R A D I O A C T I V I T Y OF T H E B O R - 6 0 R E A C T O R C O O L A N T

I. G. K o b z a r ' , V. V. K o n a s h o v , E. S. L i s i t s y n , G. I . P o z n y a k , V. I. P o l y a k o v , a n d Yu. V. C h e c h e t k i n

UDC 621.039.534.63:539.16

The widespread introduction of atomic power plants with sodium-cooled fast r eac to r s necessi ta tes a careful study of their safety and maintenance; in part icular , the sources of radioactivity and the t ranspor t and accumulat ion of radioact ive isotopes in deposits along the coolant c i rcui t and in the reac to r equipment must be examined. In order to per form such studies on the BOR-60 reac to r an experimental complex was constructed including sodium and gas spec t romete r loops fitted out with special equipment. By using interchangeable col l imators and sciniil lation and semiconductor Ge(Li) detectors the radioactivity of va r i - ous isotopes in the coolant and on the wails of the piping and equipment could be determined without taking samples [1]. In addition the radioactivi ty of the coolant was measured by radiochemical and T - s p e c t r o - met r ic analyses of samples using Ge(Li) detectors and NaI(T1) crysta ls .

During reac to r operat ion most of the ionizing radiation f rom the equipment and pipes comes f rom Na 24. The ~/-dose rate reaches 7 R / s e c in a box of technological equipment at a r eac to r power of 40 MW. The Na 24 activity measured at var ious reac to r powers and normalized to a maximum power of 60 MW r e - mains pract ical ly constant. The specific act ivi ty of the other short- l ived isotopes Ne 23 and F 2~ is approxi- mately inverse ly proportional to the coolant flow rate. Table 1 lists the values of the activit ies of the main shor t - l ived isotopes in the sodium coolant at the r eac to r outlet.

After a r eac to r shutdown of 8-12 days the activity of Na 24 becomes less than that of Na 22 (T1/2 = 2.6 yr) and Ag ll~ (T1/2 = 253 days) which subsequently determine the radiat ion environment in boxes of techno- logical equipment of the p r imary circuit. The average activit ies of the long-lived isotopes in the sodium coolant and their changes with the t ime of reac tor operation a re listed in Table 2. It is charac te r i s t i c that af ter 216 effective days of r eac to r operation at 40 MW the activity of the principal cor ros ion elements (Cr 51, Mn 54, Fe 59, Co 58, Co 6~ in the sodium coolant does not exceed 0.5-3" 10 -? C i /kg of sodium. In spite of the presence in the core of a bundle of fuel elements with assembl ies which were not gas- t ight (2.7% burnup), the activity of the f ission products in the sodium coolant while a burnup of 5.9% was being achieved in the core was lower than the sensit ivity of the method used in the analysis (_< 5" 10 "r C i /kg of sodium).

Extrapolat ion of the measured values of the Na 22 activi ty to saturat ion for a reac to r power of 60 MW gives 2 �9 10 -3 C i / k g of sodium. The re fe rence value assumed in the reac tor design was 4 - 10 -3 C i /kg of sodium [2].

Radioactive isotopes appearing in the coolant as a r e s u l t of the activation of impurit ies in the sodium are shown in Table 2. An additional source of s i lver , zinc, and antimony probably a r i ses f rom select ive co r ros ion of steel. This is confirmed by determining these elements in sodium and steel by chemical anal - ys is and by spectroscopy.

Analysis of the data in Table 2 shows that the Ag ll~ activity changes more rapidly with time, and that of Zn G5 less rapidly than follows f rom the laws of activation and decay. Calculations predict an average increase in s i lver in the c i rcui t of 4 m g / d a y and a decrease in zinc of 3 mg /day . The calculated resul ts a re in good agreement with the spect ra l analysis of the s i lver in sodium, confirming the increase of s i lver f rom 7 �9 10 -7 to 5 �9 10 -5 wt. %. The removal of zinc f rom the coolant was shown also by T - s p e c t r o m e t r i c measurements of the sodium oxides cold t rap showing a concentrat ion of Zn s5 15-20 t imes la rger than in the p r imary circui t piping.

Translated f rom Atomnaya ]~nergiya, Vol. 33, No. 6, pp. 991-992, December, 1972. Original ar t ic le submitted January 31, 1972.

C 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17thStreet, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher [or $15.00.

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T A B L E 1. R a d i o a c t i v i t y of Sodium Coolan t D e t e r m i n - ing Mos t of the Ion iz ing R a d i a t i o n f r o m P i p e s and R e - a c t o r E q u i p m e n t fo r a R e a c t o r P o w e r of 60 MW and a Coolan t F l o w R a t e of 800 m 3 / h

Isotopes

N a 24

Ne 23

FZ0

Half-life

14.8 h 38 sec 11.5 sec

Reaction forming isotope from Na z~

(n, y) (n, p) (n, cO

Experimental value of spe- cific activ- ity, Ci/kg of sodium [2]

43:~5 t.8~0.8 1.6~0.3

Calculated value of specific ac- tivity, Ci /kg [2]

57 1.5

T A B L E 2. R a d i o a c t i v i t y of Sod ium Coolant D e t e r m i n - ing M o s t of the Ion iz ing R a d i a t i o n f r o m P i p e s and E q u i p m e n t a f t e r R e a c t o r Shutdown

Time reactor Radioactivity, ci/kg of sodium was shut down Na22 Agll0m I Zn65 ] I4b86 Sb125

August 1970 November 1970 February 1971 August 1971

4.t0-~ 4,9.10-5 %3.t0-5

,9.i0 -a

1,2.10-5 0,9.t0-5 -- i ,6.10-5 0,95.10-~I 3,8.10-5 2,5.t0-~ i,2.i0-" l . i0 -~ 0,9.t0-5 2,0.i0-5 i.10-a

2. i0-7 1,4-I0 -8 i,6.10-6

M e a s u r e m e n t s on p ip ing wi thout t ak ing s a m p l e s showed tha t the Zn G5 a c t i v i t y ~ 2 . 1 0 -4 C i / m 2 and tha t of Ag l l~ -< 10 -s C i / m 2.

The i s o t o p i c c o m p o s i t i o n ob ta ined fo r the s o d i u m coo lan t d i f f e r s f r o m the va lue a s s u m e d in the c a l - c u l a t i o n f o r o p e r a t i n g wi th h e r m e t i c a l l y s e a l e d fue l e l e m e n t s (Na 22, Mn 54, Co 58) [2], and f r o m the va lue found e x p e r i m e n t a l l y f o r o t h e r r e a c t o r s (Na 22, f i s s i o n p roduc t s ) [3].

Thus the i n v e s t i g a t i o n s show tha t a f t e r long r e a c t o r o p e r a t i o n with h e r m e t i c a l l y s e a l e d fue l e l e m e n t s and wi th fuel e l e m e n t bund les hav ing a s s e m b l i e s tha t a r e not g a s - t i g h t t h e r e a r e no r a d i o a c t i v e f i s s i o n p r o d u c t s o r c o r r o s i o n p r o d u c t s f r o m the m a i n c o m p o n e n t s of the s t r u c t u r a l m a t e r i a l s in the s o d i u m coolan t . The r a d i o a c t i v i t y of the coo l an t i s due to the a c t i v a t i o n of s o d i u m and i t s i m p u r i t i e s and to the s e l e c t i v e e s c a p e of s i l v e r , z inc , and a n t i m o n y f r o m s t e e l in to the sod ium.

L I T E R A T U R E C I T E D

I. V.I. Polykov and Yu. V. Chechetkin, Atomnaya Energiya, 31, 139 (1971). 2. A . I . L e i p u n s k i i e t a l . , P r e p r i n t F E I - 1 8 7 (1969). 3. A . I . L e i p u n s k i i e t a l . , A t o m n a y a E n e r g i y a , 23, 503 (1967).

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