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Radiation chemistry related to nuclear power technology
Radiation Physics Chemt3try Vol. 22 No, 1/2, pp. 119-129, 1983
0146-5724/83/07119-11503.00/0 © 1983 Pergamon Press Ltd.
Printed in Great Britain
RADIATION
CHEMISTRY
RELATED
Kenkichi
TO
NUCLEAR
POWER
TECHNOLOGY
Ishigure
Department of N u c l e a r E n g i n e e r i n g , F a c u l t y of E n g i n e e r i n g , University of T o k y o , Tokyo, J a p a n
ABSTRACT A b r i e f r e v i e w is g i v e n to the r a d i a t i o n c h e m i c a l p r o b l e m s , e s p e c i a l l y w i t h the e m p h a s i s on w a t e r r a d i o l y s i s , in the n u c l e a r p o w e r t e c h n o l o g y . Radiation chemi stry in a q u e o u s s y s t e m is p o i n t e d out to be c l o s e l y r e l a t e d to the p r o b l e m s s u c h as c o r r o s i o n of Z i r c a l o y , the f o r m a t i o n of i n s o l u b l e c o r r o s i o n p r o d u c t s or crud, s t r e s s c o r r o s i o n c r a c k i n g of s t a i n l e s s s t e e l in B W R and the r a d i o a c t i v e waste managements. The r e s u l t s of the c o n s t a n t e x t e n t i o n r a t e t e s t s on s e n s i t i z e d 304 s t a i n l e s s s t e e l u n d e r i r r a d i a t i o n are s h o w n , and the c o m p u t e r c a l c u l a tions w e r e c a r r i e d out to s i m u l a t e the m o d e l e x p e r i m e n t s on the r e l e a s e of crud f r o m the c o r r o d i n g s u r f a c e s u n d e r i r r a d i a t i o n and a l s o the w a t e r r a d i o l y sis in c o r e of BWR. KEYWORDS Radiolysis of w a t e r , w a t e r stress corrosion cracking,
chemistry, crud, decontamination,
hydrogen peroxide, dissolved oxygen, radioactive waste management.
Currently, r a d i a t i o n c h e m i s t r y s e e m s to i n c r e a s e its i m p o r t a n c e in m a n y a s p e s t s of n u c l e a r p o w e r t e c h n o l o g y . T h e r e are m a n y r a d i a t i o n r e l a t e d p r o b l e m s , for w h i c h the r a d i a t i o n c h e m i c a l a p p r o a c h e s are i n d i s p e n s a b l e , in the f i e l d s s u c h as the r e a c t o r w a t e r c o n t r o l or the r a d i o a c t i v e w a s t e m a n a g e m e n t . In this p a p e r , a b r i e f r e v i e w is g i v e n in the n e x t s e c t i o n to the r a d i a t i o n c h e m i c a l p r o b l e m s in r e l a t i o n to w a t e r r a d i o l y s i s in the n u c l e a r p o w e r t e c h n o l o g y , and then the r e c e n t w o r k f r o m the a u t h o r ' s l a b o r a t o r y is p r e s e n t e d in m o r e d e t a i l .
Radiation
Chemistry
Related
Problems
It is w e l l k n o w n that a s i g n i f i c a n t p r o g r e s s has h e e n m a d e in the r a d i a t i o n c h e m i s t r y of the a q u e o u s s y s t e m s s i n c e the a p p e a r a n c e of the n u c l e a r r e a c t o r s w h e r e w a t e r is u s e d as c o o l a n t . Water radiolysis is s t i l l n o w a c o n t r o v e r s i a l p r o b l e m in r e l a t i o n to w a t e r c h e m i s t r y of the p r i m a r y c o o l a n t s y s t e m s in LWR, especially in BWR. W a t e r c h e m i s t r y is d e e p l y c o n c e r n e d w i t h the c o r r o s i o n of materials in the p r i m a r y s y s t e m s . It is k n o w n that the c o r r o s i o n r a t e of Z i r c a l o y is c o n s i d e r a b l y e n h a n c e d by the i n - r e a c t o r irradiation and C o x I) p o i n t e d out that b o t h of the a n o d e and the c a t h o d e r e a c t i o n s h a v e to be p r o m o t e d by the i r r a d i a t i o n . Christensen a l r e a d y r e v i e w e d 2) this p r o b l e m s and s u g g e s t e d that the r a d i a t i o n c h e m i c a l p r o d u c t s of w a t e r s u c h as 02 - m a y be involved in the c a t h o d e r e a c t i o n s of the Z i r c a l o y c o r r o s i o n . H o w e v e r , this Z i r c a l o y c o r r o s i o n p r o b l e m h a s b e e n r e c e n t l y o n c e a g a i n c o n c e r n e d in r e l a t i o n w i t h the load f o l l o w i n g o p e r a t i o n s on the n u c l e a r r e a c t o r s and the h i g h b u r n - u p u t i l i z a t i o n of the f u e l s in the r e a c t o r s 3) 4).
;20
K. ISHIGURE
Another a s p e c t of the w a t e r c h e m i s t r y is t h e s t r e s s c o r r o s i o n cracking of the stainless steel pipings in BWR. Intergranular stress corrosion cracking is affected to l a r g e e x t e n t by the o x y g e n c o n t e n t in the e n v i r o n m e n t a l water. In t h e u s u a l o p e r a t i o n of B W R the r e a c t o r w a t e r c o n t a i n s 200 to 3 0 0 ppb o x y g e n arising f r o m the r a d i o l y s i s of the r e a c t o r of w a t e r in c o r e . The corrosion potential is d e p e n d e n t on the o x y g e n c o n t e n t as s h o w n in Fig. i, a n d it w a s concluded by I n d i g 5) et al t h a t the c o r r o s i o n potential s h o u l d be l e s s t h a n - 4 0 0 m V to s u p p r e s s the i n t e r g r a n u l a r stress corrosion potential. This means t h a t the o x y g e n c o n t e n t s h o u l d be l o w e r t h a n 50 ppb as s e e n in F i g . i. To meet this requirement the r a d i o l y s i s of w a t e r in c o r e h a s to be c o n t r o l l e d to k e e p the o x y g e n l e v e l l o w e n o u g h . The successful example of the h y d r o g e n injection t e s t in the O s k a r s h a m n reactor (BWR) in 1 9 7 9 w a s a l r e a d y s h o w n by C h r i s t e n s e n 2 ) . In t h i s t e s t , h y d r o g e n was injected o n l y for v e r y s h o r t t i m e , s e v e r a l h o u r s . In 1 9 8 1 the s e c o n d h y d r o g e n injection test was performed for s e v e r a l d a y s at O s k a r s h a m n r e a c t o r w i t h the successful r e s u l t . 6) This success attracted a wide attention, a n d the h y d r o gen injection m a y be t a k e n as o n e of t h e p o s s i b l e countermeasures for the I G S C C in BWR. In c o n n e c t i o n with the hydrogen injection in BWR, it is v e r y i m p o r t a n t to k n o w the d i s t r i b u t i o n of the o x y g e n a n d the o t h e r o x d i s i n g species in each portion of the p r i m a r y s y s t e m of B W R i n c l u d i n g the r e c i r c u l a t i o n lines, the r e a c t o r core and the the sampling lines. Our k n o w l e d g e on the r e a c t o r w a t e r is a l w a y s c o l l e c t e d through the l o n g s a m p l i n g l i n e s of B W R a n d t h i s m a y not reflect the a c t u a l s i t u a t i o n s in the p r i m a r y l i n e s o w i n g to the c h e m i c a l reactions w h i c h m a y o c c u r in the l o n g s a m p l i n g lines. Thus, a computer simulation was carried in the a u t h o r ' s laboratory to k n o w t h e d i s t r i b u t i o n of o x y gen, h y d r o g e n peroxide and other chemical species involved at e a c h p o r t i o n of the primary l i n e s of BWR. The preliminary r e s u l t s h o w s t h a t the t h e r m a l decomposition reaction of h y d r o g e n peroxide is v e r y i m p o r t a n t since this react i o n y i e l d s OH r a d i c a l s even without radiation. kI H202
~
20H
(i)
However, the reliable d a t a on t h e r a t e c o n s t a n t of r e a c t i o n (I) a r e n o t a v a i l able from literature. So, a m e a s u r e m e n t w a s m a d e at h i g h t e m p e r a t u r e to determine the decomposition r a t e of h y d r o g e n peroxide. The results are given in t h e f o l l o w i n g section. If o x y g e n p l a y s an i m p o r t a n t r o l e as an o x i d i s i n g a g e n t in t h e i n t e r g r a n u l a r stress corrosion cracking, then other chemical species produced by r a d i o l y s i s of w a t e r s u c h as H 2 0 2 , H O 2 , 0 2 - a n d OH m a y a l s o a f f e c t t h e I G S C C . Hence, the constant elongation rate test (CERT) experiment was conducted to e x a m i n e the e f f e c t of r a d i a t i o n on the I G S C C of 304 t y p e s t a i n l e s s steel. It w a s f o u n d 7) that radiation accelerates the I G S C C at h i g h e r o x y g e n c o n c e n t r a t i o n but rather r e t a r d t h e I G S C C at l o w e r o x y g e n l e v e l . Water chemistry is c l o s e l y c o n n e c t e d with the behaviors of the c o r r o s i o n prod u c t s or c r u d in the p r i m a r y systems. C r u d is c o n s i d e r e d to p l a y an i m p o r t a n t r o l e in the a c c u m u l a t i o n processes of the r a d i o a c t i v e nuclides, s u c h as 60Co,58Co or 5 4 M n , on t h e s u r f a c e s of the p r i m a r y piping. It w a s f o u n d I0) that radiation has significant e f f e c t on t h e r e l e a s e of c o r r o s i o n products from stainless s t e e l a n d c a r b o n s t e e l in h i g h t e m p e r a t u r e water ; gamma-irradiation enhances t h e r e l e a s e of c r u d or i n s o l u b l e oxide particulates, w h i l e it h a s no appreciable e f f e c t on the r e l e a s e of s o l u b l e ions. This effect was explained by t h e h y p o t h e s i s that ferrous ion r e l e a s e d as a r e s u l t of the c o r r o s i o n is o x d i s e d by w a t e r r a d i o l y s i s products to f e r r i c ion l e a d i n g to h e m a t i t e (~-Fe203) through the hydrolysis reactions. A computer simulation w a s c a r r i e d o u t to confirm that the above processes actually take place under the experimental conditions. The redox reactions of m e t a l i o n s by r a d i a t i o n are excellently utilized in o n e of the c h e m i c a l decontamination methods of t h e p r i m a r y systems in n u c l e a r reactors.8) LOMI (low oxidation s t a t e m e t a l ion) r e a g e n t s a r e n e w t y p e of
Radiation chemistry related to nuclear power technology
]21
chemical solvents recently developed for the c h e m i c a l d e c o n t a m i n a t i o n by C E G B in UK. The s o l v e n t s d i s s o l v e the o x i d e l a y e r s on the s u r f a c e s of the p i p i n g by r e d u c t i o n of f e r r i c to f e r r o u s ions and are c o m p o s e d of a p o w e r f u l r e d u c i n g r e a g e n t s u c h as V ( I I ) or C r ( I I ) , a c o m p l e x i n g r e a g e n t and a b u f f e r . W h e n the decontamination is p e r f o r m e d i m m e d i a t e l y a f t e r the s h u t d o w n of the r e a c t o r , the r a d i a t i o n f i e l d in the s h u t d o w n c o r e d e c o m p o s e s the r e d u c i n g r e a g e n t s . The r a d i o l y s i s p r o d u c t s , H, OH and H202 o x i d i s e V ( I I ) ions to V ( I I I ) , w h i l e some of V ( I I I ) ions are r e d u c e d to V(II) as s h o w n by r e a c t i o n (5). H+ H + V(II)
=
H 2 + V(III)
(2)
OH + V ( I I )
m
OH-
(3)
H202
+ V(II)
m
OH + OH-
e-aq
+ V(III)
~
V(II)
+ V(III) + V(III)
(4) (5)
The a b o v e o x i d a t i o n r e a c t i o n s , (2), (3) and (4) i n d u c e the d e t e r i o r a t i o n s o l v e n t s and r e d u c e its life time. H o w e v e r , if f o r m a t e ions are a d d e d s y s t e m s , t h e y s c a v e n g e H and OH to give the c a r b o x y r a d i c a l s CO2-.
H C O 2- + OH
m
H20
H C O 2- + H
m
H 2 + CO 2-
This r a d i c a l r e d u c e s reaction product.
V(III)
ion
CO 2- + V ( I I I )
=
and
of the in the
+ CO 2-
regenerates
V(II)
(6) (7)
the
reagents,
giving
CO 2 as
+ CO 2
the
(8)
The t h i r d a s p e c t of the r a d i a t i o n c h e m i s t r y of w a t e r is the h y d r o g e n g e n e r a t i o n f o l l o w i n g a loss of c o o l a n t a c c i d e n t . W h e n the e m e r g e n c y c o r e c o o l i n g s y s t e m (ECCS) is o p e r a t e d , the m a i n s o u r c e of h y d r o g e n is the r a d i o l y s i s of w a t e r in c o r e and in the sump. It is v e r y i m p o r t a n t to d e t e r m i n e the c a p a c i t y of the hydrogen recombiner to stop h y d r o g e n r e a c h i n g the e x p l o s i o n l i m i t (4%). Thus, r e l i a b l e r e a c t i o n r a t e s of the r e l e v a n t c h e m i c a l s p e c i e s are e s s e n t i a l to s i m u l a t e the h y d r o g e n e v o l u t i o n a r i s i n g f r o m r a d i o l y s i s of w a t e r . Finally, it s h o u l d be r e f e r r e d that w a t e r r a d i o l y s i s has a m a r k e d e f f e c t on l e a c h i n g p r o c e s s of n u c l e a r w a s t e f o r m s in t h e i r land d i s p o s a l . At p r e s e n t , borosilicate g l a s s e s m a y be one of the m o s t p r o b a b l e w a s t e f o r m in the m a n a g e m e n t of h i g h l e v e l r a d i o a c t i v e w a s t e s g e n e r a t e d f r o m s p e n t f u e l s in n u c l e a r p o w e r plants. R a d i a t i o n d a m a g e has b e e n a g r e a t c o n c e r n for the g l a s s w a s t e form, and l a r g e e f f o r t has b e e n d e v o t e d to the s t u d i e s in this a s p e c t . Many works s e e m to s h o w that the e f f e c t of the r a d i a t i o n d a m a g e on the p h y s i c a l p r o p e r t i e s of the g l a s s w a s t e s m a y not be so s e r i o u s t h r o u g h a w h o l e l i f e time of the w a s t e form. H o w e v e r , r e c e n t l y M c V a y et al 9) f o u n d that g a m m a r a d i a t i o n enh a n c e s the l e a c h i n g of zinc and l a n t h a n i d e s from simulated waste glasses. The m e c h a n i s m for the e n h a n c e m e n t is not w e l l u n d e r s t o o d at the m o m e n t , b u t it w a s c o n f i r m e d that the r a d i o l y s i s of the l e a c h a t e , w a t e r i n c l u d i n g s o m e i m p u r i ties, is i n v o l v e d in the p r o c e s s . It is i n f e r r e d that some r a d i o l y s i s products, for i n s t a n c e n i t r i c acid, a c c e l e r a t e the d i s s o l u t i o n of some e l e m e n t s d e p o s i t e d on the g l a s s s u r f a c e s .
Formation It w a s
of
Crud
already
under
reported
Gamma-Irradiation by
the
present
a u t h o r l O ) -12)
that
gamma
radiation
122
K. I SHIGURE
enhances the c r u d
the r e l e a s e of i r o n (~-Fe203) formation
Fe
+
1/2
2 F e 2+ + Fe 2 +
OH
3Fe 3+
+
02
+
2H +
crud from mechanism
stainless steel and carbon was considered as f o l l o w s .
2H +
1/2
Fe 2+
02
--
= 3H20
+ H20
2Fe 3+
"
Fe203
(9) (i0)
OH-
(ii)
3Fe(OH) 3 +
2Fe(OH)3
and
+ H20
Fe 3+ + ~
steel,
+
3H+
(12)
3H20
(13)
The oxidation reaction of f e r r o u s i o n ( F e 2+) w a s a s s u m e d to be r a t e - d e t e r m i n i n g at the l o w d i s s o l v e d o x y g e n l e v e l in o r d e r to be e x p l a i n the e x p e r m e n t a l results. In the a b o v e r e a c t i o n mechanism, only a typical oxidation reaction is s h o w n , though the actual oxidation reactions are more complicated, involving other chemical species. In Fig. 2 is s h o w n the d e p e n d e n c e of the i r o n r e l e a s e levels on the r e s i d e n c e t i m e or f l o w r a t e in the l o o p e x p e r i m e n t under gamma-irradiation. As s e e n in Fig. 2, the c o n c e n t r a t i o n of the f e r r o u s ion r e l e a s e d from the c o r r o d i n g surfaces is r a t h e r low, a n d it m i g h t be d o u b t f u l t h a t the o x i d a tion reactions a r e so r a p i d to o c c u r u n d e r the e x p e r i m e n t a l conditions. Thus, a computer simulation was carried out to e x a m i n e the r e a c t i o n s of f e r r o u s ion at h i g h t e m p e r a t u r e under gamma-irradiation. 13) T h e r e s u l t is s h o w n in Fig. 3. In t h i s c a l c u l a t i o n 40 p p b of f e r r o u s ion is g i v e n as i n i t i a l condition. It is s e e n t h a t a p p r o x i m a t e l y i00 s e c o n d s a f t e r t h e e n t r a n c e of f e r r o u s i o n s to the r e a c t o r , the s y s t e m r e a c h e s the s t e a d y s t a t e a n d a l m o s t all of f e r r o u s ion is o x i d i s e d to f e r r i c ion. This redox reaction is l a r g e l y d e p e n d e n t on the i n i t i a l c o n d i t i o n , and w h e n I00 ppb of h y d r o g e n is a d d e d in the r e a c t o r , the o x i d a t i o n of f e r r o u s ion is m a r k e d l y retarded a n d a b o u t a h a l f of f e r r o u s ion is o x i d i s e d in the s t e a d y s t a t e . T h e r e w e r e two i m p o r t a n t parameters in these calculations which included some ambiguity at p r e s e n t . O n e w a s the rate constant of t h e r m a l d e c o m p o s i t i o n reaction (i) of h y d r o g e n peroxide as already mentioned earier. So t h i s r e a c t i o n rate was measured in t h e e x p e r i m e n t at 1 5 0 ~ 2 5 0 ° C by the p r e s e n t a u t h o r s l 3 ) a n d the r e s u l t is as f o l l o w s .
k I = 6.4
x
105exp
(-
17000RT )
(14)
T h i s l e a d s to the r e a c t i o n r a t e of 5.0 x 1 0 - 2 (s-l) at 2 5 0 ° C , w h i c h is u s e d in the above simulation. The second important parameters are initial G values of w a t e r r a d i o l y s i s . The reliable data are available on t h e s e G v a l u e s at ambient temperature from literatures. However, B u r n s et a l l 4 ) 15) r e p o r t e d recently t h a t the i n i t i a l G v a l u e s of w a t e r r a d i o l y s i s at h i g h t e m p e r a t u r e are somewhat different f r o m the o n e s at a m b i e n t temperature. T h e s e two k i n d s of G values are compared in T a b l e i. The calculation u s i n g the G v a l u e s at h i g h temperature is s h o w n in Fig. 3. It is n o t e d t h a t L E T e f f e c t d i s a p p e a r s at high temperature. The simulations were also conducted u s i n g t h e G v a l u e s at low temperature, and compared w i t h t h e r e s u l t s h o w n in F i g . 3. The ferrousferric oxidation reaction w a s f o u n d to be a f f e c t e d little by this variation in t h e G v a l u e u n d e r the p r e s e n t experimental condition.
Stress
Corrosion
Cracking
of
304
Stainless
Steel
under
Gamma-Irradiation
Constant extention rate testing was carried out on s e n s i t i z e d t y p e 304 s t a i n l e s s s t e e l in a h i g h t e m p e r a t u r e w a t e r l o o p s h o w n in F i g . 4. 6 ) In the t e s t loop five test specimens were mounted at o n e t i m e on the t e n s i l e m a c h i n e s , two of w h i c h w e r e i r r a d i a t e d w i t h the 6 0 C o s o u r c e at the d o s e r a t e of 4.5 x 104 R / h r , while the others were not irradiated. The tests were carried out with varying the o x y g e n c o n c e n t r a t i o n and temperature. In F i g . 5 a r e s h o w n t h e f r a c t u r e
Radiation chemistry related to nuclear power technology
123
strains with and without gamma-radiation at v a r i o u s oxygen concentrations. In T a b l e 2 a r e i n d i c a t e d the f r a c t u r e m o d e s in the f r a c t u r e faces observed by SEM. T h e e f f e c t of the g a m m a i r r a d i a t i o n on S C C s e e m s to be r a t h e r c o m p l i c a t e d It a p p a r e n t l y accelerates t h e I G S C C at 2 5 0 ° C in the p r e s e n c e of 8 p p m o x y g e n , whereas it g i v e s the s u p p r e s s i n g or p r o t e c t i v e environment to the I G S C C u n d e r the o t h e r c o n d i t i o n s . S i n c e t h e a m o u n t of o x y g e n f o r m e d by the r a d i o l y s i s of w a t e r is r a t h e r s m a l l as s h o w n in F i g . 3, the e f f e c t is n o t a t t r i b u t e d to the increase in the o x y g e n c o n c e n t r a t i o n . T h e t h r e e e f f e c t s m a y be c o n s i d e r e d : (i) the p a r t i c i p a t i o n of o x i d i s i n g species in the c a t h o d e reactions (2) the oxidation of f e r r o u s i o n s by o x i d i s i n g species leading to the f o r m a t i o n of oxide film (3) t h e c h a n g e of the m o r p h o l o g y of the o x i d e s . However, it s e e m s rather difficult at t h e m o m e n t to e x p l a i n the e x p e r i m e n t a l results reasonably.
Computer
Simulation
of
Water
Radiolysis
in
Core
of
BWR
As a l r e a d y m e n t i o n e d , a simulation w a s c a r r i e d o u t 16) to k n o w w h e t h e r the a c t u a l water chemistry in e a c h p o r t i o n of p r i m a r y l i n e s of B W R is or n o t d e v i a t e d from the o n e s we o b s e r v e at the s a m p l i n g points. T h e r a t e s of t h e r e a c t i o n s involved in the r a d i o l y s i s at 2 8 0 ° C a r e e s t i m a t e d by B u r n s et a 1 1 3 ) as s h o w n in T a b l e 3. A m o n g t h e s e the r a t e s of t h e two k e y r e a c t i o n s are somewhat modified to h a v e a g o o d a g r e e m e n t between the calculated results and observed values,as stated below. The calculations were carried out u s i n g b o t h t h e i n i t i a l G v a l u e s at l o w a n d h i g h t e m p e r a t u r e s a n d the r e s u l t s s h o w t h a t t h e G v a l u e s at high temperature g i v e the b e t t e r f i t t i n g . T h u s , the r e s u l t s obtained using the h i g h t e m p e r a t u r e G v a l u e s a r e s h o w n in t h i s p a p e r . The stripping rates of 02 a n d h y d r o g e n in c o r e a r e n o t k n o w n w e l l a n d w e r e d e t e r m i n e d so as to m a k e a good agreement between the c a l c u l a t e d and measured v a l u e s of the o f f - g a s levels. In F i g . 6 is s h o w n a t y p i c a l r e s u l t of the c a l c u l a t e d l e v e l s of 02, H 2 a n d H 2 0 2 at e a c h p o r t i o n of a i i 0 0 M W e BWR. It is s e e n in t h i s f i g u r e t h a t h y d r o g e n peroxide is n o t o b s e r v e d at t h e s a m p l i n g point because of t h e d e c o n t a m i n a t i o n in the l o n g s a m p l i n g l i n e , b u t it is p r e s e n t at a p p r e c i a b l e l e v e l in c o r e a n d the r e c i r c u l a t i o n lines. T h e o x y g e n l e v e l in t h e r e c i r c u l a t i o n l i n e is a little higher t h a n t h e v a l u e at the s a m p l i n g p o i n t a n d l o w e r t h a n t h a t in c o r e . T h e e f f e c t of the h y d r o g e n injection was also calculated and compared with the results obtained at t h e O s k a r s h a m n reactor in S w e d e n . A farely good agreement was obtained as s h o w n in Fig. 7, a l t h o u g h the r e c i r c u l a t i o n s y s t e m s a n d the power outputs are somewhat different between t h e m o d e l a n d the O s k a r s h a m n reactors. W h e n 2 5 0 p p b of h y d r o g e n is i n j e c t e d to the f e e d w a t e r , the o x y g e n a n d hydrogen peroxide l e v e l s at t h e s a m p l i n g line were calculated to r e d u c e to z e r o , w h i l e t h o s e a r e 40 a n d 73 p p b , r e s p e c t i v e l y in t h e r e c i r c u l a t i o n lines and are a l m o s t t h e s a m e as w i t h o u t hydrogen in t h e c o r e . It w a s c a l c u l a t e d that the injection of 5 p p m h y d r o g e n is n e c e s s a r y to r e d u c e the o x y g e n l e v e l in c o r e to z e r o , t h o u g h 70 p p b of h y d r o g e n peroxide is s t i l l p r e s e n t in c o r e u n d e r t h i s condition. In the a b o v e c a l c u l a t i o n it w a s f o u n d t h a t t h e r e s u l t s are rather dependent on the r a t e s of r e a c t i o n s (114) a n d (120) in T a b l e 3. A little more modification gives the better results a n d the f o l l o w i n g v a l u e s w e r e u s e d in t h e p r e s e n t calculations.
kl14
= k120
= 4.82
x 108
~ m o l -I
s e c -I
REFERENCES i)
Cox,B. Plenum
(1976). Advances Press, N.Y. p.173
in
Corrosion
Science
and
Technology.
Vol.5.
124
2 3
5 6) 7) 8) 9) i0 Ii 12 13 14 15 16
K. ISHIGURE
Christensen,H. (1981). Radiat. Phys. Chem., 18, 147. G a r z a r o l l i , F . , D. Jorde, R. Manzel, G.W. Parry and P.G. Smerd. (1980). EPRI report NP-1472. IAEA S p e c i a l i s t M e e t i n g on Influence of Water C h e m i s t r y on C l a d d i n g Reliablility. (1980). San Miniato. Indig. M.E., R.L. Cowan. (1980). Water C h e m i s t r y of Nuclear R e a c t o r Systems 2. p.105. Fejes.P. Private Communication. Muroi.M., N. Fujita, K. Ishigure, M. A k i y a m a and T. Tamura. (1982). C o r r o s i o n / 8 2 . Paper 223. Houston. Wood,C.J. (1982). Ann. Nucl. Energy., 9. 195. M c V a y , G . L . , W.J. Weber and L.P. Penderson. (1981). Nuel. Chem. Waste M a n a g e m e n t . , 2, 103. Ishigure,K., N. Fujita, T. Tamura and K. Oshima. (1981). Nucl. Tech., 50, 169. K a w a g u c h i , M . , K. Ishigure, N. Fujita and K. Oshima. (1981). Radiat. Phys Chem., 18, 733. Ishlgure,K., N. Fujita, s. Ono, H. Ikuse and K. Oshima. Radiat. Phys. Chem., in press. Takagi,J., K. Ishigure., to be published. Burns,W.G., W.R. Marsh. (1981) J. Chem. Soc., F a r a d . T r a n s . , i, 77, 197. Burns,W.G. (1981) Water C h e m i s t r y of Nuclear Reactor Systems 2. BNES, 373. Ishigure,K., J. Takagi, T . S h i g e t o and N. Fujita. (1982). Paper p r e s e n t e d at IAEA I n t e r n a t i o n a l S y m p o s i u m at Vienna.
•-OSKARSHAMH-2 O A T A 240O(:
0
- -
SCC 0
2OO
~
SCC 8WR
•
•
ESHE SCC
(mv|
-200
-400
~
NO SCC
-,lO(f -600
HIGH PURDTY
-600 mOO
STRAINING ELECTtqOOE O~T N Na2,~O4 2)40C
-aoe
I
I
J Itllil
l
I
Llilatl
10
! ~QO
I
I Ilt'tl
l,
~ a,,,ll
IOQO
IO.O00
o~J
Fig.
I.
R e l a t i o n s h i p b e t w e e n d i s s o l v e d oxygen a n d ~ p o t e n t i a l IGSCC of w e l d e d Type-304 stainless steel 5)
to
Radiation
chemistry
related to nuclear power technology
40
3O
R
S
,.=
lon
',\ I
5 10 15 Contact Time (min)
0
Fig. 2. Release of Fe ion and crud from stainless steel specimens under gamma irradiation in high temperature water loop 250°C, 4.5 x 10 ~ R/hr, 02 20 ppb
-4
-5 -6
o
o, -7
~r O
-8
=
-9
HzOz
-,o o
N
.,~ L
"~
-:2 <;/-
Fe 4 0 ppb D.O. 2 0 ppb D.H. 0 ppb
o o /
-14
.el_3
-2
HOz
-1
time(sec)
I
1
2
in
log
I
3
4
scale
Fig. 3. Model calculation of oxidation of ferrous ion under irradiation 250°C, 4.5 x 10 T R/hr
RPC 22-I/2-J
;25
126
K. ISHIGURE
,,
®
~
~ - ~ j ®
Fig. 4. Circulation loop of high temperature water for CERT with and without exposure to 60Co ?-rays: l-test channel with ?-rays 2-test channel without ?-rays 3-pressurizer. 4-circulation pump. 5-preheater. 6-cooler. 7-pump. 8-ion exchanger. 9-deaerater. ]O-water reservoir. ]]-plunger pump.
30
.c o
2c o
Lt. I0
o
o Gamma-rays e N o Gamrno-roys Temperature : 2 5 0 ° C Extension Rate : O . O 0 1 m m / m i n S t r a i n R a t e " 8 . 3 x IO-?sec -I
~
~o ~6o 260 5bo,~oo DO Concentr~ion
56~o
(pob)
Fig. 5. Variation of the fracture strain of sensitized type 304 stainless steel with DO concentration at 250°C
Radiation chemistry related to nuclear power technology
.
.
.
.
.
.
]27
ppm
~o 2.5
Feedwo'
Pump
.ine 73 DO DH in ppb H=O=
Fig. 6.
Calculated levels of 02, H 2 and H202 in BWR
~150I ~
DO
,ooI .,\ I00
Fig.
7.
Simulation a ii00 M W e •
the
HeinF.W.(ppb) 200
of 02 l e v e l s m o d e s BWR
results
SO0
400
500
in H 2 i n j e c t i o n
in O s k a r s h a m n
2 reactor.
test
in
128
K. I SHIGURE
TABLE
I. G Values of Initial Products
e aq
y
-rays
H+
in Water Radiolysis
OH
H20
H202
2.7
2.7
0.6
2.9
0.4
0.6
0.93
0.93
0.5
1.09
0.88
0.99
0.4
0.4
0.3
4.7
2.0
y.temp.
neutrons y.temp.
y - r a y s or neutrons 300-400~C
TABLE 2. Fracture Mode Observed
0
in Fracture Faced
by Scanning Electron Micrographs
tem.
pb
20
S0
200
TG
TG
TG
D
D
D
8000
500
°C IG
(88)
IG
(72)
absent TG
TG
250 present
TG
TG
TG
D
D
D
IG
(67)
IG
D
D
D
TG
TG
TG
TG
IG
TG
D
D
TG
D
TG
TG
IG
D
D
D
TG
TG
TG
TG
TG
D
D
D
D
D
TG
TG
TG
TG
TG
D
D
mode.
r l G S C C is
TG TG
absent 19)
IG
(9~
200 TG
present (8)
D
absent 150 present
D denotes
ductile
fracture
given
in
parenthese
Radiation
TABLE
3. Rate
chemistry
Coefficients
related
to n u c l e a r
of R e a c t i o n s
Involved
power
technology
in Water
]29
Radiolysis
at 280°C
I
Rate Constant (£/mol .sed)
Reaction i01
e-aq
ar H 2 0
102
e-aq
+
103
e-aq
104
e-aq
105
H
+
=
H÷
=
+
OH
=
+
H202
H
=
H
+
OH-
H OH=
OH
+
OH-
H2
1.65
x
E2
2.48
x
Ell
3.10
x
Eli
1.34
x
Eli
1.03
x
Eli
106
e-aq
+
H O 2 .= H O 2 -
2.06
x
Eli
107
e-aq
+
02
1.96
x
Ell
i08
2H20
+
2e-aq
3.00
x
E7
109
OH
OH
=
H202
4.65
x
El0
ii0
OH-
+
H
=
e-aq
+
H20
6.63
x
E8
iii
H20
+
e-aq
+
H
=
H2
6.22
x
E9
112
H20
+
e-aq
+
HO 2-
8.70
x
E8
+
+
OH
02=
113
H
114
OH
l14r
H
115
H
+
02
116
H
+
H02
=
117
iH
+
02-
= HO2-
+ +
=
=
=
H
H20
=
H2
=
H20
+
119
H
H202
120
OH
+ +
H202
+
HO 2
OH-
122r
HO 2-
+
123
H+
02-
=
123r
HO 2
=
H+
+
124
HO 2
+
02 -
2H20 H02
127
H+
127r
H20
128
OH
+ +
+
= +
2OH-
2.06
x
Ell
4.82
x
E8
OH
8.14
x
E2
02-
=
+
H20
=
HO 2 + 02
+
=
HO 2-
H20
=
OH-
+
OH-
H20
202 =
=
+
=
H20
H+
+
OH-
02 -
=
02
+
+
OH-
Ell
x
Ell
2.06
x
Ell
1.04
x
EIO
1.13
x
E9
4.82
x
E8
1.24
x
Ell
5.97
x
E9
1.89
x
E7
5.16
x
Ell
HO 2-
H202
H202
x
H20
0202
1.96 2.06
H202
+ +
HO 2
=
HO2
HO2-
H20
H202
OH-
+
H20 +
OH =
122
126
+
+
=
OH
125
OH-
OH
H202
121
+
+
=
HO 2
e-aq
+
20H-
H20
H2
118
H2+
+ 02
02
+
20H-
3.40
x
E7
4.97
x
E8
3.27
x
E5
8.95
x
E7
1.49
x
El2
1.33
x
E -I
1.24
x
Ell
at
280°C
0146-5724/83/07119-11503.00/0 © 1983 Pergamon Press Ltd.
Printed in Great Britain
RADIATION
CHEMISTRY
RELATED
Kenkichi
TO
NUCLEAR
POWER
TECHNOLOGY
Ishigure
Department of N u c l e a r E n g i n e e r i n g , F a c u l t y of E n g i n e e r i n g , University of T o k y o , Tokyo, J a p a n
ABSTRACT A b r i e f r e v i e w is g i v e n to the r a d i a t i o n c h e m i c a l p r o b l e m s , e s p e c i a l l y w i t h the e m p h a s i s on w a t e r r a d i o l y s i s , in the n u c l e a r p o w e r t e c h n o l o g y . Radiation chemi stry in a q u e o u s s y s t e m is p o i n t e d out to be c l o s e l y r e l a t e d to the p r o b l e m s s u c h as c o r r o s i o n of Z i r c a l o y , the f o r m a t i o n of i n s o l u b l e c o r r o s i o n p r o d u c t s or crud, s t r e s s c o r r o s i o n c r a c k i n g of s t a i n l e s s s t e e l in B W R and the r a d i o a c t i v e waste managements. The r e s u l t s of the c o n s t a n t e x t e n t i o n r a t e t e s t s on s e n s i t i z e d 304 s t a i n l e s s s t e e l u n d e r i r r a d i a t i o n are s h o w n , and the c o m p u t e r c a l c u l a tions w e r e c a r r i e d out to s i m u l a t e the m o d e l e x p e r i m e n t s on the r e l e a s e of crud f r o m the c o r r o d i n g s u r f a c e s u n d e r i r r a d i a t i o n and a l s o the w a t e r r a d i o l y sis in c o r e of BWR. KEYWORDS Radiolysis of w a t e r , w a t e r stress corrosion cracking,
chemistry, crud, decontamination,
hydrogen peroxide, dissolved oxygen, radioactive waste management.
Currently, r a d i a t i o n c h e m i s t r y s e e m s to i n c r e a s e its i m p o r t a n c e in m a n y a s p e s t s of n u c l e a r p o w e r t e c h n o l o g y . T h e r e are m a n y r a d i a t i o n r e l a t e d p r o b l e m s , for w h i c h the r a d i a t i o n c h e m i c a l a p p r o a c h e s are i n d i s p e n s a b l e , in the f i e l d s s u c h as the r e a c t o r w a t e r c o n t r o l or the r a d i o a c t i v e w a s t e m a n a g e m e n t . In this p a p e r , a b r i e f r e v i e w is g i v e n in the n e x t s e c t i o n to the r a d i a t i o n c h e m i c a l p r o b l e m s in r e l a t i o n to w a t e r r a d i o l y s i s in the n u c l e a r p o w e r t e c h n o l o g y , and then the r e c e n t w o r k f r o m the a u t h o r ' s l a b o r a t o r y is p r e s e n t e d in m o r e d e t a i l .
Radiation
Chemistry
Related
Problems
It is w e l l k n o w n that a s i g n i f i c a n t p r o g r e s s has h e e n m a d e in the r a d i a t i o n c h e m i s t r y of the a q u e o u s s y s t e m s s i n c e the a p p e a r a n c e of the n u c l e a r r e a c t o r s w h e r e w a t e r is u s e d as c o o l a n t . Water radiolysis is s t i l l n o w a c o n t r o v e r s i a l p r o b l e m in r e l a t i o n to w a t e r c h e m i s t r y of the p r i m a r y c o o l a n t s y s t e m s in LWR, especially in BWR. W a t e r c h e m i s t r y is d e e p l y c o n c e r n e d w i t h the c o r r o s i o n of materials in the p r i m a r y s y s t e m s . It is k n o w n that the c o r r o s i o n r a t e of Z i r c a l o y is c o n s i d e r a b l y e n h a n c e d by the i n - r e a c t o r irradiation and C o x I) p o i n t e d out that b o t h of the a n o d e and the c a t h o d e r e a c t i o n s h a v e to be p r o m o t e d by the i r r a d i a t i o n . Christensen a l r e a d y r e v i e w e d 2) this p r o b l e m s and s u g g e s t e d that the r a d i a t i o n c h e m i c a l p r o d u c t s of w a t e r s u c h as 02 - m a y be involved in the c a t h o d e r e a c t i o n s of the Z i r c a l o y c o r r o s i o n . H o w e v e r , this Z i r c a l o y c o r r o s i o n p r o b l e m h a s b e e n r e c e n t l y o n c e a g a i n c o n c e r n e d in r e l a t i o n w i t h the load f o l l o w i n g o p e r a t i o n s on the n u c l e a r r e a c t o r s and the h i g h b u r n - u p u t i l i z a t i o n of the f u e l s in the r e a c t o r s 3) 4).
;20
K. ISHIGURE
Another a s p e c t of the w a t e r c h e m i s t r y is t h e s t r e s s c o r r o s i o n cracking of the stainless steel pipings in BWR. Intergranular stress corrosion cracking is affected to l a r g e e x t e n t by the o x y g e n c o n t e n t in the e n v i r o n m e n t a l water. In t h e u s u a l o p e r a t i o n of B W R the r e a c t o r w a t e r c o n t a i n s 200 to 3 0 0 ppb o x y g e n arising f r o m the r a d i o l y s i s of the r e a c t o r of w a t e r in c o r e . The corrosion potential is d e p e n d e n t on the o x y g e n c o n t e n t as s h o w n in Fig. i, a n d it w a s concluded by I n d i g 5) et al t h a t the c o r r o s i o n potential s h o u l d be l e s s t h a n - 4 0 0 m V to s u p p r e s s the i n t e r g r a n u l a r stress corrosion potential. This means t h a t the o x y g e n c o n t e n t s h o u l d be l o w e r t h a n 50 ppb as s e e n in F i g . i. To meet this requirement the r a d i o l y s i s of w a t e r in c o r e h a s to be c o n t r o l l e d to k e e p the o x y g e n l e v e l l o w e n o u g h . The successful example of the h y d r o g e n injection t e s t in the O s k a r s h a m n reactor (BWR) in 1 9 7 9 w a s a l r e a d y s h o w n by C h r i s t e n s e n 2 ) . In t h i s t e s t , h y d r o g e n was injected o n l y for v e r y s h o r t t i m e , s e v e r a l h o u r s . In 1 9 8 1 the s e c o n d h y d r o g e n injection test was performed for s e v e r a l d a y s at O s k a r s h a m n r e a c t o r w i t h the successful r e s u l t . 6) This success attracted a wide attention, a n d the h y d r o gen injection m a y be t a k e n as o n e of t h e p o s s i b l e countermeasures for the I G S C C in BWR. In c o n n e c t i o n with the hydrogen injection in BWR, it is v e r y i m p o r t a n t to k n o w the d i s t r i b u t i o n of the o x y g e n a n d the o t h e r o x d i s i n g species in each portion of the p r i m a r y s y s t e m of B W R i n c l u d i n g the r e c i r c u l a t i o n lines, the r e a c t o r core and the the sampling lines. Our k n o w l e d g e on the r e a c t o r w a t e r is a l w a y s c o l l e c t e d through the l o n g s a m p l i n g l i n e s of B W R a n d t h i s m a y not reflect the a c t u a l s i t u a t i o n s in the p r i m a r y l i n e s o w i n g to the c h e m i c a l reactions w h i c h m a y o c c u r in the l o n g s a m p l i n g lines. Thus, a computer simulation was carried in the a u t h o r ' s laboratory to k n o w t h e d i s t r i b u t i o n of o x y gen, h y d r o g e n peroxide and other chemical species involved at e a c h p o r t i o n of the primary l i n e s of BWR. The preliminary r e s u l t s h o w s t h a t the t h e r m a l decomposition reaction of h y d r o g e n peroxide is v e r y i m p o r t a n t since this react i o n y i e l d s OH r a d i c a l s even without radiation. kI H202
~
20H
(i)
However, the reliable d a t a on t h e r a t e c o n s t a n t of r e a c t i o n (I) a r e n o t a v a i l able from literature. So, a m e a s u r e m e n t w a s m a d e at h i g h t e m p e r a t u r e to determine the decomposition r a t e of h y d r o g e n peroxide. The results are given in t h e f o l l o w i n g section. If o x y g e n p l a y s an i m p o r t a n t r o l e as an o x i d i s i n g a g e n t in t h e i n t e r g r a n u l a r stress corrosion cracking, then other chemical species produced by r a d i o l y s i s of w a t e r s u c h as H 2 0 2 , H O 2 , 0 2 - a n d OH m a y a l s o a f f e c t t h e I G S C C . Hence, the constant elongation rate test (CERT) experiment was conducted to e x a m i n e the e f f e c t of r a d i a t i o n on the I G S C C of 304 t y p e s t a i n l e s s steel. It w a s f o u n d 7) that radiation accelerates the I G S C C at h i g h e r o x y g e n c o n c e n t r a t i o n but rather r e t a r d t h e I G S C C at l o w e r o x y g e n l e v e l . Water chemistry is c l o s e l y c o n n e c t e d with the behaviors of the c o r r o s i o n prod u c t s or c r u d in the p r i m a r y systems. C r u d is c o n s i d e r e d to p l a y an i m p o r t a n t r o l e in the a c c u m u l a t i o n processes of the r a d i o a c t i v e nuclides, s u c h as 60Co,58Co or 5 4 M n , on t h e s u r f a c e s of the p r i m a r y piping. It w a s f o u n d I0) that radiation has significant e f f e c t on t h e r e l e a s e of c o r r o s i o n products from stainless s t e e l a n d c a r b o n s t e e l in h i g h t e m p e r a t u r e water ; gamma-irradiation enhances t h e r e l e a s e of c r u d or i n s o l u b l e oxide particulates, w h i l e it h a s no appreciable e f f e c t on the r e l e a s e of s o l u b l e ions. This effect was explained by t h e h y p o t h e s i s that ferrous ion r e l e a s e d as a r e s u l t of the c o r r o s i o n is o x d i s e d by w a t e r r a d i o l y s i s products to f e r r i c ion l e a d i n g to h e m a t i t e (~-Fe203) through the hydrolysis reactions. A computer simulation w a s c a r r i e d o u t to confirm that the above processes actually take place under the experimental conditions. The redox reactions of m e t a l i o n s by r a d i a t i o n are excellently utilized in o n e of the c h e m i c a l decontamination methods of t h e p r i m a r y systems in n u c l e a r reactors.8) LOMI (low oxidation s t a t e m e t a l ion) r e a g e n t s a r e n e w t y p e of
Radiation chemistry related to nuclear power technology
]21
chemical solvents recently developed for the c h e m i c a l d e c o n t a m i n a t i o n by C E G B in UK. The s o l v e n t s d i s s o l v e the o x i d e l a y e r s on the s u r f a c e s of the p i p i n g by r e d u c t i o n of f e r r i c to f e r r o u s ions and are c o m p o s e d of a p o w e r f u l r e d u c i n g r e a g e n t s u c h as V ( I I ) or C r ( I I ) , a c o m p l e x i n g r e a g e n t and a b u f f e r . W h e n the decontamination is p e r f o r m e d i m m e d i a t e l y a f t e r the s h u t d o w n of the r e a c t o r , the r a d i a t i o n f i e l d in the s h u t d o w n c o r e d e c o m p o s e s the r e d u c i n g r e a g e n t s . The r a d i o l y s i s p r o d u c t s , H, OH and H202 o x i d i s e V ( I I ) ions to V ( I I I ) , w h i l e some of V ( I I I ) ions are r e d u c e d to V(II) as s h o w n by r e a c t i o n (5). H+ H + V(II)
=
H 2 + V(III)
(2)
OH + V ( I I )
m
OH-
(3)
H202
+ V(II)
m
OH + OH-
e-aq
+ V(III)
~
V(II)
+ V(III) + V(III)
(4) (5)
The a b o v e o x i d a t i o n r e a c t i o n s , (2), (3) and (4) i n d u c e the d e t e r i o r a t i o n s o l v e n t s and r e d u c e its life time. H o w e v e r , if f o r m a t e ions are a d d e d s y s t e m s , t h e y s c a v e n g e H and OH to give the c a r b o x y r a d i c a l s CO2-.
H C O 2- + OH
m
H20
H C O 2- + H
m
H 2 + CO 2-
This r a d i c a l r e d u c e s reaction product.
V(III)
ion
CO 2- + V ( I I I )
=
and
of the in the
+ CO 2-
regenerates
V(II)
(6) (7)
the
reagents,
giving
CO 2 as
+ CO 2
the
(8)
The t h i r d a s p e c t of the r a d i a t i o n c h e m i s t r y of w a t e r is the h y d r o g e n g e n e r a t i o n f o l l o w i n g a loss of c o o l a n t a c c i d e n t . W h e n the e m e r g e n c y c o r e c o o l i n g s y s t e m (ECCS) is o p e r a t e d , the m a i n s o u r c e of h y d r o g e n is the r a d i o l y s i s of w a t e r in c o r e and in the sump. It is v e r y i m p o r t a n t to d e t e r m i n e the c a p a c i t y of the hydrogen recombiner to stop h y d r o g e n r e a c h i n g the e x p l o s i o n l i m i t (4%). Thus, r e l i a b l e r e a c t i o n r a t e s of the r e l e v a n t c h e m i c a l s p e c i e s are e s s e n t i a l to s i m u l a t e the h y d r o g e n e v o l u t i o n a r i s i n g f r o m r a d i o l y s i s of w a t e r . Finally, it s h o u l d be r e f e r r e d that w a t e r r a d i o l y s i s has a m a r k e d e f f e c t on l e a c h i n g p r o c e s s of n u c l e a r w a s t e f o r m s in t h e i r land d i s p o s a l . At p r e s e n t , borosilicate g l a s s e s m a y be one of the m o s t p r o b a b l e w a s t e f o r m in the m a n a g e m e n t of h i g h l e v e l r a d i o a c t i v e w a s t e s g e n e r a t e d f r o m s p e n t f u e l s in n u c l e a r p o w e r plants. R a d i a t i o n d a m a g e has b e e n a g r e a t c o n c e r n for the g l a s s w a s t e form, and l a r g e e f f o r t has b e e n d e v o t e d to the s t u d i e s in this a s p e c t . Many works s e e m to s h o w that the e f f e c t of the r a d i a t i o n d a m a g e on the p h y s i c a l p r o p e r t i e s of the g l a s s w a s t e s m a y not be so s e r i o u s t h r o u g h a w h o l e l i f e time of the w a s t e form. H o w e v e r , r e c e n t l y M c V a y et al 9) f o u n d that g a m m a r a d i a t i o n enh a n c e s the l e a c h i n g of zinc and l a n t h a n i d e s from simulated waste glasses. The m e c h a n i s m for the e n h a n c e m e n t is not w e l l u n d e r s t o o d at the m o m e n t , b u t it w a s c o n f i r m e d that the r a d i o l y s i s of the l e a c h a t e , w a t e r i n c l u d i n g s o m e i m p u r i ties, is i n v o l v e d in the p r o c e s s . It is i n f e r r e d that some r a d i o l y s i s products, for i n s t a n c e n i t r i c acid, a c c e l e r a t e the d i s s o l u t i o n of some e l e m e n t s d e p o s i t e d on the g l a s s s u r f a c e s .
Formation It w a s
of
Crud
already
under
reported
Gamma-Irradiation by
the
present
a u t h o r l O ) -12)
that
gamma
radiation
122
K. I SHIGURE
enhances the c r u d
the r e l e a s e of i r o n (~-Fe203) formation
Fe
+
1/2
2 F e 2+ + Fe 2 +
OH
3Fe 3+
+
02
+
2H +
crud from mechanism
stainless steel and carbon was considered as f o l l o w s .
2H +
1/2
Fe 2+
02
--
= 3H20
+ H20
2Fe 3+
"
Fe203
(9) (i0)
OH-
(ii)
3Fe(OH) 3 +
2Fe(OH)3
and
+ H20
Fe 3+ + ~
steel,
+
3H+
(12)
3H20
(13)
The oxidation reaction of f e r r o u s i o n ( F e 2+) w a s a s s u m e d to be r a t e - d e t e r m i n i n g at the l o w d i s s o l v e d o x y g e n l e v e l in o r d e r to be e x p l a i n the e x p e r m e n t a l results. In the a b o v e r e a c t i o n mechanism, only a typical oxidation reaction is s h o w n , though the actual oxidation reactions are more complicated, involving other chemical species. In Fig. 2 is s h o w n the d e p e n d e n c e of the i r o n r e l e a s e levels on the r e s i d e n c e t i m e or f l o w r a t e in the l o o p e x p e r i m e n t under gamma-irradiation. As s e e n in Fig. 2, the c o n c e n t r a t i o n of the f e r r o u s ion r e l e a s e d from the c o r r o d i n g surfaces is r a t h e r low, a n d it m i g h t be d o u b t f u l t h a t the o x i d a tion reactions a r e so r a p i d to o c c u r u n d e r the e x p e r i m e n t a l conditions. Thus, a computer simulation was carried out to e x a m i n e the r e a c t i o n s of f e r r o u s ion at h i g h t e m p e r a t u r e under gamma-irradiation. 13) T h e r e s u l t is s h o w n in Fig. 3. In t h i s c a l c u l a t i o n 40 p p b of f e r r o u s ion is g i v e n as i n i t i a l condition. It is s e e n t h a t a p p r o x i m a t e l y i00 s e c o n d s a f t e r t h e e n t r a n c e of f e r r o u s i o n s to the r e a c t o r , the s y s t e m r e a c h e s the s t e a d y s t a t e a n d a l m o s t all of f e r r o u s ion is o x i d i s e d to f e r r i c ion. This redox reaction is l a r g e l y d e p e n d e n t on the i n i t i a l c o n d i t i o n , and w h e n I00 ppb of h y d r o g e n is a d d e d in the r e a c t o r , the o x i d a t i o n of f e r r o u s ion is m a r k e d l y retarded a n d a b o u t a h a l f of f e r r o u s ion is o x i d i s e d in the s t e a d y s t a t e . T h e r e w e r e two i m p o r t a n t parameters in these calculations which included some ambiguity at p r e s e n t . O n e w a s the rate constant of t h e r m a l d e c o m p o s i t i o n reaction (i) of h y d r o g e n peroxide as already mentioned earier. So t h i s r e a c t i o n rate was measured in t h e e x p e r i m e n t at 1 5 0 ~ 2 5 0 ° C by the p r e s e n t a u t h o r s l 3 ) a n d the r e s u l t is as f o l l o w s .
k I = 6.4
x
105exp
(-
17000RT )
(14)
T h i s l e a d s to the r e a c t i o n r a t e of 5.0 x 1 0 - 2 (s-l) at 2 5 0 ° C , w h i c h is u s e d in the above simulation. The second important parameters are initial G values of w a t e r r a d i o l y s i s . The reliable data are available on t h e s e G v a l u e s at ambient temperature from literatures. However, B u r n s et a l l 4 ) 15) r e p o r t e d recently t h a t the i n i t i a l G v a l u e s of w a t e r r a d i o l y s i s at h i g h t e m p e r a t u r e are somewhat different f r o m the o n e s at a m b i e n t temperature. T h e s e two k i n d s of G values are compared in T a b l e i. The calculation u s i n g the G v a l u e s at h i g h temperature is s h o w n in Fig. 3. It is n o t e d t h a t L E T e f f e c t d i s a p p e a r s at high temperature. The simulations were also conducted u s i n g t h e G v a l u e s at low temperature, and compared w i t h t h e r e s u l t s h o w n in F i g . 3. The ferrousferric oxidation reaction w a s f o u n d to be a f f e c t e d little by this variation in t h e G v a l u e u n d e r the p r e s e n t experimental condition.
Stress
Corrosion
Cracking
of
304
Stainless
Steel
under
Gamma-Irradiation
Constant extention rate testing was carried out on s e n s i t i z e d t y p e 304 s t a i n l e s s s t e e l in a h i g h t e m p e r a t u r e w a t e r l o o p s h o w n in F i g . 4. 6 ) In the t e s t loop five test specimens were mounted at o n e t i m e on the t e n s i l e m a c h i n e s , two of w h i c h w e r e i r r a d i a t e d w i t h the 6 0 C o s o u r c e at the d o s e r a t e of 4.5 x 104 R / h r , while the others were not irradiated. The tests were carried out with varying the o x y g e n c o n c e n t r a t i o n and temperature. In F i g . 5 a r e s h o w n t h e f r a c t u r e
Radiation chemistry related to nuclear power technology
123
strains with and without gamma-radiation at v a r i o u s oxygen concentrations. In T a b l e 2 a r e i n d i c a t e d the f r a c t u r e m o d e s in the f r a c t u r e faces observed by SEM. T h e e f f e c t of the g a m m a i r r a d i a t i o n on S C C s e e m s to be r a t h e r c o m p l i c a t e d It a p p a r e n t l y accelerates t h e I G S C C at 2 5 0 ° C in the p r e s e n c e of 8 p p m o x y g e n , whereas it g i v e s the s u p p r e s s i n g or p r o t e c t i v e environment to the I G S C C u n d e r the o t h e r c o n d i t i o n s . S i n c e t h e a m o u n t of o x y g e n f o r m e d by the r a d i o l y s i s of w a t e r is r a t h e r s m a l l as s h o w n in F i g . 3, the e f f e c t is n o t a t t r i b u t e d to the increase in the o x y g e n c o n c e n t r a t i o n . T h e t h r e e e f f e c t s m a y be c o n s i d e r e d : (i) the p a r t i c i p a t i o n of o x i d i s i n g species in the c a t h o d e reactions (2) the oxidation of f e r r o u s i o n s by o x i d i s i n g species leading to the f o r m a t i o n of oxide film (3) t h e c h a n g e of the m o r p h o l o g y of the o x i d e s . However, it s e e m s rather difficult at t h e m o m e n t to e x p l a i n the e x p e r i m e n t a l results reasonably.
Computer
Simulation
of
Water
Radiolysis
in
Core
of
BWR
As a l r e a d y m e n t i o n e d , a simulation w a s c a r r i e d o u t 16) to k n o w w h e t h e r the a c t u a l water chemistry in e a c h p o r t i o n of p r i m a r y l i n e s of B W R is or n o t d e v i a t e d from the o n e s we o b s e r v e at the s a m p l i n g points. T h e r a t e s of t h e r e a c t i o n s involved in the r a d i o l y s i s at 2 8 0 ° C a r e e s t i m a t e d by B u r n s et a 1 1 3 ) as s h o w n in T a b l e 3. A m o n g t h e s e the r a t e s of t h e two k e y r e a c t i o n s are somewhat modified to h a v e a g o o d a g r e e m e n t between the calculated results and observed values,as stated below. The calculations were carried out u s i n g b o t h t h e i n i t i a l G v a l u e s at l o w a n d h i g h t e m p e r a t u r e s a n d the r e s u l t s s h o w t h a t t h e G v a l u e s at high temperature g i v e the b e t t e r f i t t i n g . T h u s , the r e s u l t s obtained using the h i g h t e m p e r a t u r e G v a l u e s a r e s h o w n in t h i s p a p e r . The stripping rates of 02 a n d h y d r o g e n in c o r e a r e n o t k n o w n w e l l a n d w e r e d e t e r m i n e d so as to m a k e a good agreement between the c a l c u l a t e d and measured v a l u e s of the o f f - g a s levels. In F i g . 6 is s h o w n a t y p i c a l r e s u l t of the c a l c u l a t e d l e v e l s of 02, H 2 a n d H 2 0 2 at e a c h p o r t i o n of a i i 0 0 M W e BWR. It is s e e n in t h i s f i g u r e t h a t h y d r o g e n peroxide is n o t o b s e r v e d at t h e s a m p l i n g point because of t h e d e c o n t a m i n a t i o n in the l o n g s a m p l i n g l i n e , b u t it is p r e s e n t at a p p r e c i a b l e l e v e l in c o r e a n d the r e c i r c u l a t i o n lines. T h e o x y g e n l e v e l in t h e r e c i r c u l a t i o n l i n e is a little higher t h a n t h e v a l u e at the s a m p l i n g p o i n t a n d l o w e r t h a n t h a t in c o r e . T h e e f f e c t of the h y d r o g e n injection was also calculated and compared with the results obtained at t h e O s k a r s h a m n reactor in S w e d e n . A farely good agreement was obtained as s h o w n in Fig. 7, a l t h o u g h the r e c i r c u l a t i o n s y s t e m s a n d the power outputs are somewhat different between t h e m o d e l a n d the O s k a r s h a m n reactors. W h e n 2 5 0 p p b of h y d r o g e n is i n j e c t e d to the f e e d w a t e r , the o x y g e n a n d hydrogen peroxide l e v e l s at t h e s a m p l i n g line were calculated to r e d u c e to z e r o , w h i l e t h o s e a r e 40 a n d 73 p p b , r e s p e c t i v e l y in t h e r e c i r c u l a t i o n lines and are a l m o s t t h e s a m e as w i t h o u t hydrogen in t h e c o r e . It w a s c a l c u l a t e d that the injection of 5 p p m h y d r o g e n is n e c e s s a r y to r e d u c e the o x y g e n l e v e l in c o r e to z e r o , t h o u g h 70 p p b of h y d r o g e n peroxide is s t i l l p r e s e n t in c o r e u n d e r t h i s condition. In the a b o v e c a l c u l a t i o n it w a s f o u n d t h a t t h e r e s u l t s are rather dependent on the r a t e s of r e a c t i o n s (114) a n d (120) in T a b l e 3. A little more modification gives the better results a n d the f o l l o w i n g v a l u e s w e r e u s e d in t h e p r e s e n t calculations.
kl14
= k120
= 4.82
x 108
~ m o l -I
s e c -I
REFERENCES i)
Cox,B. Plenum
(1976). Advances Press, N.Y. p.173
in
Corrosion
Science
and
Technology.
Vol.5.
124
2 3
5 6) 7) 8) 9) i0 Ii 12 13 14 15 16
K. ISHIGURE
Christensen,H. (1981). Radiat. Phys. Chem., 18, 147. G a r z a r o l l i , F . , D. Jorde, R. Manzel, G.W. Parry and P.G. Smerd. (1980). EPRI report NP-1472. IAEA S p e c i a l i s t M e e t i n g on Influence of Water C h e m i s t r y on C l a d d i n g Reliablility. (1980). San Miniato. Indig. M.E., R.L. Cowan. (1980). Water C h e m i s t r y of Nuclear R e a c t o r Systems 2. p.105. Fejes.P. Private Communication. Muroi.M., N. Fujita, K. Ishigure, M. A k i y a m a and T. Tamura. (1982). C o r r o s i o n / 8 2 . Paper 223. Houston. Wood,C.J. (1982). Ann. Nucl. Energy., 9. 195. M c V a y , G . L . , W.J. Weber and L.P. Penderson. (1981). Nuel. Chem. Waste M a n a g e m e n t . , 2, 103. Ishigure,K., N. Fujita, T. Tamura and K. Oshima. (1981). Nucl. Tech., 50, 169. K a w a g u c h i , M . , K. Ishigure, N. Fujita and K. Oshima. (1981). Radiat. Phys Chem., 18, 733. Ishlgure,K., N. Fujita, s. Ono, H. Ikuse and K. Oshima. Radiat. Phys. Chem., in press. Takagi,J., K. Ishigure., to be published. Burns,W.G., W.R. Marsh. (1981) J. Chem. Soc., F a r a d . T r a n s . , i, 77, 197. Burns,W.G. (1981) Water C h e m i s t r y of Nuclear Reactor Systems 2. BNES, 373. Ishigure,K., J. Takagi, T . S h i g e t o and N. Fujita. (1982). Paper p r e s e n t e d at IAEA I n t e r n a t i o n a l S y m p o s i u m at Vienna.
•-OSKARSHAMH-2 O A T A 240O(:
0
- -
SCC 0
2OO
~
SCC 8WR
•
•
ESHE SCC
(mv|
-200
-400
~
NO SCC
-,lO(f -600
HIGH PURDTY
-600 mOO
STRAINING ELECTtqOOE O~T N Na2,~O4 2)40C
-aoe
I
I
J Itllil
l
I
Llilatl
10
! ~QO
I
I Ilt'tl
l,
~ a,,,ll
IOQO
IO.O00
o~J
Fig.
I.
R e l a t i o n s h i p b e t w e e n d i s s o l v e d oxygen a n d ~ p o t e n t i a l IGSCC of w e l d e d Type-304 stainless steel 5)
to
Radiation
chemistry
related to nuclear power technology
40
3O
R
S
,.=
lon
',\ I
5 10 15 Contact Time (min)
0
Fig. 2. Release of Fe ion and crud from stainless steel specimens under gamma irradiation in high temperature water loop 250°C, 4.5 x 10 ~ R/hr, 02 20 ppb
-4
-5 -6
o
o, -7
~r O
-8
=
-9
HzOz
-,o o
N
.,~ L
"~
-:2 <;/-
Fe 4 0 ppb D.O. 2 0 ppb D.H. 0 ppb
o o /
-14
.el_3
-2
HOz
-1
time(sec)
I
1
2
in
log
I
3
4
scale
Fig. 3. Model calculation of oxidation of ferrous ion under irradiation 250°C, 4.5 x 10 T R/hr
RPC 22-I/2-J
;25
126
K. ISHIGURE
,,
®
~
~ - ~ j ®
Fig. 4. Circulation loop of high temperature water for CERT with and without exposure to 60Co ?-rays: l-test channel with ?-rays 2-test channel without ?-rays 3-pressurizer. 4-circulation pump. 5-preheater. 6-cooler. 7-pump. 8-ion exchanger. 9-deaerater. ]O-water reservoir. ]]-plunger pump.
30
.c o
2c o
Lt. I0
o
o Gamma-rays e N o Gamrno-roys Temperature : 2 5 0 ° C Extension Rate : O . O 0 1 m m / m i n S t r a i n R a t e " 8 . 3 x IO-?sec -I
~
~o ~6o 260 5bo,~oo DO Concentr~ion
56~o
(pob)
Fig. 5. Variation of the fracture strain of sensitized type 304 stainless steel with DO concentration at 250°C
Radiation chemistry related to nuclear power technology
.
.
.
.
.
.
]27
ppm
~o 2.5
Feedwo'
Pump
.ine 73 DO DH in ppb H=O=
Fig. 6.
Calculated levels of 02, H 2 and H202 in BWR
~150I ~
DO
,ooI .,\ I00
Fig.
7.
Simulation a ii00 M W e •
the
HeinF.W.(ppb) 200
of 02 l e v e l s m o d e s BWR
results
SO0
400
500
in H 2 i n j e c t i o n
in O s k a r s h a m n
2 reactor.
test
in
128
K. I SHIGURE
TABLE
I. G Values of Initial Products
e aq
y
-rays
H+
in Water Radiolysis
OH
H20
H202
2.7
2.7
0.6
2.9
0.4
0.6
0.93
0.93
0.5
1.09
0.88
0.99
0.4
0.4
0.3
4.7
2.0
y.temp.
neutrons y.temp.
y - r a y s or neutrons 300-400~C
TABLE 2. Fracture Mode Observed
0
in Fracture Faced
by Scanning Electron Micrographs
tem.
pb
20
S0
200
TG
TG
TG
D
D
D
8000
500
°C IG
(88)
IG
(72)
absent TG
TG
250 present
TG
TG
TG
D
D
D
IG
(67)
IG
D
D
D
TG
TG
TG
TG
IG
TG
D
D
TG
D
TG
TG
IG
D
D
D
TG
TG
TG
TG
TG
D
D
D
D
D
TG
TG
TG
TG
TG
D
D
mode.
r l G S C C is
TG TG
absent 19)
IG
(9~
200 TG
present (8)
D
absent 150 present
D denotes
ductile
fracture
given
in
parenthese
Radiation
TABLE
3. Rate
chemistry
Coefficients
related
to n u c l e a r
of R e a c t i o n s
Involved
power
technology
in Water
]29
Radiolysis
at 280°C
I
Rate Constant (£/mol .sed)
Reaction i01
e-aq
ar H 2 0
102
e-aq
+
103
e-aq
104
e-aq
105
H
+
=
H÷
=
+
OH
=
+
H202
H
=
H
+
OH-
H OH=
OH
+
OH-
H2
1.65
x
E2
2.48
x
Ell
3.10
x
Eli
1.34
x
Eli
1.03
x
Eli
106
e-aq
+
H O 2 .= H O 2 -
2.06
x
Eli
107
e-aq
+
02
1.96
x
Ell
i08
2H20
+
2e-aq
3.00
x
E7
109
OH
OH
=
H202
4.65
x
El0
ii0
OH-
+
H
=
e-aq
+
H20
6.63
x
E8
iii
H20
+
e-aq
+
H
=
H2
6.22
x
E9
112
H20
+
e-aq
+
HO 2-
8.70
x
E8
+
+
OH
02=
113
H
114
OH
l14r
H
115
H
+
02
116
H
+
H02
=
117
iH
+
02-
= HO2-
+ +
=
=
=
H
H20
=
H2
=
H20
+
119
H
H202
120
OH
+ +
H202
+
HO 2
OH-
122r
HO 2-
+
123
H+
02-
=
123r
HO 2
=
H+
+
124
HO 2
+
02 -
2H20 H02
127
H+
127r
H20
128
OH
+ +
+
= +
2OH-
2.06
x
Ell
4.82
x
E8
OH
8.14
x
E2
02-
=
+
H20
=
HO 2 + 02
+
=
HO 2-
H20
=
OH-
+
OH-
H20
202 =
=
+
=
H20
H+
+
OH-
02 -
=
02
+
+
OH-
Ell
x
Ell
2.06
x
Ell
1.04
x
EIO
1.13
x
E9
4.82
x
E8
1.24
x
Ell
5.97
x
E9
1.89
x
E7
5.16
x
Ell
HO 2-
H202
H202
x
H20
0202
1.96 2.06
H202
+ +
HO 2
=
HO2
HO2-
H20
H202
OH-
+
H20 +
OH =
122
126
+
+
=
OH
125
OH-
OH
H202
121
+
+
=
HO 2
e-aq
+
20H-
H20
H2
118
H2+
+ 02
02
+
20H-
3.40
x
E7
4.97
x
E8
3.27
x
E5
8.95
x
E7
1.49
x
El2
1.33
x
E -I
1.24
x
Ell
at
280°C