking behavior, but no funding or plans exist
19

Yl3534

(b)

 

Fig. 10. Hastelloy N Exposed 500 hr at 700°C to MSBR Fuel Salt
Containing CrFe; and CryTey With U(IV)/U(III) Equal to {(a) 90 and (b) 7.

100,
20

B 141273

 

(c)

e Y -143477

 

E Y-143472

   
 

(e)

Fig. 11. Hastelloy N Exposed About 260 hr at 700°C to MSBR Fuel
Salt Containing CrFe; and CrTei.2ss with U(IV)/U(ITI) Equal to (a) 10,
(b) 30, (c) 60, (d) 85, and (e) 300. 100x.
21

ORNL~-DWG T77-4680

1 l l ] l
900 |- REDUCING OXIDIZING

——ee ———————-

 

CRACKING 600 |— -
PARAMETER
[FREQUENCY (cm™
X AVG. DEPTH {um)] u 7

300 ~

 

 

®
o L-#————t———r"" | | !
10 20 40 70 100 200 400
SALT OXIDATION POTENTIAL [U(IV)/UUIL)]

Fig. 12. Cracking Behavior of Hastelloy N Exposed 260 hr at 700°C
to MSBR Fuel Salt Containing CrTei,266-

 

Table 4. Intergranular Crack Behavior of Hastelloy N Specimens Exposed
in Cr-Te-U Interaction Experiment at 700°C

 

 

 

Exposure Conditions Crack Average . ‘
Specimen | Density Crack Depth Density Times
Time U(IV) Te (m..ul) (Um) DEPth
(br) TOID) Additive
1 504 90 CrsTey 12,200 74.8 0.9126
2 502 i CraTey 0 0.0 0.0
3 257 MO CrTei, 266 200 3.6 0.0007
4 257 ~10 CrTe1,. 2656 400 8.6 0.0034
5 257 30 CrTe; . 266 200 - 12.7 0.0025
6 257 30 CrTei.266 300 12.2 0.0037
7 257 ~60 CrTes.z266 500 13,0 0.0065
8 257 ~85 CrTei1.z256 6,800 10.7 0.0728
9 262 300 CrTe;.266 6,000 13.6 0.0816

 

4511 were standard Hastelloy N, heat 405065.
22

for further investigation of this possibility. (Also different was the
telluride added, CrTe:1.333 vs CrTei1.266, but from previous experience
we would not expect it to make such a large difference).

Controlling the oxidation potential of the salt coupled with the
presence of chromium ions in the salt appears to be an effective means
of limiting tellurium embrittlement of Hastelloy N. However, further
studies are needed to assess the effects of longer exposure times and
to measure the interaction parameters for chromium and tellurium under

varying salt oxidation potentials.

SUMMARY

As a result of these studies, we have found that Hastelloy N exposed
in salt containing metal tellurides such as LimTe and CryTez undergoes
grain boundary embrittlement like that observed in the MSRE. The
embrittlement is a function of the chemical activity of tellurium
associated with the telluride. The degree of embrittlement can be
reduced by alloying additions to the Hastelloy N. The addition of 1 to
2 at. % Nb significantly reduces embrittlement, but small additions of
titanium or additions of up to 15 at. %Z Cr do not affect embrittlement.

We have found that if the U(IV)/U(III) ratio in fuel salt is kept
below about 60, embrittlement is essentially prevented when CrTei 2656

is used as the source of tellurium.

ACKNOWLEDGMENTS

This work was supported by the Molten-Salt Reactor Program and
was carried out under the program direction of H. E. MeCoy. His strong
support and encouragement are very much appreciated. The assistance
of the following is gratefully appreciated: E. J. Lawrence for operating
the experiments, B. McNabb and J. C. Feltner for preparation and evalua-
tion of the specimens, W. H. Farmer for carrying out the metallographic
examination of the specimens, J. R. DiStefano and J. H. DeVan for advice
and encouragement, and H. E. McCoy and R. E. Clausing for review of the

manuscript.
23

Some of the experiments were carried out as a project of the tellu-

rium working group involving S. L. Bennett, D. N. Braski, J. Brynestad,

R. E. Clausing, J. R. Keiser, J. M. Leitnaker, H. E., McCoy, and C. L. White.

This group met periodically and made valuable input into these experiments.

The report was edited by R. R. Thrig and prepared for reproduction

by K. A. Witherspoon.

1.

REFERENCES

H. E. McCoy and B. McNabb, Intergranular Cracking of INOR-8 in the
MSRE, ORNL-4829 (November 1972).

R. E. Clausing and L. Heatherly, "Examination of a Hastelloy N Foil
Sample Embrittled in the Molten Salt Reactor Experiment' paper in
preparation.

J. R. Keiser, D. L. Manning, and R, E. Clausing, "Corrosion Resistance
of Some Nickel-Base Alloys to Molten Fluoride Salts Containing UFy

and Tellurium, pp 315-28 in Molten Salts, the Electrochemical Society,
New York, 1976.

H. E. McCoy, J. Brynestad, D. Kelmers, and B. McNabb, ORNL, private
communication, May 31, 1975.

D. N. Braski, ORNL private communication, March 1976.

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J. Brynestad, ORNL, private communication, September 30, 1975.

D. L. Manning and G. Mamantov, ORNL, private communication, November
30, 1975.
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14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24,
25.
26.
27.
28.
29.
30.
31.
32.
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34.
35,
36.

37-39.
40,
41—50.

79-80.
81-82,

83-192.

25

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