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Report No. BMI-1468
UC-25 Metallurgy and Ceramics (TID-4500, 15th Ed. )
PROGRESS ON CERAMIC COATED FUEL PARTICLES AT BATTELLE
P r e s e n t e d at the AEC Information Meeting Held at
Battel le Memor ia l Institute July 12, I960
September l 6 , I960
P r e p a r e d by Battelle Memor ia l Institute Contract No. •W-7405-eng-92
for UNITED STATES ATOMIC ENERGY COMMISSION
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.
TABLE OF CONTENTS
Page
INTRODUCTION 1
PRESENTATIONS AND DISCUSSIONS AT MEETING 2
Background Information 2 Coa ted -Pa r t i c l e Fabr ica t ion 3 Fue l -E l emen t Fabr ica t ion 15 In -P i le Tes t s 19 Final R e m a r k s 30
REFERENCES 31
PROGRESS ON CERAMIC COATED FUEL PARTICLES AT BATTELLE
INTRODUCTION
W. S. Diethorn
Fue l p a r t i c l e s individually clad with an imperv ious coating and d i spersed in a suitable m a t r i x may have many impor tan t appl icat ions in h igh - t empera tu re fuel-elennent technology. A number of s i tes a r e invest igating this fuel concept. On July 12, I960, an information meet ing on coated fuel pa r t i c l e s was held at Battel le Memoria l Institute at the reques t of the Commiss ion . The purpose of the meet ing was to review the work on nonmetal l ic coat ings for fuel pa r t i c l e s and to encourage an informal discuss ion of th is fuel concept in h igh - t empera tu re fuel -e lement appl icat ions . B. W. Dunnington of Battel le was m o d e r a t o r of the mee t ing . Represen ta t ives from the following organ izat ions attended:
Organizat ion
AEC
Aerojet
Battel le
Repre sentative
R. F . Kirkpat r ick (Washington) M. F . Whitman (Washington) J . Mart in (Oak Ridge) V. E. Adler (New York) J . Wise (New York) J . F . Weissenberg (LAROO) H. Reynolds (Savannah River)
R. L. P e a r s o n
B. W. Dunnington W. S. Diethorn J . M. Blocher W. C. Riley G. E . Raines
General Atomic G. R. Tully L. R. Zumwalt
GE-ANPD
Genera l Nuclear Engineer ing Corpora t ion
Lawrence Labora tory
Oak Ridge National Labora tory
C. C. Browne
W. A. Maxwell
C. Z. Hoenig
W. R. Gr imes
Sanderson & P o r t e r L. Stoughton
2
This r e p o r t , which s t immar izes the Battel le contr ibut ion to this mee t ing , is being i ssued because the information it contains is not available e l sewhere in a concise form to the gene ra l r e a d e r s of a tomic -ene rgy l i t e r a t u r e . Hi ther to , this information has been p r e sen t ed only in s ca t t e r ed p r o g r e s s r e p o r t s . Some of the d iscuss ion following the p r e sentat ions in this meet ing is also given because the quest ions and answer s add information about the subject.
PRESENTATIONS AND DISCUSSIONS AT MEETING
Background Information
W. S. Diethorn
About 500,000 p a r t i c l e s of fully enr iched UO2, each coated with 40 ji of a luminum oxide, a re cur ren t ly being i r r ad i a t ed at 1500 F in the Battelle R e s e a r c h Reac tor . After 70 days for a burnup of 3 pe r cent of the u r a n i u m - 2 3 5 , or 30,000 MWD/T of u r an ium, these p a r t i c l e s continue to show excel lent f i s s ion-gas re tent ion. In the sess ion this morning we a r e going to review the work on a lumina-coa ted UO2 pa r t i c l e s and, in addit ion, the l e s s extensive work on ca rbon-coa ted UC^. Only the a lumina-coa ted UO2 has been t e s t ed in -p i l e .
If an imperv ious coating with good h igh - t empera tu re p rope r t i e s can be applied to fuel p a r t i c l e s , a d i spers ion of these pa r t i c l e s in a m a t r i x offers a pronnising solution to severa l h igh - t empera tu re fuel-elennent p r o b l e m s . Among these a re f iss ion-product r e l e a s e , c o r r o s i o n , and fuel naigration. Battel le has been invest igating coated fuel pa r t i c l e s for seve ra l g r o u p s , including Sanderson & P o r t e r , Genera l A tomic , and Aerojet . This p resen ta t ion will not include s tudies of me ta l - coa ted p a r t i c l e s .
The h i s to ry of our coated pa r t i c l e work may be of some in t e r e s t to you. About 3 y e a r s ago a c e r a m i c p r o c e s s of coating UO2 pa r t i c l e s with AI2O3 was studied. ^^' In this p r o c e s s UO2 p a r t i c l e s a re spray coated with a slip containing AI2O3, compac ted , and then s in tered . Labora tory evaluation of the pa r t i c l e s was encouraging. In ear ly 1959 a second method , the so-ca l led f luidized-bed method , of coating UO2 pa r t i c l e s with AI2O3 was developed. In this technique UO2 pa r t i c l e s a r e coated with AI2O3 by react ing Al2Cl^ and water vapor in a fluidized bed of the p a r t i c l e s . Labora tory evaluation of this coated m a t e r i a l showed sat is factory behav ior , encouraging a snnall ra te of effort d i rec ted toward fuel-e lement appl icat ions . In late 1959 it was decided to inves t igate the coated fuel par t ic le as one approach to the p rob lem of f i ss ion-product r e l e a se from fueled graphite in Sanderson & P o r t e r ' s Pebble Bed Reac tor . The core of the hel i \ im-cooled Pebble Bed Reactor cons i s t s of a bed of severa l hundred thousand fueled-graphite sphe re s . (2) P romis ing surface coatings have been studied for the r e tention of fission products in the s p h e r e s . The advantages of individually coated fuel p a r t i c l e s suggested that a study be made of coated fuel pa r t i c l e s in Ba t te l l e ' s fuel-e lement evaluation p r o g r a m for Sanderson & P o r t e r . Uranium dioxide p a r t i c l e s coated with AI2O3 by the f luidized-bed p r o c e s s were se lec ted for study. By ea r l y Apr i l of this y e a r , this m a t e r i a l had passed successfully l abora to ry evalua t ions , and was incorpora ted into graphi te sphe res and i r r a d i a t e d in a h igh - t empera tu re capsule exper iment in the BRR.
(1) References at end.
3
The potential advantages of the coa ted- fue l -par t ic le concept , and in par t i cu la r the encouraging r e su l t s with AI2O3-UO2J has genera ted cons iderable in t e re s t in other types of coated fuel pa r t i c l e s such as ca rbon-coa ted UC2- A carbon coating is readily applied to UC2 by the f luidized-bed p r o c e s s . Out-of-pi le evaluation of this m a t e r i a l i s encouraging. I r rad ia t ion of ca rbon-coa ted UC2 will await the completion of p resen t out-of-pi le evaluat ions .
The Battel le work on coated pa r t i c l e s will be d i scussed under the following th ree topics :
(1) Coa ted -Pa r t i c l e Fabr ica t ion
(2) Fue l -E lemen t Fabr ica t ion
(3) In-Pi le T e s t s .
Coa t ed -Pa r t i c l e Fabr ica t ion
John M. B loche r , J r .
It is a p l ea su re to review our exper ience in the coating of UO2 and UC2 fuel p a r t i c l e s . My presen ta t ion will cover the h i s to ry of our work in chemical vapor deposit ion, desc r ibe Ai203 and carbon coatings which have been applied by this technique, show equipment for l a b o r a t o r y - s c a l e p r e p a r a t i o n , and d i scuss out-of-pile evaluation r e su l t s .
Chemical vapor deposit ion can be defined a s the formation of a solid deposit by chemica l reac t ion of vapors at a heated surface . A wide var ie ty of reac t ions a re ava i l able for the chemica l vapor deposit ion of m e t a l s , ox ides , n i t r i d e s , b o r i d e s , c a r b i d e s , and s i l i c ides . Battel le has been active in this field since 1938, both in the p repara t ion of r e f rac to ry coat ings and in the p r e p a r a t i o n of h igh-pur i ty re f rac to ry m e t a l s . In 1957, while cons ider ing chemica l vapor deposit ion in a fluidized bed of seed pa r t i c l e s for the p repa ra t i on of h igh-pur i ty m e t a l s . Dr . Joseph Oxley, of our g roup , suggested that this technique would be ideally suited for coating nuc lear fuel p a r t i c l e s . Dense coatings would be expected to provide good f i ss ion-product re tent ion and prevent contact between fuel and c o r r o s i v e env i ronments .
The f i rs t task undertaken was the difficult one of coating m i c r o n - s i z e d UO2 p a r t i c les with tungsten. In this c a s e , the UO2 powder tended to coat as agglomera tes r a the r than as d i s c r e t e p a r t i c l e s . However , it is poss ible to coat d i sc re te pa r t i c l e s in d i amet e r s g r e a t e r than about 20 ju with e a s e . Subsequent work demons t ra ted that we could coat fuel p a r t i c l e s with ca rbon , c h r o m i u m , chromium-n icke l a l loys , molybdenum, n icke l , n iobium, niobium-vanadium a l loys , t an ta lum, AI2O3, C r 2 0 3 , NbC, SiC, and Z r 0 2 . In addit ion, l imi ted work on the deposit ion of z i rconium has shown p r o m i s e . Chemical vapor deposition of these coatings may be c a r r i e d out in a ro t a ry k i ln , on a vibrat ing pan , o r in a fluidized bed. We p re fe r to use the fluidized bed because it yields a m o r e uniform product and, under p rope r condi t ions , avoids the formation of agg lomera t e s .
4
H» supply
Water 5 vaporizer 5
Hg supply
Dust trap Dust
trap
Thermocouple
Filter Filter
To vent
Scrubber
Water supply
Hg supply AlgClglg) +3H20(g) 1 0 0 ^ Alg O3 (a) +6 HCI (g)
Al CI3 vaporizer A-35164
FIGURE 1. SCHEMATIC DIAGRAM OF APPARATUS FOR THE LABORATORY PRODUCTION OF AI2O3-COATED UO2
5
AI2O3-UO2
The work which has led d i rec t ly to this meet ing today s ta r ted in March of 1959 with our deposition of AI2O3 on UO2 as a reac t ion b a r r i e r . The f i rs t few runs were thought to be unsuccessful since the re was no change in appearance of the UO2 powder. It went in black and came out black. Closer examination revealed that the coating was indeed p r e s e n t as dense , t r a n s p a r e n t a lpha -Al203 . In J anua ry , I960, we began to p r e p a r e Al203-coa ted UO2 a s p a r t of Sanderson & P o r t e r ' s fuel-e lement evaluation p r o g ram at Bat te l le . La ter on in the sess ion this morning Gil Raines will d i scuss the ex cellent i r r ad ia t ion per fo rmance of this m a t e r i a l .
F igure 1 i s a schemat ic d iagram of appara tus for the deposition of AI2O3 on UO2 p a r t i c l e s . The react ion is c a r r i e d out in an external ly heated quar tz tube provided with a conical bottom to support the bed of powder. Hydrogen, par t ia l ly sa tura ted with aluminum ch lo r ide , const i tutes the ma jo r port ion of fluidizing gas . A separa te s t r e a m of hydrogen i s p a s s e d through a water vapo r i ze r and through an axial tube which t e r m i na tes in the lower p a r t of the fluidized bed where mixing of the reac tan t s occurs at a t e m p e r a t u r e of about 1000 C. A 100-g batch size is convenient to work with and is a d e quate for our evaluation s tudies . With a 100-g bed of UO2 powder in a 1-in. -d iamete r r e a c t o r , about one-half of the AI2O3 fornaed by hydrolys is is deposited on the p a r t i c l e s , the other half is formed as a fine powder and is c a r r i e d over into the t r a p s . P r o g r e s s of the hydrolys is is followed by t i t ra t ing the HCI collected in the water sc rubber . F igu re 2 i s a photograph of the appara tus set up in a labora tory hood.
FIGURE 2. LABORATORY APPARATUS FOR THE PREPARATION OF Ai203-COATED UO2 PARTICLES
6
lOOX N65246 a. UOg Particles With 20-/n Coating
50 OX N65247 b . View of a Coated Particle
FIGURE 3 . TYPICAL EXAMPLES OF AI2O3-COATED 100 TO 150-fi UO2 POWDER
7
It should be pointed out that no effort has been made to inc rease the ra te of coa t ing formation to a max imum consis tent with good coating p r o p e r t i e s . The highest ra te of formation used in our work is 4 JJL pe r h r . There is no apparent change in coating m i c r o s t r u c t u r e when m a t e r i a l s p r e p a r e d at this ra te and lower r a t e s a r e compared.
In work with coated pow^ders at Bat te l le , we have used th ree s izes of fluidized-bed r e a c t o r , the 1-in. -d i ame te r r e a c t o r mentioned above, a 2-in. -d iamete r r eac to r for p repa ra t ion of l a r g e r quanti t ies of m a t e r i a l , and a 5-in. -d i ame te r p i lo t -sca le r e ac tor provided with faci l i t ies for the rec i rcu la t ion of hydrogen. It i s in teres t ing to note that this 5-in. -d i ame te r r eac to r is p rac t i ca l ly a prototype for commerc i a l units handling fully enr iched powder , because the diajneter mus t be l imited to some value near 5 in. to avoid cr i t ica l i ty p r o b l e m s . This l imitat ion significantly affects the eco nomics of the coating p r o c e s s . The cost of coating na tura l or slightly enriched m a t e r ia l should be substantial ly l e s s than that of coating enr iched m a t e r i a l because l a rge r fluidized-bed r e a c t o r s can be used.
F igure 3 shows 100 to 150-jU UO2 powder coated with 20 /i of AI2O3. Figure 4 is an e lec t ron mic rog raph of the AI2O3 coating. The granular s t ruc ture of the UO2 core is shown in the lower pa r t of F igure 4. The dense alpha altimina coating is shown just above the U02 ' Notice that the re is no evidence of a gra in s t ruc ture in the AI2O3 at this magnification.
A
"N
*-^ 17,200X J422
FIGURE 4. ELECTRON MICROGRAPH OF AI2O3 COATING ON UO2
Labora tory evaluation of coated UO2 pa r t i c l e s consis ts of a i r -oxidat ion t e s t s , ni t r ic acid leaching, m e a s u r e m e n t s of uranium contaminat ion, and neutron activation followed by po s t i r radia t ion heat ing. Contamination is de te rmined by counting the alpha radioactivi ty from a nnonolayer of the coated pa r t i c l e s in an internal-Sctmple p ropo r tional counter . An alpha assay is a m o r e sensi t ive method of measur ing uranium
8
contaminat ion than an a i r -ox ida t ion t e s t but does not dist inguish among core uranium exposed by a c rack in the coat ing, u ran ium on the outside surface of the coat ing , or u ran ium in the coating within 10 jU of the coating surface . F i s s i o n - g a s r e l e a s e is det e rmined by heating the neu t ron-ac t iva ted p a r t i c l e s in flowing he l ium, absorbing the l ibe ra ted xenon-133 in a cha rcoa l t r a p , and moni tor ing the cumulat ive t r a p activity during the heat t r e a t m e n t .
Some rep resen ta t ive r e s u l t s of the evaluation a r e summar i zed in Table 1. Compa r i son of the leach data for the f i r s t two lots shows the effect of coat ing-deposi t ion t e m p e r a t u r e on coating poros i ty . Dense alpha alumina is deposited at 1000 C, but at 700 C l e s s dense gamma and kappa alumina a re deposi ted. Lots 15638-13A and 15638-14C were ve ry r e s i s t an t to oxidation at 1000 C. Oxidation values in Table 1 a r e open to some ques t ion , however , because during the t e s t with the fourth lot the platinum crucible gained weight equivalent to oxidation of 0.7 per cent of the U02. Lot 16587-2 showed an inc rease in alpha activity after severe t h e r m a l cycling to 1370 C, but the r e sults of an oxidation t e s t at 650 C appear to exclude coating fai lure during the t h e r m a l cycling.
In p repa ra t ion for the capsule i r r ad ia t ion of a graphite sphere fueled with AI2O3-coated UO2 the a s -p roduced p a r t i c l e s were neutron act ivated to de te rmine p o s t i r r a d i a -tion f i s s ion-gas r e l e a s e . About 10"'^ of the xenon-133 in the pa r t i c l e s was r e l ea sed during seve ra l hours at 2300 F .
The coefficient of expansion of UO2 is somewhat g r e a t e r than that of AI2O3. Accordingly, the coating is p laced in tension as the pa r t i c l e s a r e heated above the coat ing-deposi t ion t e m p e r a t u r e . It i s , t h e r e f o r e , of i n t e r e s t to de te rmine the rat io of coating th ickness to par t i c le d iamete r n e c e s s a r y to withstand the t h e r m a l s t r e s s . This is being done with a s e r i e s of ba tches containing 22 to 150-jLi coat ings . These samples will be alpha counted and oxidation tes ted before and after t h e r m a l cycling.
Other a spec t s of the pe r fo rmance of Al203-coa ted p a r t i c l e s will be d i scussed by the other speake r s this morn ing .
Carbon-Coated UC2
Pyro ly t i c - ca rbon coatings can be applied to uranium carbide by the fluidized-bed vapor-depos i t ion technique. When a hydrocarbon is decomposed on a surface at t e m p e r a t u r e s of 1000 C and above, a ha rd form of carbon commonly cal led "pyrolytic ca rbon" deposi ts with the planes of the graphi t ic r ings usually lying p a r a l l e l with the surface on which the deposit ion o c c u r s . In con t ras t with the o r d e r e d s t ruc tu re of g raph i t e , adjacent p lanes a re randomly or iented . If the graphi t ic planes grow c i r c u m -ferent ial ly on the fuel p a r t i c l e s , the coating will be an iso t rop ic .
F igure 5 i s a schemat ic d iagram of appara tus used to apply pyro ly t i c -ca rbon coatings to fuel p a r t i c l e s . It i s ve ry s imi l a r to the appara tus used to produce AI2O3 coa t ings , except for the m a t e r i a l s of cons t ruc t ion . In ca r ry ing out the reac t ion at 1300 to 1400 C, a MuUite r eac to r is used. At higher t e m p e r a t u r e s , a carbon or graphite r eac to r is n e c e s s a r y . At 1350 C, ca rbon i s deposited in our appara tus at a ra te of 20 fj. pe r h r on 150 to 175-/i UC2 p a r t i c l e s when the vapor feed cons i s t s of 10 to 20 vo l ume p e r cent acetylene in an iner t gas .
TABLE 1 . SUMMARY OF LABORATORY EVALUATION OF AI2O3-COATED UO2 PARTICLES
Lot
Fuel Particle Diameter, n
149-250
105-149
44-53
44-53
Coating
Thickness, ^
8
8
9 .5
15
Deposition
Temperature, C
1000
700
1000
1000
Treatment
As prepared
As prepared
16 hr at 1000 C
in a n
23 hr at 480 C
in air
+16.5 hr at 1000 C in air
+64 hr at 1000 C in am
+160 hr at 1000 C
in air
Alpha Assay ' , cpm per g
of clad particles
—
--
--
--
--
--
--
Uranium Contamination^ ',
mg per g of clad particles
--
--
--
--
--
--
--
UO2 Oxidized^'^^ per cent
Uranium
Leached^ ' ,
per cent
15638-5
15638-8A
15638-13A
15638-14C
0 .1
0.3
0.0
0.3
0.7
0.3
65 .
vO
(Platinum crucible)
16587-lB
16587-2
--
105-149
105-149
--
20
35-40
--
1000
1000
--
As prepared
As prepared
Nine (20 mm) thermal cycles
to 1370 C in air
--
7 .3 ± 1.3
1. 4 ± 1.0
76 ± 5.2
--
0.01
0.002
0 .1
--
—
—
0.0 (5 hr at 650 C)
(a) Corrected for background.
(b) Corrected for counting geometry. Contammauon assumed to be in top 10 fi of coat ing. (c) Per cent of UO2 in par t ic les . (d) Per cent of uranium leached by 1:1 HNO3 m 18 hr at room temperature .
10
^Thermocouple well
To vent
Dust Filter trap
^2^2 i o o o c ^ ^ ••" ^^
A-35165
FIGURE 5 . SCHEMATIC DIAGRAM OF APPARATUS FOR THE PREPARATION OF CARBON-COATED UCg
11
Figure 6 shows 150 to 175-/ i -diameter pa r t i c l e s of UC2 coated with 80 ji of p y r o lytic carbon. Coating deposition t e m p e r a t u r e was 1350 C. The cha rac t e r i s t i c "onionskin" s t ruc tu re i s ba re ly d iscernib le in the coating in Figure 6b. This s t ruc tu re may possibly resu l t from the difference in the coefficients of expansion in the radial and c i rcumferen t ia l d i rec t ions coupled with wealc in te rp lanar bonding. The data in Table 2 show that the poros i ty of the carbon coat ings is genera l ly very low, and that the coa t ings can be ruptured by heating to t e m p e r a t u r e s in excess of the coating deposition t e m p e r a t u r e . Fo r example , the r e l e a s e of xenon-133 inc reased sharply on p o s t i r r a d i -ation heating of Lot SPIC above the coating t e m p e r a t u r e . Alpha a s says and leach tes t s on the other samples after nine 20-min t h e r m a l cycles between 300 and 2000 C in argon also showed this same relat ionship between deposition t empera tu re and coating fai lure. Coating failure undoubtedly occur s as the r e su l t of the la rge difference in the coefficients of expansion of UC2 (10 x 10~6 per C) and pyrolytic carbon (2 x 10"" per C).
There a r e seve ra l approaches to the t h e r m a l - s t r e s s problem at high t empe ra tu r e . One approach is to coat the fuel par t ic le at a t e m p e r a t u r e higher than the maximum opera t ing t e m p e r a t u r e expected in the coa ted-par t i c l e application. However, any ro t a t ion, re la t ive to the she l l , of a l oose , nonspher ica l core on cooling after coating could resu l t in t h e r m a l s t r e s s e s at t e m p e r a t u r e s below the coating t empera tu re as the p a r t i cle is reheated . Another approach i s to provide a layer of porous carbon adjacent to the UC2 to se rve as a cushion for the expansion. It is believed that conditions can be found for the deposition of porous carbon of the d e s i r e d s t ruc tu re . In the near future we plan to de te rmine whether higher coat ing-deposi t ion t empera tu re s will extend the useful t e m p e r a t u r e range of the coating. We intend also to follow up some promis ing r e su l t s on the modification of the bas ic pyrolyt ic carbon s t ruc tu re .
Summary
In s u m m a r y , the following four points dese rve emphas i s :
(1) Dense , n e a r - t r a n s p a r e n t coatings of alpha alumina can be applied to fuel pa r t i c l e s by hydrolys is of al\imin\im chloride in a fluidized bed of UO2 powder p a r t i c l e s . This coating provides good oxidation res i s t ance and f i ss ion-gas retent ion.
(2) Pyro ly t i c -ca rbon coatings can be easi ly applied by the fluidized-bed p r o c e s s . The very low^ coefficient of expansion of pyrolytic carbon, its anisot ropic c h a r a c t e r , and its poor rad ia l s t rength a re potential p rob l e m s . Several p romis ing approaches to these p rob lems a re avai lable.
(3) Chemical vapor deposition offers the p r o m i s e of cus tom-bui l t m a s s -produced nuclear- fuel p a r t i c l e s . A wide var ie ty of m a t e r i a l s can be deposited singly, s imul taneously , or a l t e rna te ly . Void s t r u c t u r e s , burnable po i sons , or fer t i le m a t e r i a l s for breeding can also be deposited.
12
lOOX N70051 a. Typical Appearance of Particles
50 OX N70053 b . Typical Particle Enlarged to Show Onionskin
Structure of Coating
FIGURE 6. SECTIONS OF 150 TO 175-^ UC2 POWDER PARTICLES WITH 20-;/COATINGS OF PYROLYHC CARBON
TABLE 2. SUMMARY OF LABORATORY EVALUATIONS OF CARBON-COATED UC„
Fuel Particle Coating Lot Diameter, ft Thickness, fi
Deposition Temperature,
C Treatment
Xenon-133 Released(^\
per cent
Alpha Assay, cpm per g of clad particles
Uranium Contamination C'),
mg per g of clad
particles
Uranium Leached ' ^, per cent
IHr 7Hr
SPIC 177-250 40 1100 As prepared
Neutron activated, 875 C 4,6 x 10 '* (160 min)
Neutron activated, 1090 C 4.6 x 10"* (93 min)
Neutron activated, 1315 C 4.8 x 10"^ (65 min)
0,004 <0.00l(^)
SP2 177-250
SP5 149-177
SP6 149-177
17
160
80
1015
1350
1350
As prepared 2000 C thermal cycles (^^
As prepared 2000 C thermal cycles
As prepared 2000 C thermal cycles
244 ± 7
3680 ± 3
1.9± 1.3 21.4 ±3.7
1.9 ± 1.3
65. 6 ± 5.2
0.33
4.94
0.005
0.06
0.005
0.18
0.004
—
0.001
0.321
0.009
-
<0.001
—
<0.001
0.60
0.001
~
(a) Per cent of total in particles at beginning of heat treatment released in indicated time. (b) Same basis as in Table 1. (c) 1:1 HNO3 at 90 C. (d) Nine 20-min thermal cycles in argon between 300 and 2000 C. (e) Detection limit is 0. 001 per cent.
14
Discuss ion
Quest ion, L. R. Zumwalt (GA):
Reply:
Are the coated pa r t i c l e s sc reened after vapor depos i tion of the carbon coat ing?
Yes , not to b reak up a g g l o m e r a t e s , but to remove soot and chips of carbon which may have broken loose from the r e a c t o r wal l s .
Ques t ion , R. Ki rkpa t r ick (AEC):
Reply:
Quest ion, L. R. Zumwalt (GA):
Reply:
In the neut ron-ac t iva t ion r e su l t s with ca rbon-coa ted UC2 t h e r e appea r s to be some cracking of the coa t ings between 2000 and 2400 F .
That i s c o r r e c t . Some of the coatings have c racked . There is a l a rge difference in the coefficients of e x pansion of the co re and the coating which r e su l t s in l a rge tens i le s t r e s s e s in the coating as the pa r t i c l e s a r e heated above the coat ing-deposi t ion t e m p e r a t u r e .
Is t he re apprec iable adherence between the UC2 pa r t i c l e s and the carbon coat ing?
The fuel pa r t i c l e shr inks away from the coating when the coated pa r t i c l e i s brought to room t e m p e r a t u r e . During meta l lographic p repa ra t i on the carbon coating frequently chips off at the in ter face . These cons ide ra t ions suggest that the a d h e r e n c e , if any, is -weak.
Quest ion , C. Hoenig (Lawrence) : Is t he re u ran ium in the coa t ings?
Reply:
Quest ion, G. Tully (GA):
Reply:
Quest ion, L. R. Zumwalt (GA):
Reply:
Quest ion, L. R. Zumwalt (GA):
Reply:
To min imize u ran ium contaminat ion in our AI2O3 work for Sanderson & P o r t e r , we stop the coating p r o c e s s after a 5 to 8-ju coating i s deposi ted, remove the p a r t i c l e s , leach them in n i t r i c ac id , and continue the coating p r o c e s s in a clean r e a c t o r .
Is the vapor feed at 1 a t m ?
Yes .
Did you have t rouble with deposition on the wal l?
A smal l amount of AI2O3 fo rms on the wall . Howe v e r , this does not appear to cause any difficulty.
Have you ever coated i r r e g u l a r l y shaped fuel p a r t i c l e s ?
We p re fe r s p h e r e s , but we have coated nonspher ica l p a r t i c l e s a l so . The coating i s fairly uniform at the c o r n e r s .
15
Ques t ion , L. R. Zumwalt (GA): Have you worked with z i rconium ca rb ide?
Reply: No, we have not invest igated this coating. However, t he re a r e suitable react ions for the deposition of z i rconium c a r b i d e , and the development of this coat ing should be feas ible .
F u e l - E l e m e n t Fabr ica t ion
W. C. Riley
Many of the fuel appl icat ions cu r ren t ly of i n t e re s t involve the use of coated fuel pa r t i c l e s d i spe r sed in a suitable m a t r i x . Graphite is the logical naatrix choice for ca rbon-coa ted UC2^ and AI2O3 or graphite is the choice for UO2 pa r t i c l e s coated with AI2O3. Some explora tory work on p r e s s i n g oxide-coated UO2 shows that the a s -produced p a r t i c l e s can be aggregated to form a pel le t .
Our work on the fabr icat ion and evaluation of bodies containing coated fuel p a r t i c l e s has been l imi ted .
Graph i t e -Mat r ix Studies
Graphite has been fueled with Al203-coa ted UO2 as pa r t of our ass i s tance p r o g r a m to Sanderson & P o r t e r . Most of the work has been done with bodies made with 2301 graphite powder and s tandard pi tch. A few bodies containing Texas coke and s tandard pi tch or AGOT graphi te and a the rmose t t ing r e s in have also been p repa red . During fabr icat ion of the bodies t e m p e r a t u r e s have been kept below 2300 F to prevent poss ible reac t ion between the graphi te and the oxide coat ings. Oxide loadings up to about 7. 5 volume p e r cent a r e of i n t e r e s t in the Pebble-Bed Reac to r . This l a t t e r loading does not consti tute a l imit on the amount of oxide that can be d i spersed in graphi te .
A brief study was under taken to de te rmine whether the p re sence of the coated pa r t i c l e s changes significantly the p r o p e r t i e s of the graphite m a t r i x . Compress ion t e s t s were run on bodies containing about 7. 5 volume p e r cent oxide in the form of Al203-coa ted UO2 and bodies containing the same amount of fuel (about 2 voltime per cent) but no coating. Resul ts indicate that the p re sence of the coated pa r t i c l e s does not dec r ea se compres s ive s t rength. Resul t s r epor t ed by National Carbon a re also of int e r e s t . Table 3 show^s the effect of u ran ium oxide content on the physical p rope r t i e s of graphi te , v- ) D e c r e a s e s in t h e r m a l conductivi ty, f lexural s t rength , and compress ive s t rength a r e smal l with fuel loadings as high as about 20 w / o .
16
TABLE 3. PROPERTIES OF MOLDED URANIUM OXIDE-BEARING GRAPHITE IvIATERIALS BAKED AT 1450 C (3)
Uranium Content
G per Cm3
0 0.067 0.220 0.384
w / o
0 3 . 8
11.7 19.9
Bulk Density,
g p e r cm^
1.72 1.75 1.88 1.93
T h e r m a l Conductivity (3-.
w/(cm)(K)
0 .22 0.20 0. 17 0. 18
, b ) . Flexural(a) Strength,
ps i
6600 5470 5070 4290
Compres s ive Strength,
p s i
15,800 18,600 18,990 14, 100
(a) Measurements made against grain. (b) Measurements made at room temperature.
In p r e p a r a t i o n fo r a c a p s u l e i r r a d i a t i o n of a f u e l e d - g r a p h i t e body a s p e c i m e n w a s e v a l u a t e d by the fo l lowing p r o c e d u r e s :
(1) Radiography
(2) Alpha a s s a y s
(3) Neutron act ivat ion followed by po s t i r radiat ion heating
(4) Thermal cycling
(5) Heat t r e a t m e n t followed by meta l lography.
Radiography showed that the pa r t i c l e s were uniformly d i spe r sed in the graphi te . About 10~6 to 10~7 of the u ran ium in the body was on the sur face . This amount of contamination was expected on the bas i s of an alpha a s s a y of the a s -p roduced p a r t i c l e s . Additional alpha a s s a y s on f ragments of the body after impact f rac ture showed that the coa t ings were not damaged when the body f rac tured . Neutron-act ivat ion r e su l t s showed that the AI2O3 coatings re ta ined fission gas . At the highest t e m p e r a t u r e (2000 F ) , the total r e l ea se of xenon-133 from the body after s eve ra l hours at this t e m p e r a t u r e was no g r e a t e r than the cimount observed in a neut ron-ac t iva t ion t e s t of p a r t i c l e s alone. T h e r mal cycling the body seve ra l t imes between 2000 F and room t e m p e r a t u r e during the po s t i r radia t ion heating did not i nc r ea se f i ss ion-gas r e l e a s e . These t e s t s demons t ra te that the AI2O3 coat ings a r e not damaged during the 2300 F heat t r e a t m e n t given the graphite body during fabricat ion.
The evaluation work repor ted above suggests that l i t t le or no react ion occur s b e tween graphite and the AI2O3 coatings at the t e m p e r a t u r e s occur r ing during fabrication of the body (2300 F) . At some higher t e m p e r a t u r e , l e s s than 3000 F , react ion will occur and damage the AI2O3 coat ings . A brief invest igat ion has been made of the t ime and t e m p e r a t u r e r equ i red for the reac t ion to take p lace . F igure 7a shows a lumina-coated UO2 d i spe r sed in a graphi te body fabr icated at 2300 F . The cen t ra l white a r e a is UO2 and the surrounding gray a r e a is the coating. F igure 7b shows the same spec i men after a 1000-hr heat t r ea tmen t at 2500 F in flowing he l ium. Longer t imes and higher t e m p e r a t u r e s have not been studied. Reaction between the coating and the
17
^^m^'^.
N67648
b . After 1000 Hr in 2500 F
Flowing Helium
FIGURE 7. EFFECT OF HEAT TREATMENT ON AlgOg-COATED UOg PARTICLES DISPERSED IN GRAPHITE
Very litt le evidence of a reaction can be observed in the specimens.
18
graphite is suggested by the voidlike appearance at the graphite-coating boundary in Figure 7b. The severity of the reaction between the coating and graphite depends on the location of the particle in the body. Generally, particles near or on the surface of the body show more coating deterioration than those inside the body. We have not investigated the effect of fabrication variables^ degree of particle-graphite contact, and choice of graphite starting materials on the kinetics of reaction. All these variables are expected to influence the course of the reaction.
In the limited work completed so far on graphite bodies containing ALzOs-coated U02> encouraging results promise important high-temperature fuel-element applications. Two aspects require immediate attention: (1) determination of the meiximum amount of oxide that can be dispersed in graphite without affecting the physical properties and (Z) deternaination of the effect of chemical reaction between the coating and graphite on the fission-gas release.
Oxide-Matrix Studies
To date, studies on incorporation of coated particles in oxide matrices have been confined to preliminary work on fabrication techniques.
The preparation of dense oxide matrices containing coated fuel particles may be difficult with vapor-deposited coatings. These coatings are not readily sinterable, and when a sinterable matrix shrinks onto them, some cracking of the matrix is likely. There are several possible methods for solving this problem. One involves creating a suitable burned-out layer around the vapor-deposited particles that will accommodate the shrinkage of the matrix. A porous coating applied to the outside of a dense coating may have a similar effect. Both the fluidized-bed and spray-tumbling(^) methods of coating UO2 particles can provide tailored porosity in the coating and thus permit a matching of coating and matrix. Another approach is to aggregate the coated fuel particles themselves by hot-pressing techniques. Some preliminary trials have been made with Al203-coated UO2. It has been found that at 2800 F and 7000 psi , flow can be obtained in these vapor-deposited coatings. The resulting bodies maintain structural integrity, and their density is perhaps 90 per cent of theoretical. This fragmentary result is encouraging but additional work is required to establish the best fabrication method and the quality of the bodies.
Discussion
Question, R. Kirkpatrick (AEC): Do you have any results on the properties of the hot-pressed AI2O3 body?
Reply; No, this body was made only a few days ago. The body is spherical in shape, and contains about 80 per cent by volume of fuel. *
•A few days later several hot-pressed bodies were heated In 1200 F ait for several hours. These bodies were made by hot-pressing 100 to 150-^ UO2 coated with 40 ^ of AI2O3. Hot-pressing temperatures and pressures up to 2900 F and 7000 psi were used. The porosities of the bodies, determined by liquid displacement, ranged from 20 to 1 per cent. AH the bodies crumbled to a fine powder In this oxidation test. In fabricating these bodies, pressure was applied before the compact reached temperature. It Is probable that oxldatlon-teslstant bodies can be made by heating the compacts before applying pressure.
19
Quest ion, L. Zumwalt (GA): Can fuel pe l le t s 1/2 in. in d iameter be coated?
Reply: It appea r s poss ib le to coat a pel let this large but ab ras ion is expected to be a difficult p rob lem.
In-Pi le Tes t s
G. E. Raines
Based on the encouraging r e su l t s of the out-of-pi le evaluation of Al203-coated UO2 and graphi te fueled with these p a r t i c l e s , i r r ad ia t ion of a fueled-graphite sphere in the BRR was ini t iated in ea r ly Apr i l , I960. This capsule exper iment was c a r r i e d out in •support of Sanderson & P o r t e r ' s fuel -e lement evaluation p r o g r a m for the Pebble-Bed Reac tor . In the following d iscuss ion the purpose and r e su l t s of this capsule exper iment will be reviewed.
SP-5 Capsule Exper iment
The purpose of the SP-5 exper iment is to de te rmine the effect of burnup on the h igh - t empera tu re in-pi le r e l e a se of shor t - l ived and long-lived fission gases from a number of fueled-graphi te spheres of in te res t in the Pebble-Bed Reactor p r o g r a m . St ructura l and phys ica l changes in the sphe res will be de te rmined by pos t i r rad ia t ion examination on complet ion of the i r r ad ia t ion . The capsule is a so-ca l led sweep capsule . F i ss ion gase s a r e c a r r i e d out of the capsule by a once- through strecim of helium at low p r e s s u r e and low flow r a t e . A continuous flow of helium p a s s e s over the spheres du r ing i r r ad ia t ion . Long-lived fission gases in the hel ium a re m e a s u r e d by gamma-spec t romet ry methods after collect ion of a known fraction of the exit helium flow in charcoa l t r a p s above the pool of the BRR. Short - l ived fission gases a re determined by shunting the exit hel ium through a tube packed with s ta in less s teel cloth. During the 90-sec res idence t ime of the helium in the tube (daughter t r a p ) , shor t - l ived fission gases decay and deposit long-l ived gamma-emi t t ing daughte rs . Radiochemical analysis of the daughter t r ap provides the n e c e s s a r y information for the calculat ion of the ra te of escape of the shor t - l ived p r e c u r s o r s from the sphe re . The following f i ss ion-gas species have been m e a s u r e d in the sweep hel ium:
Long-Lived Short-Lived
Species Half-Life Species Half-Life
Krypton-85m 4 . 4 h r Xenon-135 9. 2 h r
Krypton-87 78 min Krypton-89 3. 2 min
Krypton-88 2. 8 hr Xenon-140 16 sec
Xenon-133 5. 2 day Xenon-141 1. 7 sec
Iodine-131 and iodine-133 a r e also observed in the daughter t r a p . Other f iss ion-gas species a r e undoubtedly p r e s e n t in the sweep hel ium but no thorough sea rch for additional species has been m a d e . Half-life cons ide ra t i ons , decay p r o p e r t i e s , and fission yields r e s t r i c t the number of species which can be eas i ly assayed in the gas or in the daughter t r a p .
20
Figu re s 8 and 9 show the SP-5 capsule and the gas t r a in for a s say of long-l ived fission gases in the exit he l ium. The daughter t r ap (not shown) i s located above the capsule ajid i s in p a r a l l e l with the exit line from the capsule . F igure 10 shows a photograph of the a s sembled capsule and daughter t r a p s . F i s s ion heat i s the only source of heat in the capsule . The capsule contains six fueled 1. 5 - in . -d i amete r graphite s p h e r e s , two sweep spec imens and four stat ic (no hel ium flow) s p e c i m e n s , mounted in hoUowed-out graphi te b locks . The sweep hel ium p a s s e s through the thin spher ica l annulus separating the sphere from the graphi te block. This annulus is filled with graphite powder . Thermocouples posi t ioned in the graphi te blocks in the two sweep compar tmen t s r e c o r d t e m p e r a t u r e s . Hea t - t r ans fe r cons idera t ions allow us to es t ima te sweep-spec imen t e m p e r a t u r e s from the thermocouple data. The graphite sphere fueled with AI2O3-coated UO2 is located at the top of the capsu le . An identical sphere is also being i r r ad i a t ed in the stat ic connpartment. The remaining four spheres contain b a r e fuel pa r t i c l e s and will not be d i scussed this morn ing .
The 1. 5 - in . -d i amete r graphi te sphere fueled with Al203-coa ted UO2 p a r t i c l e s i s designated F A - 2 2 , No. 471. Coating th ickness i s 40 ju. This sphere contains about 4. 7 g of fully enr iched UO2 in a total of about 500,000 coated p a r t i c l e s . The sphere was heated to a max imum t e m p e r a t u r e of 2300 F during fabr icat ion. Heat -genera t ion ra te is 1. 5 kw in an effective flux of about 10l3 nv. Sphere - sur face and - cen te r t e m p e r a t u r e s a re running about 1300 and 1500 F , respec t ive ly . Thermocouple data during a typical BRR cycle (Figure 11) a r e c o r r e c t e d for the t e m p e r a t u r e drop a c r o s s the graphite block and annulus to give these l a t t e r e s t i m a t e s . F igure 11 also shows the thermocouple h i s to ry of the second sweep specimen which is of no i n t e r e s t h e r e . As of July 8, i960 , the FA-22 sphere has accumula ted a 70-day i r rad ia t ion exposure for an es t ima ted u ran ium-235 f iss ion burnup of about 3 pe r cent .
Resul ts
We e x p r e s s the m e a s u r e d r e l ea se of both the long- and shor t - l ived fission gases by the t e r m R / B , where R is the ra te of r e l e a s e of a given f i s s ion-gas species from FA-E2 in a toms per second and B i s the equi l ibr ium ra te of production of this species in the sphere in a toms per second. The value of B is calcula ted from the neutron flux, fission y ie ld , u ran ium-235 fission c r o s s sec t ion , and uran ium-235 content. The R / B value is a pa r t i cu la r ly good index to use in descr ib ing the f i s s i o n - g a s - r e l e a s e r e su l t s b e c a u s e , when radioact ive equi l ibr ium is achieved both inside and outside a fuel spec i m e n , R / B is the rat io of the number of c u r i e s outside the specimen to the number inside the specimen. Accordingly, R / B values e x p r e s s d i rect ly the amount of con tami nation which this fuel specimen cont r ibutes in a c losed sys tem. Another in te rpre ta t ion of R / B i s simply the fraction of power from a fissioning source w^hich is effective in l ibera t ing fission p roduc t s . For example , if R / B is 10~6 for a 1-megawatt f ission s o u r c e , the effective f i ss ion-product source is 1 w.
F igure 12 shows a pa r t i a l h i s to ry of R / B values for the graphite sphere conta ining Al203-coa ted UO2 p a r t i c l e s . Sphere - su r face t e m p e r a t u r e is indicated above each point. There is some sca t te r in the points for individual s p e c i e s , so we have omit ted k ryp ton -87 , k ryp ton -88 , and xenon-133 to simplify the f igure. These la t te r th ree species show a long- t e rm var ia t ion s imi l a r to those for k ryp ton-85m and xenon-135. F igure 12 does not include the R / B values from the mos t recent gas sample (70 days) , which fall in the 10"6 region. The solid points a r e the R / B values for the th ree l i s ted shor t - l ived spec ies . These la t te r data a r e d i scussed below. During the f i rs t 20 days of i r rad ia t ion the re appeared to be a co r r e l a t i on between R / B and t e m p e r a t u r e , but this
C«nterline of reactor
Helium out
f -Fins Helium in
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tSJ
FIGURES. CAPSULE SP-5
DMWN . T
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MTTQxc •aioRuu. wrrmm wtamecnuG MECHANICS DIV.
^ COLUMIW, owo 'SP--S COMPOSITf SWFFP-GAS bPSULI f HOJOCT N0,
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22
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i l
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Air monitor reactor scram ond close No 1 valves
Line moniter— close No. 4 valves
Exhaust-ga s monitor — close No. 2 valve and op en No. 3 valva
1 Activity Instrumentation
FIGURE 9 . GAS TRAIN FOR S P - 5 CAPSULE EXPERIMENT
23
Valves associated with daughter traps
Short-half-life daughter traps
Manual shutoff valves
Two sweep compartnnents
Static compartment
Grid positioner
FIGURE 10. ASSEMBLED SP-5 CAPSULE WITH DAUGHTER TRAPS
THERMOCOUPLE TEMPERATURES, F
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25
irradiation Exposure,days
FIGURE 12. HISTORY OF FISSION-GAS RELEASE FROM FA-22
26
was not verif ied by the subsequent R / B va lue s . The R / B values from two success ive gas saxnples col lected on the same day show ag reemen t within a factor of two. E x a m i nation of F igure 12 shows that f i s s ion-gas re tent ion in the AI2O3-coated UO2 p a r t i c l e s is excel lent .
Four s p e c i e s , bar i \am-140, ce r i i am-141 , i od ine -131 , iod ine -133 , and s t ron t ium-89, together with t r a c e s of ce r ium-137 and t e l l u r i um-132 , were found in the daughter t r ap after the exit hel ium had p a s s e d through the t r ap for 24 h r . B a r i u m - 1 4 0 , c e r i u m -141, and s t ron t ium-89 a re deposi ted in the t r ap when the i r shor t - l ived p r e c u r s o r s , xenon-140, xenon -141 , and krypton-89> r e spec t ive ly , decay in the t r ap . F igure 13 sunnmarizes the r e su l t s for b a r i u m - 1 4 0 , c e r i u m - 1 4 1 , and stronti\u:n-89. If the only source of these th ree species i s decay of xenon-140 , xenon-141 , and krypton-89^ r e spect ive ly , and if all daughters stick where they a re fo rmed, the s t ra ight l ines give the expected dis t r ibut ions for a 90-sec re s idence t ime of the helixom in the t r a p . The exper imenta l points a r e in excel lent ag reemen t with the p red ic ted d is t r ibu t ions . Iodine r e su l t s a r e shown in F igure 14. The t a i l s of the dis t r ibut ions for iodine-131 and iodine-133 a r e probably the resialt of poor counting s t a t i s t i c s . The R / B values for iodine-131 and iodine-133 a r e difficult to e s t ima te because the fraction of iodine l i b e ra t ed from the sphere which e n t e r s the t r ap is not known. Assuming that all of it i s col lected in the t r ap gives R / B va lues of about 10"^ for both iodine spec ies . Daughter-t r ap activity is compared with the act ivi ty in the FA-22 sphere in Table 4.
TABLE 4. SUMMARY OF DAUGHTER-TRAP RADIOACTIVITY
Radioact ivi ty , cu r i e s Species
St ront ium-89
Bar ium-140
Cer ium-141
Iodine-131
Iodine-133
Trap
2 . 4 x lO-"?
3.1 X 10-7
5. 0 x 10-8
4 X 10-8
1 X 10-7
FA-22(a)
19
64
37
37
97
(a) Total activity collected in trap during 23-hr period.
P r i o r to the SP-5 exper iment Steve Beck of Battelle developed a theore t ica l diffusion model descr ib ing the r e l e a se of a decaying fission gas from coated fuel p a r t i c l e s . F igure 15 shows a plot of R / B v e r s u s \ , the decay cons tant , for diffusion constants of 10-11 and 10-12 cm^ p e r sec in UO2 and AI2O3. Krypton and xenon a r e assunned to have the same diffusion constant in both m a t e r i a l s . The solid curves a r e p red ic ted by the mode l . Our exper imenta l data a r e shown a l s o , the ve r t i c a l l ines indicating the range of R / B values for the long-l ived g a s e s . The single points a re the R /B values from the daughter t r a p . Exper imenta l R / B values do not d e c r e a s e rapidly with d e c reas ing X as p red ic ted by the model .
27
5-
4
X Strontium - 89 , I = 0.00193 j4c/in.
• Barium-140, 1= 0,0280 >4c/in.
0 Cerium-141 , 1= 0.0202 Mc/m-
"?4 32 40
Length of Travel , in. 56
FIGURE 1 3 . DISTRIBUTION OF RADIOACTIVITY IN DAUGHTER TRAP FOR FA-22
28
3£H^
Iodine-133
lodineH3l
8 16 24 32 Length of Trap, in.
40
FIGURE 14. DISTRIBUTION OF IODINE RADIOACTIVITY IN DAUGHTER TRAP FOR FA-22
29
o
a> (0 o ®
cr o c o o o
10-2
10
10-6
10-8
lo'^
10'2
in-"J lU
I0'6
in-18
t
\
\
\
\
D= lO'A
Xe-i33 1
^
\ , \
C
{
\
\
>=io' \ c n
\
:m^sec \
\
\
\
\
\
Xe-135 \
J6f- V'
i^/sec
\
\
\
\
\
\
88 \
*
• »
Note: Solid Curves are graph of
3 r V - i _ f . \l—h
B v s i n h y t i ' 1*
^ sinh (v VTH)] for the values
of D shown where v = — = a
radius of fuel core . ^ a radius of element "'"^ ' ~ D
Kr-89 Xe-wo Xei4i
10-6 10-5 lO-** 10-3 10-2 10-'
X, Disintegration Constant,sec"'
FIGURE 1 5 . FRACTIONAL RELEASE RATIO VERSUS DISINTEGRATION CONSTANT
30
At p r e s e n t , we believe that the major pa r t of the observed f iss ion-gas re lease from FA-22 is the resu l t of u ran ium contamination on or c lose to the sphere surface. F i s s ion gases genera ted in the contamination would be expected to pa s s freely out of the porous graphite with l i t t le effect of half-life on R / B . A measurenaent of the alpha radioactivi ty on the surface of the FA-22 sphere before i r rad ia t ion gives a value of 10"" to 10"^ for the fract ional amount of uran ium in the sphere which i s on the surface. If this contamination i s the major source of f ission gas in the sphe re , the expected R /B is IQ-o to 10-7. The alpha as say g ives , t h e r e f o r e , some support to our belief that a major source of the observed f i ss ion-gas r e l e a s e is the r e su l t of u ran ium contamination. This in te rpre ta t ion of the R / B values and the absence of a strong half-life effect does not , however , explain the var ia t ion of R / B values (Figure 12) with t ime .
P r e s e n t plans a r e to continue the SP-5 exper iment through October , I960.
Final R e m a r k s
W. S. Die thorn
In our p resen ta t ion this morning covering the coated fuel -par t ic le development the m e r i t of Al203-coa ted UO2 has been demons t ra ted at modera te ly high t e m p e r a t u r e s , and the explora tory r e su l t s with ca rbon-coa ted UC2 look promis ing . However , it is evident how very li t t le is rea l ly known about the p r o p e r t i e s and behavior of coated p a r t i c l e s , alone or in a m a t r i x . The l imi ta t ions of e i ther of the two coated pa r t i c l e s a re undefined. Defining these l imi ta t ions is the f i rs t step in the design of the optimum coated fuel par t ic le for a specific fuel -e lement application.
It is appropr ia te here to d iscuss briefly a few of the major unknow^ns which a re ahead in the development of the coa ted- fue l -par t ic le concept. This is pa r t i cu la r ly de s i rable with Al203-coa ted UO^ because this coated par t i c le has been advanced rapidly to the i r r ad ia t ion stage with good r e su l t s but w^ith only a modes t understanding of its l imi ta t ions .
In g e n e r a l , any coating on a fuel pa r t i c l e is expected to leak fission products at some combination of t e m p e r a t u r e and burnup. Lecikage may resu l t from the buildup of f i ss ion-gas p r e s s u r e , damage to the coating by fission r e c o i l s , or both. Coating and fuel -par t ic le p r o p e r t i e s a re expected to influence these two types of failure but , at p r e s e n t , l i t t le is know^n about how these p r o p e r t i e s affect the behavior of the coated pa r t i c l e . Some leakage of fission gases will be to le rable if the leakage is ent i re ly the resu l t of diffusion through a mechanica l ly sound coating. Under these conditions shor t lived fission gases -will decay in the coat ing, reducing the amount of contamination from this pa ren t source of long-l ived f iss ion-product gamma e m i t t e r s .
One of the impor tant applicat ions of coated fuel pa r t i c l e s is in a graphite ma t r ix . F r o m the explora tory carbur iza t ion studies completed with Al203-coa ted UO2 in graphite there is evidence that these coatings will not be suitable for long t ime serv ice at t e m p e r a t u r e s over 3000 F . Other oxide coatings may have higher t empe ra tu r e capabi l i t ies in graphi te . Carbon-coated UC2 pa r t i c l e s avoid the graphi te-compat ib i l i ty problem cha rac t e r i s t i c of the oxide coat ing, but at some t e m p e r a t u r e , fuel migra t ion through the coating may l imit h igh - t empera tu re se rv ice . Anisotropy in the carbon coat ing may have a de t r imenta l effect on i r rad ia t ion pe r fo rmance . It is not known whether fission product diffusion is sensi t ive to this p roper ty of carbon coat ings .
31 and 32
REFERENCES
(1) Smal ley, A. K. , Ri ley , W. C. , and Duckworth, W. A. , "Alumina-Clad UO2 for Fuel Appl ica t ions" , BMI-1321 (Februa ry 18, 1959).
(2) "Design and Feas ib i l i ty Study of a Pebble Bed Reactor - Steam Power P lan t " , NYO-8753 (May 1, 1958).
(3) Ea the r ly , W. P . , J a n e s , M. , Mansfield, R. L.. , Bourdeau, R. A. , and Meyer , R. A. , "Phys ica l P r o p e r t i e s of Graphite Mate r i a l s for Special Nuclear Applicat i o n s " , P roceed ings of the Second tJnited Nations International Conference on the Peaceful Uses of Atomic Ene rgy , Geneva (1958), Vol 7 , A/Conf. 1 5 / P / 7 0 8 , pp 389-401.
"WSD:ims/pa
