LOCALIZING FISSION PRODUCTS IN IRRADIATED NUCLEAR FUEL PARTICLES WITH THE UNSHIELDED ELECTRON MICROPROBE

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LOCALIZING FISSION PRODUCTS IN

IRRADIATED NUCLEAR FUEL PARTICLES WITH THE UNSHIELDED ELECTRON MICROPROBE

H. Grübmeier

To cite this version:

H. Grübmeier. LOCALIZING FISSION PRODUCTS IN IRRADIATED NUCLEAR FUEL PAR-

TICLES WITH THE UNSHIELDED ELECTRON MICROPROBE. Journal de Physique Colloques,

1984, 45 (C2), pp.C2-833-C2-836. �10.1051/jphyscol:19842191�. �jpa-00223867�

JOURNAL DE PHYSIQUE

Colloque C2, supplkment au nb2, Tome 45, fkvrier 1984 page C2-833

L O C A L I Z I N G F I S S I O N PRODUCTS I N I R R A D I A T E D NUCLEAR F U E L P A R T I C L E S W I T H THE UNSHIELDED ELECTRON MICROPROBE

H . B . Griibmeier

KemforschungsanZage JZfZich GmbH, I n s t i t u t fW Reaktorwerkstoffe, JZfZich, F.R.G.

R@sum& - L ' u t i l i s a t i o n d'une microsonde non blindee pour examiner l e s produits d e f i s s i o n dans des p e t i t s echantillons de materiaux i r r a d i e s e s t montree par

quelques exemples d'examens de particules combustibles nucleaires enrobees.

Abstract - The application of an unshielded microprobe t o examine f i s s i o n pro- ducts in small samples of- i r r a d i a t e d material i s demonstrated by some examples of investigations i n coated nuclear fuel p a r t i c l e s .

I - IRRADIATED MATERIAL IN THE UNSHIELDED MICROPROBE

The coated nuclear fuel p a r t i c l e s - with diameter < ¹ mm - which contain the f i s s i l e material of the high-temperature reactor in form of t i n y fuel kernels, surrounded by pyrocarbon and s i l i c o n carbide coatings, a r e the object of extensive investigations.

Irradiation t e s t s a r e performed t o verify the general i n t e g r i t y of coated p a r t i c l e s , and especially t o study the chemical behaviour of f i s s i o n products and t h e i r trans- port properties.

X-ray microanalysis i s an important method f o r the chemical examination a f t e r i r r a - diation. Because only a small portion of nuclear material i s concentrated i n a s i n g l e p a r t i c l e , a f t e r cooling-time periods of 1-2 years the radioactive background level can be decreased t o practicable low values of some mCi (Fig. I ) , thus giving us the unique p o s s i b i l i t y t o work with an unshielded electron microprobe.

L i Fission product \

2 3-

15

lo6 2 5 lo7 5 loe sec

I I I I

, 2

Cooling time

3 years

Fig. 1 - Effect of cooling-time on the a c t i v i t y and P / r - r a t i o of f i s s i o n products generated during 1 y i r r a d i a t i o n of 1 mg U 235 assumed to be t o t a l l y fissioned (100 % f i f a ) .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842191

C2-834 JOURNAL DE PHYSIQUE

I t f o l l o w s f r o m F i g . 1 t h a t f o r t y p i c a l v a l u e s o f 0 . 1 mg U 235 p e r c o a t e d p a r t i c l e t h e a c t i v i t y can be reduced a f t e r 50 % f i f a burnup and 2 y c o o l i n g - t i m e t o about 2.5 mCi. T h i s i s i n t h e o r d e r o f magnitude which has been p r a c t i c a l l y measured on specimens used f o r t h e m i c r o p r o b e examination. The r e s i d u a l a c t i v i t y does n o t i n - f l u e n c e t h e d e t e c t i o n l i m i t s o f v a r i o u s f i s s i o n p r o d u c t s v e r y s e r i o u s l y i . e . by a f a c t o r o f 2-3. The u s e f u l n e s s o f t h e above c o n s i d e r a t i o n s i s demonstrated i n t h e f o l l o w i n g examples.

I 1 - FISSION PRODUCTS I N FUEL KERNELS

The p r i m a r y o b j e c t o f t h e e l e c t r o n m i c r o p r o b e i n v e s t i g a t i o n s i s t h e k e r n e l o f t h e c o a t e d f u e l p a r t i c l e , i n i t i a l l y c o n s i s t i n g o f n u c l e a r m a t e r i a l o f t h e o x i d e (UO ) o r c a r b i d e (UC2) t y p e , and t h e n a d d i t i o n a l l y e n r i c h e d w i t h f i s s i o n p r o d u c t s dur?ng t h e i r r a d i a t i o n t e s t . The f i s s i o n p r o d u c t s can be c l a s s i f i e d by t h e i r chemical be- h a v i o u r i n groups o f elements which a r e s o l u b l e i n t h e k e r n e l m a t r i x o r r e a c t i n g w i t h t h e uranium c o n t a i n i n g f u e l m a t e r i a l and o t h e r s which do n o t f o r m s t a b l e com- pounds o r p r e c i p i t a t e i n s e p a r a t e phases. From t h e r e s u l t s o f m i c r o a n a l y s e s some c h a r a c t e r i s t i c a l d i s t r i b u t i o n s o f t y p i c a l f i s s i o n p r o d u c t s b e h a v i o u r i n t h i s r e s - p e c t a r e shown i n F i g . 2 and 3, a l s o d e m o n s t r a t i n g t h e i n f l u e n c e o f t h e i n i t i a l k e r n e l c o m p o s i t i o n ( o x i d e o r c a r b i d e ) .

d ) Molybdenum-Mo LA

F i g . 2 - T y p i c a l f i 8 s i o n p r o d u c t d i s t r i b u t i o n s o f UO c o a t e d p a r t i c l e s a f t e r i r r a d i a -

t i o n a t 1300. ..I150 C and 50 % f i m ) ( = f i s s i o n s p e r i n i t i a l c e t a l a t o m s ) . Scanning

area o f X-ray images: 400 x 400 um _/ . -

1. Oxide F u e l s (UO?) :

C e ~ j q ~ ( F i g . 2a) does n o t f o r m s t a b l e compounds and as a v o l a t i l e element i s depo- s i t e d p r e f e r e n t i a l l y i n t h e b u f f e r l a y e r s u r r o u n d i n g t h e c e n t r a l k e r n e l . fjgodymjum ( F i g . 2b) l i k e t h e o t h e r r a r e e a r t h elements i s s o l u b l e as o x i d e i n t h e uranium o x i d e m a t r i x . BaEilum ( F i g . 2c) p r e c i p i t a t e s i n a s e p a r a t e d phase c o n s i s t i n g predo- m i n a n t l y o f barium-strontium-zirkonate. Molybdenum ( F i g . 2d) i s found i n m e t a l l i c

i n c l u s i o n s c o n t a i n i n g a l s o t e c h n e t i u m and n o b l e metals.

a) Molybdenum-Mo LL b ) Neodymium-Nd L&

F i g . 3 - T y p i c a l f i s s d o n p r o d u c t d i s t r i b u t i o n s o f UC2 c o a t e d p a r t i c l e s a f $ e r i r r a - d i a t i o n a t 1030 ... ⁹⁴⁰ C and 68 % f i m a burnup. Scanning area: 100 x 100/um .

2. C a r b i d e F u e l s (UC2):

P4olyb+nurl, ( F i g . 3a) forms t e r n a r y c a r b i d e s o f t y p e UMC (M = M e t a l ) w i t h uranium and technetium. I t s d i s t r i b u t i o n i s a l s o c o r r e l a t e d w i t $ t h e n o b l e m e t a l s M = Ru,Rh, Pd which a r e p o s s i b l y f o r m i n g a n o t h e r complex c a r b i d e phase o f t y p e U2MC . A c c o r d i n g t o t h e r e s u l t s o f s i m u l a t i o n experiments /1/ b o t h phases can be mixed t o 6 e t h e r i n a f i n e d i s p e r s i o n u n r e s o l v e d by t h e e l e c t r o n microprobe. fjggdy~jum ( F i g . 3b) l i k e t h e o t h e r r a r e e a r t h elements i s i n s o l u b l e i n t h e UC2 m a t r i x and p r e c i p i t a t e s i n a sepa- r a t e c a r b i d e phase. The c o r r e l a t i o n o f a l o w i n t e n s i t y l e v e l o f N d - L d w i t h t h e u r a - nium d i s t r i b u t i o n i n d i c a t e s t h e f o r m a t i o n o f a s o l i d s o l u t i o n w i t h UC c o n t a i n i n g s m a l l c o n c e n t r a t i o n s of r a r e e a r t h s as d e s c r i b e d i n /1,2/.

I 1 1 - FISSION PRODUCTS I N THE COATINGS

The moveable f i s s i o n p r o d u c t s must be r e t a i n e d w i t h i n t h e f u e l p a r t i c l e s , f o r which

purpose t h e c o a t i n g s s u r r o u n d i n g t h e k e r n e l have been o p t i m i z e d t o a c t as t h e main

d i f f u s i o n b a r r i e r . By q u a n t i t a t i v e measurement o f c o n c e n t r a t i o n p r o f i l e s o f v o l a t i -

l e s o l i d f i s s i o n p r o d u c t s such as cesium w i t h a s t e p s c a n n i n g t e c h n i q u e t h r o u g h t h e

c r o s s s e c t i o n s o f i n t e r e s t i n g c o a t i n g s a d a t a b a s i s can be o b t a i n e d f o r r e t e n t i o n

p r o p e r t i e s o f t h e f u e l k e r n e l and t r a n s p o r t c o e f f i c i e n t s o f c o a t i n g m a t e r i a l .

The method i s demonstrated f o r cesium i n c o a t e d p a r t i c l e s w i t h o x i d e k e r 1s which

have been m o d i f i e d by s i 1 ic a - a 1 umina a d d i t i v e s t o improve t h e r e t e n t i o n F f i s s i o n

p r o d u c t s /3/. The amount r e l e a s e d f r o m t h e k e r n e l d u r i n g t h e i r r a d i a t i o n can be ob-

t a i n e d by i n t e g r a t i o n o f t h e c o n c e n t r a t i o n p r o f i l e s o f cesium ( F i g . 4 ) p r e f e r e n t i a l -

l y found i n t h e b u f f e r l a y e r n e i g h b o u r i n g t h e k e r n e l . A f t e r some r e c a l c u l a t i o n s bas-

ed on t h e d i f f u s i o n model o f A.H. Booth /4/, an e f f e c t i v e k e r n e l t r a n s p o r t c o e f f i -

c i e n t f o r Cs 137 can be determined ( F i g . 5 ) .

JOURNAL DE PHYSIQUE

1 Buffer layer Pyrocarbon layer

I I

I

Oxide kernels with alumina-silica additives I

I I I I I

.+,I250

I

\*..-* I -0

11 50°C'~\ ^{\ ' e '} \ ^{e ' Detection}

- - - - - - - - - - 9 ,- ^{--- + ---} limit I.

I I 1 I I

0 20 LO 60 80 100 120 1LO

Layer thickness r / ~ m

F i g . 4 - Cesium c o n c e n t r a t i o n p r o f i l e s measured by a stepscanning technique i n t h e coatings o f n u c l e a r f u e l p a r t i c l e s i r r a d i a t e d a t d i f f e r e n t temperatures t o a burnup o f 4-5 % fima.

Forthmom. R.. Nwl. Tech". 56

119821

I I I & I I

5.0 5.5 6.0 6.5 20 a ao

$/Io-'K-~

Temperature T/OC

1600 1100 1200 1000

F i g . 5 - Arrhenius p l o t o f e f f e c t i v e t r a n s p o r t c o e f f i c i e n t s f o r cesium i n oxide ker- n e l s w i t h a l u m i n a - s i l i c a a d d i t i v e s i r r a d i a t e d a t d i f f e r e n t temperatures 151. Data obtained from microprobe measurements are compared w i t h a chemical e t c h i n g technique and show e x c e l l e n t agreement.

10-lo

10-l2

lo-' :

10-l6

REFERENCES

/ I / L o r e n z e l l i , N. , Thesis U n i v e r s i t k de P a r i s , Report CEA-R-4465 (1973) /2/ Smailos, R., D i s s e r t a t i o n U n i v e r s i t a t Karlsruhe, Report KFK-1953 (1974) /3/ FBrthmann, R., Report 581-1620 (1979)

/4/ Booth, A.H., Report AECL-496 (1958)

/5/ Ftrthmann, R., Nucl. Technology 56 (1982) 81-92

\.

Cs 137

O\

microprobe

chemical

=\