Elsner, N.

Holzl, R.

Lindgren, J.

Weinberg, A.

English

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GENERAL ATOMIC 
November 6, 1963 
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GENERAL ATOMIC 
D I V I S I O N  OF 
GENERAL DYNAMICS 
JOHN JAY HOPKINS LABORATORY FOR PURE AND APPLIED SCIENCE 
P.O. BOX 608. SAN DlEGO 12. CALIFORNIA 
GA-4764 
TUNGSTEN CLADDING OF REACTOR FUET.S* 
J. Udgren, A. Weinberg, N. Elsner, 
and R. Holzl t 
i -  
This paper will be presented at the Symposium on Applications 
of Refractory Metals, to be held December 9 and 10, 1963, in Los 
Angeles, California, and will be published in the Proceedings of 
the Symposium. 
* 
This work was sponsored in part by the National Aeronautics 
and Space Adnainistration under Contract NAS 3-2532. 
tSan Fernando Laboratories, Pacoima, California 
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November 6, 1963 
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TUNGSTEN CLADDING OF REACTOR FUELS 
INTRODUCTION 
The need for  cladding of reactor fue l s  arises because bare fue ls  
operated a t  temperatures of 180OoC i n  reactor environments usually a r e  not 
strong, often swell, suf fe r  weight losses  from vaporization, and permit a 
r e l a t ive ly  large amount of fission-product release.  
l khe  fue ls  under consideration a t  General Atomic a s  candidate nuclear 
Studies a t  General Atomic and elsewhere of the compatibility 
fue l s  t o  be operated a t  temperatures up t o  18OO0C a r e  UC, UC-ZrC, W-UC, and 
W-UO cermets 
of refractory metals and these fuels  have indicated that of four metals 
studied (W, Ta, Mo, mb) 
ab ie  fo r  use a s  a dire& c l ~ ~ I d i n g . ~  Tungsten has considerable strength a t  
18OO0C ('), and l i k e  other pure metals does not exhibi t  s ignif icant  dimen- 
s iona l  changes due t o  the reactor environment ( 2 ) J  and according t o  re fer -  
ence (3)  i ts  physical properties a re  not expected t o  be s igni f icant ly  
changed a t  these temperatures. Tungsten can add s igni f icant  strength t o  
the fue ls  and hence enhance the i r  dimensional s t a b i l i t y ,  and can prevent 
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pure tungsten is, t o  date, the material  most s u i t -  
-- 1 
the  f u e l  element from losing w e i g h t  a s  a r e s u l t  of vaporization. 
fission-product re lease is impeded by the cladding, 
Further, 
Tungsten is a f a i r l y  d i f f i c u l t  material  t o  fabr icate  by conventional 
techniques. 
tes t  specimens. 
indicated the t r ans i t i on  tempemlure t o  be a function of grain s i ze .  
s t en  can be vapor-deposited i n  f ine g r a i n s  and, because of i t s  resis tance t o  
grain growth, w i l l  r e t a i n  its duc t i l i t y  and low t r ans i t i on  temperature even 
vhen heated t o  temperatures higher than the recrys ta l l iza t ion  temperatures 
of s intered or  cas t  tungsten. 
Vapor deposition was one log ica l  choice as a method fo r  cladding 
ies on duc t i le - to-br i t t l e  t r ans i t i on  temperatures have 
Tung- 
In  order t o  develop improved tungsten cladding of reactor fuels ,  
General Atomic undertook a program i n  close cooperation w i t h  San Fernando 
Laboratories. 
car r ied  out a t  San Fernando Laboratories, and the research and development 
data analysis and fuel fabrication were done a t  General Atomic. 
Most of the vapor deposition p r t  of the fabricat ion work was 
1 
DESIGN OF FUEL-SPECIMEN CLADDING 
Two di f fe ren t  methods of tungsten cladding were employed i n  the 
s tudies  a t  General Atomic. [One method u t i l i z e d  vapor deposition, the other 
a machined W-2Mo a l l o y  cup (ins€de surface clad w i t h  vapor-deposited tungsten) 
with a diffusion-bonded cap. 
Figures 1 and 2. It w i l l  be noted tha t  i n  both designs there i s  a void a t  
the top, The void was incorporated f o r  two reasons: (1) The void allows 
room for  the expansion of escaped f i ss ion  gases, and less stress i s  put on 
the cladding, The design gives each specimen the capabi l i ty  of being 
used t o  measure the volume of f i ss ion  gas release.  Figure 3 shows a cross 
sect ion of one finished vapor-depocited-tungsten-clad specimen and a top and 
a s ide  view of another vapor deposited specimen. The specimen has a tungsten 
pin brazed i n  a t  one end. A hole was d r i l l e d  i n  t he  top, a s  the specimen had 
t o  be out-gassed in te rna l ly  t o  remove the residue from vapor deposition; the 
pin sea ls  the hole. The W-2Mo clad specimens were out-gassed in te rna l ly  
pr ior  t o  diffusion-bonding the cap t o  the specimen i n  vacuum. 
_ j .  
*_ 
?he designs of the par ts  f o r  each a r e  shown i n  
*, 1 
(2) 
As a preliminary investigation in to  the f e a s i b i l i t y  of the method, 
0 several  carbides were coated; the par t s  were out-gassed a t  -1800 C and were 
sectioned and found t o  be in t ac t ,  with no evidence of s ign i f icant  damage a s  
a r e s u l t  of t he  vapor-deposition coating process, It was then assumed tha t  
the ordinary vapor-deposition techniques were f eas ib i l e  for  coating the 
carbides t o  be used i n  tes t ing  of tungsten-clad emitters.  
A s l i g h t  change i n  design of the preforms used t o  enclose the void 
WAS introduced t o  make t h e  cladding process more economical. The first de- 
s ign i s  shown a t  the top of Figure b ,  and the newer design, i n  which the 
tungsten preform was made i n  two parts and extended the length of the speci- 
men, is shown a t  the bottom. 
When ac tua l  cladding of the carbides t o  be used i n  the t e s t  speci- 
mens was begun, a s e r i e s  of unanticipated d i f f i c u l t i e s  i n  the vapor deposition 
process were encountered. Every carbide reacted with the atmosphere during 
deposition--the degree varying from slight a t tack  t o  such severeat tack t h a t  
the carbide turned t o  powder. After a number of specimens had been ruined i n  
t,hese attempts, it became apparent t h a t  the process would have t o  be changed, 
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W-UO cermet clad with vapor-deposited tungsten 2 
Top view Side view 
90 mol-$ U C 4 0  mol-$ ZrC (depleted) clad with vapor-deposited tungsten 
6 
FUEL 
VAPOR DEPOSITED 
Vapor Deposited Tungsten Emitter ( 4  x 1 = scale) 
(old design) 
VAPOR DEPOSITED 
TUNGSTEN PREM>RM 
VAPOR DEPOSITED 
TUNGSTEN ( .020" 
FUEL (uc-zrc) 
Vapor Deposited Tungsten Emitter (4 x 1 = scale) 
(new design) 
Fig. 4 
It was a l so  apparent t ha t  the successful coating of the carbides during the 
preliminary investigation was due par t ly  t o  the design of the specimen (which 
prevented trapping of corrosive gases i n s i d e  the tungsten preform) and per- 
haps because the plat ing atmosphere was cleaner than i s  usual during ordin- 
a r y  tungsten-vapor deposition. 
the prac t ica l  minimum of moisture, oxygen, or  other contaminants. 
A clean atmosphere is  one which contains 
DEVELOPMENT OF SATISFACTORY CLADDING PROCEDURE 
' A Program t o  develop a more su i tab le  method fo r  coating the carbides 
was s t a r t d  Since UC-ZrC specimem a re  attacked rapidly i n  moist atmos- 
pheres and the a t tack  i s  accelerated by ra i s ing  the temperature, the first 
s tep  was t o  minimize contamination from impurities i n  gases used i n  the 
plat ing process Hg, He, and WF6, and t o  eliminate leaks i n  the p la t ing  
system equipment. 
Even a f t e r  impurities were eliminated from the system it was deter-  
mined that the carbides a re  attacked by reaction products formed during the 
plat ing operation, To prevent t h i s  type of a t tack  on the carbides during 
f i n a l  encapsulation, it was necessary t o  develop techniques f o r  seal ing 
the carbide specimens w i t h  a tungsten coating applied under conditions which 
eliminated o r  minimized such at tack,  The specimen could then be t reated 
much l i k e  an ordinary piece of tungsten. 
Several dozen specimens were coated under various plat ing conditions 
and evaluated by metallographic examimt io~ of the tungsten-carbide i n t e r -  
faces.  
the other photomicrograph ( i n  Figure 5 )  shows no s igni f icant  a t tack .  
These r e su l t s  indicated tha t  a vapcr-deposited tungsten coating could 
i n  f a c t  be applied w i t h  no s ignif icant  a t tgck-on the carbide. 
with protective _-- coatings 1-1/2 t o  5 m i l s  thick were heated 
A photomicrograph showfng a t tack  is given a t  the top of Figure 5; 
. - - - - - __ __ - .------_---- __L--- - I - 
When carbides 
I - I___  
- -- -- - --- ---- --_. 
i n  HF f o r  one- - -- 
hal f  -hour a t  - 7OO0C, it was found tha t  a 3-mil coating gave adequate pro- 
~ __ __---- - 
t ec t ion  provided t h a t  the coating had no holes. 
f o r  applying the sea l  coating; 
encapsulation. 
Figure 6 shows the setup 
Figure 7 shows the arrangement f o r  f i n a l  
Thirty-five specimens were successf i l ly  coated by these techniques. 
The re su l t s  showed tha t  tungsten-vapor deposition is  a fabr icat ion method 
amenable t o  cladding of complex shapes; tungsten can be vapor-deposited 
a 
I 
._ 
( 25ox ) 
Vapor deposited tungsten on gOUC-10ZrC specimen. 
IJote attack of carbide with reaction products , 
(400x1 Vapor deposited tungsten on 3OUC-'7OZrC specimen, 
Note no attack of carbide from reaction products 
of vapor deposition. 
Fig. 5 
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GAS FEED TUBE CARBIDE 
UF6+ H2 SPEC IMEN 
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FOR POINT CONTACT 
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GAS FEED TUBE 
TUNGSTEN 
CARB I DE SPECIMEN PREFORM 
SEAL f-4 
y i 0 GRAPH IT€ HOLDER 
VYCOR TUBE J 
Fig. I-(--Arrangement f o r  f i n a l  encapsulation 
11 
over voids such as are shown i n  Figure 4, and applied t o  extremely react ive 
materials such a s  UC. 
handling, and out-gassing without being damaged. All of the th i r ty- f ive  
specimens remined i n  t a c t  and suitable f o r  i r r ad ia t ion  testing. 
raphic evaluation of several  of the specimens after fabricat ion and out- 
gassing showed there was l i t t le ,  i f  any, evidence of react ion o r  a t tack  on 
the carbide i n  a l l  of the specimens examined. 
All of the  specimens coated withstood machining, 
Metallog- 
References: 
(1) WSA TND 1094 
(2) 
( 3 )  
Radiation Damage i n  Solids, Bellington, D. S. and 
Crawford, J. 8. Jr., Princeton University Press, 1961 
Deformation, Fracture, and Radiation Dermage i n  Body 
Centered Cubic Transition Metals, Johnson, A .  A .  Jnl.  
Less Common Metals, Vol. 2, pgs. 194-200 (1960) 


Tungsten cladding of reactor fuels

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