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Comparison of neutron embrittlement of steel in different reactor spectra
NUCLEAR STRUCTURAL ENGINEERING 1 (1965) 76-82. NORTH-HOLLAND PUBLISHING COMPANY, AMSTERDAM
COMPARISON OF NEUTRON EMBRITTLEMENT OF STEEL IN DIFFERENT REACTOR S P E C T R A * ** A. D. ROSSIN
Argonne National Laboratory, Argonne, Illinois, USA Received 20 October 1964
A procedure has been developed for measuring and reporting neutron exposure for radiation damage studies. The neutron spectrum shape is calculated and its magnitude determined by the reaction F e ~ (n,p)Mn 54. A unit, called RDU, is used for reporting exposure. To test this procedure, steel specimens were irradiated in three very different reactor spectra. The exposures were expressed in RDU's and the resulting embrittlement compared. Satisfactory agreement was obtained. The procedure proved to be easy to use. A new method for reducing a limited amount of irradiation data was devised to handle irradiations done in a facility where the neutron flux varied appreciably from place to place.
1. INTRODUCTION
2. M E A S U R E M E N T OF EXPOSURE
This study is an a t t e m p t to v e r i f y a m e t h o d f or m e a s u r i n g n eu tr o n e x p o s u r e in r a d i a t i o n d a m a g e e x p e r i m e n t s . E m b r i t t l e m e n t of c a r b o n s t e e l by n e u t r o n i r r a d i a t i o n has been studied e x t e n s i v e l y , but the e x p e r i m e n t a l data g e n e r a l l y do not s a t i s f y the r e a c t o r d e s i g n e r b e c a u s e he cannot apply the e x p o s u r e r e p o r t e d to his own needs. T h e r e is no need to look f o r a s o - c a l l e d " n e u t r o n s p e c t r u m e f f e c t " . A p p a r e n t d i f f e r e n c e s of r e s u l t s in d i f f e r e n t s p e c t r a a r e the r e s u l t of i n a d e q u a c i e s in d o s i m e t r y . T h e r e is a need f o r a dose unit, and a method of m e a s u r i n g and r e p o r t i n g n eu t r o n d o s e , that will w o r k in any n e u t r o n s p e c t r u m . Such a unit is the RDU. Its d e v e l o p m e n t and u s e a r e d e s c r i b e d i n A N L - 6 8 2 6 [1]. This unit t a k e s into account the effect of n e u t r o n s of di f f eren t e n e r g y , and g i v e s an unambiguous va l ue of e x p o s u r e f o r any i r r a d i a t i o n . To t e s t this method the e x p o s u r e r a t e s w e r e m e a s u r e d in t h r e e d i f f e r e n t r e a c t o r s y s t e m s , s a m p l e s w e r e i r r a d i a t e d in each, and the r e s u l t s a r e c o m p a r e d in t h i s p a p e r . Since the s a m e e f f e c t was m e a s u r e d in each set, the s l o p e s of c u r v e s of d a m a g e v e r s u s e x p o s u r e should be the s a m e . If all e x p o s u r e s a r e m e a s u r e d in RDU, the data f r o m al l t h r e e s y s t e m s should coincide.
A c c u r a t e d o s i m e t r y depends on the knowledge of the magnitude and shape of the n e u t r o n s p e c t r u m . S p e c t r a l s h a p e s have been d e t e r m i n e d by m e a n s of d e t a i l e d n eu t r o n t r a n s p o r t t h e o r y c a l c u l a t i o n s . The e n t i r e f a s t p a r t of the s p e c t r u m is divided into 20 n a r r o w e n e r g y i n t e r v a l s to c o v e r the r a n g e f r o m 0.067 to 10 MeV, which is b e l i e v e d to account f o r m o s t of the f a s t n e u t r o n d am ag e. The magnitude of the s p e c t r u m is d e t e r m i n e d by a c t i v a t i o n of f o i l s , and r e s p o n s e s of s e v e r a l d i f f e r e n t f o i l s p r o v i d e a rough ch eck on the c a l c u l a t e d s p e c t r u m shape. The RDU (Radiation D a m a g e Unit) is p r e s e n t ed as a function of e n e r g y in table 1 and it is d e fined such that 1018 n e u t r o n s , f o r e x a m p l e , h a v ing t h e i r e n e r g i e s d i s t r i b u t e d as in the f i s s i o n s p e c t r u m , d e l i v e r 1018 RDU to an i r o n - b a s e t a r get m a t e r i a l . In other w o r d s , in an i r r a d i a t i o n w h e r e the r e p o r t e d e x p o s u r e is 1018 RDU, the r e s u l t i n g d a m a g e would be the s a m e as that i n c u r r e d by an i r r a d i a t i o n of 1 0 1 8 f i s s i o n n e u t r o n s . The a c c u r a c y depends on the a c c u r a c y of the e n e r g y - d e p e n d e n t m o d e l u s e d f o r developing the RDU. This m e t h o d f o r m e a s u r i n g n e u t r o n e x p o s u r e w as d e v e l o p e d [2] to o v e r c o m e the s h o r t c o m i n g s of the f a m i l i a r "> 1 MeV" notation. The f i r s t is that t h e r e is no d i r e c t way to m e a s u r e the n u m b e r of n e u t r o n s above 1 MeV. The m o s t c o n v e nient d et ect i n g f o i l s have si g n i f i can t c r o s s s e c tion v a l u e s only above 3 MeV or so. C o n s e q u e n t -
* Work performed under the auspices of the U.S. Atomic Energy Commission. ** Accepted by T. A. Jaeger.
NEUTRON EMBR1TTLEMENT OF STEEL
ly, a s p e c t r u m shape has to be a s s u m e d . T h e f i s s i o n s p e c t r u m is often chosen, even though a n u n d e g r a d e d f i s s i o n s p e c t r u m n e v e r e x i s t s in a r e a c t o r . Actually only a s m a l l f r a c t i o n of the n e u t r o n s p e c t r u m is above 3 MeV and the shape of the r e s t of the s p e c t r u m is v e r y dependent on the g e o m e t r y and m a t e r i a l s in the r e a c t o r . Thus, the r e p o r t e d n u m b e r of n e u t r o n s above 1 MeV is always an a p p r o x i m a t i o n , the a c c u r a c y being different for different r e a c t o r s , or between a c o r e i r r a d i a t i o n f a c i l i t y and p r e s s u r e v e s s e l wall. Secondly, the choice of 1 MeV as a r e f e r e n c e e n e r g y is p u r e l y a r b i t r a r y . If it takes about 25 eV to knock an atom out of its lattice site, a n e u t r o n with a few h u n d r e d eV is capable of d i s p l a c i n g an i r o n atom. It is not c o r r e c t , t h e r e fore, to neglect the vast n u m b e r s of b o m b a r d i n g n e u t r o n s with 100000 eV and m o r e ' The f a s t e r the b o m b a r d i n g n e u t r o n s , the higher the e n e r g y of the p r i m a r y k n o c k - o n atom is likely to be. So it is not quite p r o p e r to a s s i g n the s a m e effect i v e n e s s to a 100000 eY n e u t r o n as to one with 20 t i m e s as much energy. The RDU method takes these f a c t o r s into account, and a l s o allows for the d i f f e r e n c e s in the e l a s t i c s c a t t e r i n g c r o s s s e c t i o n of i r o n with n e u t r o n energy. The n e u t r o n s p e c t r u m can be tabulated (see s e c t i o n 4) to show the n u m b e r of n e u t r o n s / c m 2sec in each e n e r g y i n t e r v a l at the n o r m a l o p e r ating power level of the r e a c t o r . Multiplying the flux in each i n t e r v a l by the RDU value for that e n e r g y , and adding, gives the RDU e x p o s u r e r a t e . It is u s u a l l y convenient to r e d u c e the expos u r e r a t e to RDU p e r M W - s e c of r e a c t o r o p e r a tion. Then the i n t e g r a t e d e x p o s u r e of a s a m p l e is e a s i l y obtained by m u l t i p l y i n g this e x p o s u r e r a t e by the M W - s e c of r e a c t o r operation while the s a m p l e was in the r e a c t o r .
3. EXPERIMENTAL FACILITIES
3.1. Irradiation The t h r e e r e a c t o r f a c i l i t i e s chosen differ enough to give s p e c t r a with s u b s t a n t i a l l y d i f f e r ent c h a r a c t e r i s t i c s . These d i f f e r e n c e s demand that c a r e be taken in d o s i m e t r y p r o c e d u r e s in o r d e r to obtain an a c c u r a t e value of the m a g n i tude of the fast n e u t r o n flux. The r e a c t o r s used were the C P ° 5 at Argonne, I l l i n o i s , and the E B R - I , at NRTS, Idaho. T h e s e r e a c t o r s a r e a m ply d e s c r i b e d in the l i t e r a t u r e [3,4], and in a p r e v i o u s p a p e r [5] the f e a t u r e s significant to this study a r e d e s c r i b e d in detail. C P - 5 is a heavy water m o d e r a t e d and cooled
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t a n k - t y p e r e s e a r c h r e a c t o r . The fuel e l e m e n t c o n s i s t s of t h r e e c o n c e n t r i c t u b e s of a l u m i n i u m that contain the u r a n i u m fuel, and D 2 0 flows b e tween and a r o u n d the tubes. The s p e c i m e n s r e f e r r e d to as " C P - 5 fuel" w e r e tightly held in a l u m i n i u m f i x t u r e s to a s s u r e heat t r a n s f e r and i r r a d i a t e d in the c e n t r a l t h i m b l e of a fuel e l e ment. The m a j o r p o r t i o n of the s p e c t r u m c o m e s f r o m fast n e u t r o n s f r o m f i s s i o n s in the s u r r o u n d i n g fuel tubes. S p e c i m e n t e m p e r a t u r e was about 65°C. The set called " C P - 5 dummy" was i r r a d i a t e d in identical c a p s u l e s in a s i m i l a r a r r a n g e m e n t of a l u m i n i u m t u b e s , but these tubes contained no u r a n i u m . The d u m m y e l e m e n t is located in one of the outer r i n g of p o s s i b l e fuel e l e m e n t l o c a t i o n s in C P - 5 and f o r m s p a r t of the r e f l e c t o r r e gion for the r e a c t o r . F a s t n e u t r o n s in this l o c a tion a r e due to the leakage f r o m the core. The t h e r m a l flux in this r e f l e c t o r r e g i o n is a bit higher than in the c e n t e r of the core. With the fast flux in the C P - 5 d u m m y only about ~ of that in the C P - 5 fuel location, the high t h e r m a l flux t h e r e c a u s e s m o r e p r o b l e m s in d o s i m e t r y . S p e c i m e n t e m p e r a t u r e was 60oc. E B R - I Mark III was a compact NaK cooled fast r e a c t o r . Its s m a l l core was made of u r a n i u m alloy r o d s clad with Z i r c a l o y - 2 . Steel s p e c i m e n s w e r e placed in hollow tubes and four of t h e s e tubes were s u b s t i t u t e d for four of the fuel rods. Holes in the tubes allowed NaK to cool the s p e c i m e n s directly. S p e c i m e n t e m p e r a t u r e was about 140°C. While this t e m p e r a t u r e was about 80oc above that of the C P - 5 s p e c i m e n s , it is g e n e r a l l y a g r e e d that the r a t e of damage r e c o v e r y in s t e e l does not begin to i n c r e a s e s u b s t a n t i a l l y until the t e m p e r a t u r e r e a c h e s 200oc or higher. The E B R - I s p e c t r u m is fast, but s i g n i f i c a n t l y degraded f r o m a f i s s i o n s p e c t r u m . T h e r e a r e no t h e r m a l n e u t r o n s at all due to the heavy a b s o r p t i o n of the c o r e m a t e r i a l s and the complete a b s e n c e of light moderator elements. 3.2. Testing The m a t e r i a l and the impact s p e c i m e n s used in this study a r e d e s c r i b e d in detail in ref. [5]. The s t e e l w~ts A212B f r o m a documented heat f u r n i s h e d by U.S. Steel C o r p o r a t i o n to ASTM C o m m i t t e e E l 0 . The s u b s i z e Izod impact s p e c i m e n s w e r e notched r o u n d s a p p r o x i m a t e l y 0.5 c m (0.200 in.) d i a m e t e r , 9.5 cm (3~ in.) long and have t h r e e notches m a c h i n e d c i r c u m f e r e n t i a l l y . All s p e c i m e n s w e r e taken f r o m plate such that t h e i r long d i m e n s i o n was p a r a l l e l to the final rolling direction. S p e c i m e n s w e r e b r o k e n with a Baldwin p e n d u -
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A. D. ROSSIN Table 1 Neutron spectra and cross sections. Fast neutron flux (n/cm2-sec) a) Group
EL
(MeV)
~Mn54
(barns)
CP-5 fuel VT 10 4.6 MW
RDU
CP-5 dummy VT 22 4.6 MW 0.147 × 1 0 1 1 0.358 0.806 0.980 1.234
EBR-I C 17 midplane 1.2 MW 0.406 x 1012 1.096 3.106 4.718 7.025
0.650 0.600 0.530 0.375
1.223 1.213 1.121 1.643
2.865
0.132x1012 0.346 0.946 1.417 0 . 1 5 0 1.601 2.074
6 7 8 9 10
2.231 1.738 1.353 1.054 0.821
0.075 0.030 0.005 0 0
1.416 1.164 0.991 0.736 0.609
2.867 3.017 3.238 3.269 3.276
2.126 1.934 2.045 2.040 1.989
8.849 10.264 12.026 12.193 12.821
11 12 13 14 15
0.639 0.498 0.388 0.302 0.235
0 0 0 0 0
0.749 0.475 0.647 0.358 0.291
3.488 3.216 2.448 3.237 2.950
2.841 2.757 1.857 3.334 3.512
15.413 16.254 14.551 13.271 11.990
16 17 18 19 20
0.183 0.143 0.111 0.086 0.067
0 0 0 0 0
0.240 0.234 0.156 0.170 0.120
2.647 2.292 2.547 2.108 2.226
3.544 3.679 3.870 4.039 4.178
1 2 3 4
7.788 6.065 4.724 3.679
5
RDU/sec Allowance for neutrons < 0.067 MeV RDU/sec RDU/MW-sec Mn54 act/sec-1024 atoms
10.252 7.949 6.774 4.531 3.834 32.406x 1012 25.463 x 1011 100.87 x 1012 +3%
+7%
+1%
33.38 x l012 27.245 x 1011~ 101.88
x 1012
7.26 x 1012
5.922 × 1011
84.90 x 1012
4.26 x 1011
3.30 x 1010
5.35 x 1012
a) Calculated numbers l u r e - t y p e i m p a c t m a c h i n e in a h i g h - l e v e l hot c e l l at Argonne. Cooling f o r t e s t s below a m b ie n t w a s obtained by holding the s p e c i m e n in a f i x t u r e and i m m e r s i n g it in liquid nitrogen. T h i s f i x t u r e w a s used to hold the s p e c i m e n s f o r i m p a c t t e s t i n g . It was equipped with a t h e r m o c o u p l e , and s p e c i m e n t e m p e r a t u r e s w e r e e s t a b l i s h e d by c o r r e l a t i o n s with a s p e c i a l i n s t r u m e n t e d s p e c i m e n . T h i s f i x t u r e has r e s i s t a n c e h e a t e r s imbedded in it, and w a s a l s o u s e d to heat s p e c i m e n s f o r h i g h e r t e m p e r a t u r e i m p a c t t e s t s . A f t e r b r e a k i n g at the f i r s t notch the s p e c i m e n was r a i s e d in the f i x t u r e so that the second notch w a s in p o s i t i o n f o r b r e a k ing, etc. In no c a s e w a s the t e m p e r a t u r e f o r a t e s t l e s s than that of the p r e c e d i n g t e s t on a given s p e c i m e n b a r . 4. DOSIMETRY Fast neutron spectra were calculated for each
f a c i l i t y and the a c t i v a t i o n r a t e s of v a r i o u s f o i l s w e r e m e a s u r e d in e a c h location. In a r e a c t o r the m a t e r i a l s p r e s e n t d e t e r m i n e the shape of the f a s t s p e c t r u m at any location, but b e c a u s e the m e a n f r e e path of t h e s e n e u t r o n s is of the o r d e r of c e n t i m e t e r s , the s p e c t r u m shape does not v a r y d r a s t i c a l l y through c a p s u l e s of the s i z e u s e d in t h e s e e x p e r i m e n t s . The d o s i m e t r y p r o c e d u r e s w e r e d e s c r i b e d in a p a p e r g i v en at the IAEA C o n f e r e n c e on N e u t r o n D o s i m e t r y in D e c e m b e r 1962 [6] and p l o t s of the d i f f e r e n t s p e c t r a a r e shown. Sulfur a c t i v a t i o n r a t e s w e r e u s e d in that p a p e r to e s t a b l i s h the magnitude of the f l u x e s , and the d e g r e e of a g r e e ment obtained f r o m i r o n , nickel, and U238 f o i l s g i v e s confidence that the c o m p u t a t i o n s a r e r e a sonably a c c u r a t e . Although the c r o s s s e c t i o n f o r Fe54(n,p)Mn54 as a d e t a i l e d function of n e u t r o n e n e r g y is not well known, its v a l u e s w e r e e s t i m a t e d b a s e d on its r a t i o to n i c k e l [7] and sulfur [6]
NEUTRON EMBRITTLEMENT OF STEEL a c t i v a t i o n in the E B R - I , and its shape was a s s u m e d to be s i m i l a r to that of the Ni58(n,p)Co58 r e a c t i o n . The m u l t i g r o u p a c t i v a t i o n c r o s s s e c tions u s e d in this study give the following v a l u e s when i n t e g r a t e d o v e r the f i s s i o n s p e c t r u m : S32(n,p)P 32 Ni58(n, p)Co58 Fe54(n,p)Mn 54 RDU
0.065 b a r n s 0.116 b a r n s 0.09! b a r n s 1.0
Although the a b s o l u t e c r o s s s e c t i o n v a l u e s m a y s o m e d a y be changed when new c r o s s s e c t i o n s a r e m e a s u r e d , the Mn 54 r e s u l t s w e r e u s e d c o n s i s t e n t l y to c o r r e l a t e all e x p e r i m e n t s d e s c r i b e d in t h i s p a p e r . The s p e c i m e n s i r r a d i a t e d in the C P - 5 d u m m y had a v e r y high r e l a t i v e l e v e l of Fe 59 a c t i v i t y f r o m t h e r m a l n e u t r o n c a p t u r e in Fe58. The' Mn 54 was s e p a r a t e d c h e m i c a l l y f r o m the r e m a i n i n g i r o n to p e r m i t a c c u r a t e counting [7]. The p r o c e d u r e was shown to be q u a n t i t a t i v e by p a s s i n g E B R - I m a t e r i a l containing no F e 59 a c t i v i t y t h r o u g h the ion exchange c o l u m n [8]. S p e c t r a f o r the t h r e e r e a c t o r f a c i l i t i e s at t h e i r n o r m a l o p e r a t i n g p o w e r l e v e l a r e given in t a bl e 1. C a l c u l a t e d s p e c t r a l shapes w e r e n o r m a l i z e d to a g r e e with the m e a s u r e d Mn54 a c t i vation r a t e . The c a l c u l a t e d a c t i v a t i o n r a t e is obtained u s i n g the c r o s s s e c t i o n s l i s t e d in table 1 and the r e s u l t s a r e shown at the bottom. The c o n t r i b u t i o n s to the RDU r a t e f o r a ll e n e r g y g r o u p s a r e s u m m e d to give the e x p o s u r e r a t e . T h e s e v a l u e s a r e given at the bottom of the t a b l e , both f o r the n o m i n a l r e a c t o r power l e v e l and the R D U / M W - s e c . Although the o r i g i n a l c a l c ula ti o n s only went down to 0.067 MeV, l a t e r c a l c u l a t i o n s extended the r a n g e . The contribution due to f a s t n e u t r o n s below 0.067 MeV i s added to e a c h column.
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F o r t y t e s t s w e r e used to e s t a b l i s h the b a s e c u r v e for unirradiated material. CP-5 dummy irradiations covered nominal p e r i o d s of 4, 6, 10, 18, and 30 days. The s a m e p e r i o d s w e r e used f o r C P - 5 fuel i r r a d i a t i o n s in addition to 1 and 2 day e x p o s u r e s . The t r a n s i t i o n t e m p e r a t u r e shift was m e a s u r e d at the a r b i t r a r i l y c h o s e n l e v e l of 2 f t - l b (2.72 n t - m ) . Other c h o i c e s of i m p a c t e n e r g y l e v e l s gave v e r y s i m i l a r r e s u l t s . Fig. 1 shows the r e s u l t s f o r the d u m m y and fuel positions. The a b s c i s s a is in M W - s e c of r e a c t o r o p e r a t i o n ; no dose m e a s u r e m e n t s a r e involved. Note that the flux l e v e l f o r the fuel position is at l e a s t an o r d e r of m a g n i tude higher than that of the dummy. The R D U / M W - s e c r a t e s at the bottom of table 1 can be ap p l i ed d i r e c t l y to the c u r v e s to c o n v e r t the i n t e g r a t e d e x p o s u r e to RDU f o r c o m p a r i s o n . I o
'
' IF"~
I
r
'
I ['"T
I
I
f
_
u. 150
--
b~m t 0 0 O.
CP-5
CP-5
i
id
r
I
llJJ
i
I
1 iJKllJ
id REACTOR
DUMMY
I
I
ioT EXPOSURE,
MW - s e c
Fig. 1. Transition temperature shift as a function of megawatt-seconds of CP-5 operation L~ two i r r a d i a t i o n
facilities. 5. R E S U L T S 5.1. C P - 5 irradiations F o r e a c h d e s i r e d l e v e l of i r r a d i a t i o n one c a p sule containing four s p e c i m e n s was used. T h e s e f u r n i s h e d 12 i m p a c t t e s t v a l u e s f o r e a c h l e v e l , s i n c e c h e c k s f o r Mn 54 a c t i v i t y at e a c h notch showed the f a s t flux v a r i a t i o n withirr a c a p s u l e in the C P - 5 fuel p o s i t i o n to be ± 2%. C h e m i c a l s o p a r a t i o n was r e q u i r e d to a n a ly z e the C P - 5 d u m m y s a m p l e s but the s t a n d a r d d e v i a t io n w a s s t i l l only ~10%. T w e l v e i m p a c t t e s t s w e r e sufficient to g e n e r ate a b r i t t l e - d u c t i l e t r a n s i t i o n c u r v e . H o w e v e r , in a few s e t s the s c a t t e r of data points c a u s e d u n c e r t a i n t y in the t r a n s i t i o n t e m p e r a t u r e shift.
5.2. E B R - I irradiations The c o r e of the E B R - I is so s m a l l (it can be a p p r o x i m a t e d by a s p h e r e 10 c m in r ad i u s) that ax i al d i f f e r e n c e s in the neutron flux cannot be ignored. E v e n in a single s p e c i m e n t h e r e is about a 10% drop in e x p o s u r e r a t e b et w een the notch c l o s e s t to the c o r e midplane and the next one, only 2.5 c m away. An additional drop of 15 to 20% o c c u r s b et w een the second and t h i r d notches. F o u r fuel r o d l o c a t i o n s w e r e c o n v e r t e d to i r r a d i a t i o n t h i m b l e s and t h e r e a r e d i f f e r e n c e s in flux l e v e l between them as well. C a l c u l a t i o n s indicate s o m e d i f f e r e n c e s in s p e c t r a as the d i s t an ce f r o m the c e n t e r of the c o r e i n c r e a s e s , but they a r e much l e s s i m p o r t a n t than the d i f f e r e n c e s in magnitude.
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v a l u e s w e r e p l o t t ed ag ai n st e x p o s u r e , the r e s u l t s f o r al l t e s t s at a given t e m p e r a t u r e f o r m e d a s t r a i g h t line on a s e m i - l o g plot. Some t y p i c a l s e t s a r e shown in fig. 2 * With such a set of s t r a i g h t l i n e s , it b e c o m e s p o s s i b l e to i n t e r p o l a t e to find the i m p a c t e n e r g y at a given t e s t t e m p e r a t u r e f o r any v al u e of e x posture. The i m p a c t e n e r g i e s f o r a given v a l u e of exposure form familiar S-shaped brittle-ductile t r a n s i t i o n c u r v e s , as showr~ in fig. 3. Th e t r a h sition t e m p e r a t u r e shift as a function of e x p o s u r e can be r e a d d i r e c t l y f r o m fig. 3 *. The a r b i t r a r y c h o i c e of the 2 f t - l b c r i t e r i o n f o r m e a s u r i n g
When all of t h e s e d i f f e r e n c e s w e r e taken into account it b e c a m e c l e a r that it would not be p o s sible to find enough n o t c h e s with the s a m e e x p o s u r e f o r the c o n s t r u c t i o n of c o m p l e t e b r i t t l e ductile t r a n s i t i o n c u r v e s . P r e l i m i n a r y a t t e m p t s using n o m i n al e x p o s u r e v a l u e s showed a p p r e c i a ble s c a t t e r of i m p a c t t e s t r e s u l t s . To m a k e u s e of the a v a i l a b l e data a new method of a n a l y s i s was em p l o y ed . The flux s p e c t r u m c a l c u l a t i o n s give the r a t i o of the RDU r a t e to the Mn 54 a c t i v a t i o n r a t e at any point in the r e a c t o r . Mn 54 a c t i v i t y w a s m e a s u r e d f r o m a chip of m a t e r i a l taken f r o m notch, so a v a l u e of i n t e g r a t e d e x p o s u r e in RDU was obtained f o r e a c h notch b r o k e n in i m pact t e s t i n g . When the m e a s u r e d i m p a c t e n e r g y t
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NEUTRON EMBRITTLEMENT OF STEEL t r a n s i t i o n t e m p e r a t u r e shift was also applied to these c u r v e s .
6. DISCUSSION The p u r p o s e of this study is to c o m p a r e the damage produced in t h r e e different s y s t e m s to see if the method used for d e t e r m i n i n g flux gives the s a m e damage for the s a m e m e a s u r e d expos u r e . Each set of damage r e s u l t s is plotted a g a i n s t RDU and they all a r e shown together in fig. 4. Although the data f r o m C P - 5 f o r m e d s u r p r i s ingly s t r a i g h t l i n e s when plotted a g a i n s t the l o g a r i t h m of exposure, the c r o s s - p l o t t e d E B R - I data did not do so. (The log dependence may a c tually be f o r t u i t o u s s i n c e the impact e n e r g y t r a n sition is an e m p i r i c a l m e a s u r e m e n t of a complex p r o p e r t y . Other i n v e s t i g a t o r s have suggested a ½ or ~ power law. T h e s e data plotted on log-log paper give a slope between ½ and 3.) C a r e f u l a n a l y s i s of the impact t e s t s and the influence of the choice of c r i t e r i o n and s c a t t e r e d data on the c r o s s - p l o t t e d r e s u l t s suggest that the low and high ends of the E B R - I c u r v e a r e not too m e a n ingful. The u n c e r t a i n t y in the shape of each d u c t i l e b r i t t l e t r a n s i t i o n c u r v e is enough that shifts of l e s s than 30°C cannot be c o n s i d e r e d quantitative. At the high e x p o s u r e end the 2 f t - l b c r i t e r i o n l o s e s s i g n i f i c a n c e . Highly i r r a d i a t e d s p e c i m e n s n e v e r showed high impact e n e r g i e s even at high test t e m p e r a t u r e s and the t r a n s i t i o n c u r v e s b e come v e r y much flattened. This effect is exagg e r a t e d by the c r o s s - p l o t t e d technique, making the last two points shown for E B R - I data much too high. F o r t h e s e r e a s o n s the low and high ends of the E B R - I r e s u l t s c u r v e a r e shown dotted in fig. 4. Fig. 4 shows that the d o s i m e t r y p r o c e d u r e s outlined above give s u b s t a n t i a l a g r e e m e n t for r e s u l t s f r o m different f a c i l i t i e s . On the b a s i s of these r e s u l t s alone one cannot say that the e n e r g y - d e p e n d e n t model used for c o n s t r u c t i n g the RDU is c o r r e c t . However, it is b a s e d on logical a r g u m e n t s and gives s a t i s f a c t o r y r e s u l t s . P o s s i b l e r e a s o n s for d i s c r e p a n c i e s in the c o r r e l a t i o n include t e m p e r a t u r e , effect of dose r a t e , and c o n t r i b u t i o n s to damage by t h e r m a l neutrons. T e m p e r a t u r e . No significant d i f f e r e n c e in d e fect a n n e a l i n g r a t e is expected at these t e m p e r a t u r e s , but if one w e r e to exist, it could c a u s e E B R - I impact e n e r g i e s to a bit lower than the C P - 5 v a l u e s . On the other hand, some i n v e s t i g a -
81
t o r s suggest that i n c r e a s e d atomic m o b i l i t y may c a u s e the damage to be g r e a t e r for i r r a d i a t i o n s between 150 and 200oc than at r o o m t e m p e r a ture. R a t e . If any dose r a t e effect w e r e to exist, those r e c e i v i n g the lowest dose r a t e would show the g r e a t e s t p r o p e r t y change. The r e l a t i v e dose r a t e s a r e about 1 to 12 to 37 for C P - 5 dummy, C P - 5 fuel, and E B R - I , r e s p e c t i v e l y . It is v e r y unlikely that any r e a l dose r a t e effect would be p r e s e n t at these t e m p e r a t u r e s . If it w e r e , it would cause the C P - 5 d u m m y v a l u e s to be high, a s observed. T h e r m a l n e u t r o n s . A p o s s i b l e model for c o m puting the n u m b e r of d i s p l a c e m e n t s caused by the r e c o i l n u c l e u s after t h e r m a l n e u t r o n c a p t u r e is given by W e c h s l e r [9]. H a r r i e s applied this to i r o n and p r e d i c t s about 8 d i s p l a c e m e n t s per c a p t u r e [10]. On this b a s i s a value of 0.015 RDU per unit t h e r m a l n e u t r o n flux was obtained. The t h e r m a l flux in C P - 5 fuel was m e a s u r e d to be 1.1 × 1013 n / c m 2 - s e c , thus adding about 2.8% to the c a l c u l a t e d RDU r a t e . The flux of 1.25 x 1013 n / c m 2 - s e c in the C P - 5 d u m m y a d d s 23% to the fast n e u t r o n RDU r a t e . Since t h e r e a r e no t h e r m a l n e u t r o n s in the E B R - I , this c o r r e c t i o n is only significant in the C P - 5 d u m m y data. Moving the C P - 5 d u m m y line 23% f u r t h e r to the r i g h t (dashed line in fig. 4) i m p r o v e s the c o r r e l a t i o n of r e sults. The r e s u l t s indicate that if the t e m p e r a t u r e and r a t e v a r i a b l e s were r e a l l y e l i m i n a t e d in the e x p e r i m e n t s , a b e t t e r c o r r e l a t i o n might be obtainable. If t h e r m a l n e u t r o n s a r e m o r e i m p o r t a n t than suggested above, the d i s c r e p a n c y could be p a r t i a l l y explained. It is also p o s s i b l e that the model chosen to compute the RDU values as a function of energy could be improved. T h e r e is c e r t a i n l y r o o m for i m p r o v e m e n t in methods of c a l c u l a t i o n , c r o s s s e c t i o n s , and r a d i o a n a l y s i s t e c h n i q u e s , which should i m p r o v e the c o r r e l a tion. F i n a l l y , it is admitted that the impact test used in this work is not well s t a n d a r d i z e d . Being totally e m p i r i c a l it cannot be used to provide a good c o r r e l a t i o n with other t e s t s or p r a c t i c a l design p r o b l e m s . S t a n d a r d i z e d charpy impact t e s t s , while also e m p i r i c a l , would have produced data with l e s s s c a t t e r .
7. CONCLUSIONS Although the l i n e s for the t h r e e sets of r e sults do not c o r r e l a t e p e r f e c t l y , it is believed that the method is sound and p r o d u c e s dependable
82
A. D. ROSSIN
r e s u l t s . The c o n s i s t e n c y of m e a s u r e d Mn 54 a c tivation r a t e s among v a r i o u s s p e c i m e n s shows the great potential a v a i l a b l e i n the use of this r e a c t i o n for d o s i m e t r y . With m o r e e x p e r i e n c e it is b e l i e v e d that the RDU method can e l i m i n a t e the wide degree of u n c e r t a i n t y that now e x i s t s in r a d i a t i o n damage studies. One vital need is indicated by this i n v e s t i g a tion. The n u c l e a r i n d u s t r y would be well s e r v e d ff the c r o s s s e c t i o n for the r e a c t i o n Fe54(n,p) Mn 54 w e r e well known as a function of n e u t r o n energy. It is a difficult r e a c t i o n to m e a s u r e , but its potential u s e f u l n e s s j u s t i f i e s the effort r e qiJired. An attempt was made to evaluate the u n c e r t a i n t y of exposure r e s u l t s u s i n g the v a r i o u s t h r e s h o l d c r o s s section v a l u e s found in the l i t e r a t u r e for Mn 54 a c t i v a t i o n u s i n g c a l c u l a t e d and f i s s i o n s p e c t r a [11]. The u n c e r t a i n t y r a n g e for a single piece of data was well in e x c e s s of 2500/0. The u s e of r e a l i s t i c r e a c t o r s p e c t r a r a t h e r than the f i s s i o n s p e c t r u m can be e x t r e m e l y i m p o r t a n t in p r e d i c t i n g damage to r e a c t o r p r e s s u r e v e s s e l s . R e c e n t advances i n computing make s p e c t r a l c a l c u l a t i o n s r e l a t i v e l y e a s y for the r e a c t o r p h y s i c i s t , and the computation of RDU r a t e s r e q u i r e s but m i n u t e s . It can even be obt a i n e d a u t o m a t i c a l l y with the s p e c t r u m c a l c u l a tion. The amount of effort involved is not much g r e a t e r than if t h r e s h o l d c r o s s s e c t i o n s a r e used, but the r e s u l t s a r e much m o r e dependable.
But m o s t i m p o r t a n t is the logic of the p r o c e d u r e , allowing i m p r o v e m e n t m a c c u r a c y a s b e t t e r n u m b e r s b e c o m e available without changing the p r o c e d u r e itself. RE FERENCES [1] A.D.Rossin, Dosimetry for radiation damage studies, ANL-6826 (1964). [2] A.D.Rossin, Nucl. Sci. Eng. 9 (1961) 137. [3] W.H.McCorkle and H.C.Stevens, The proposed change of operating power level and fuel assembly design for the Argonne Research Reactor CP-5, ANL-5553 (1956). [4] R.E.Rice et al., EBR-I Mark HI design report, ANL-5636 (1958). [5] A.D.Rossin, Significance of neutron spectrum on radiation effects studies, ASTM-STP 341 (1963) p. 115. [6] A.D. Rossin and R. J. Armani, Neutron dosimetry, vol. II (IAEA, Vienna, 1963) p. 293. [7] M.Kangilaski, F.Shober and J.Gates, Effect of simulated in-reactor annealing of an irradiated type 347 stainless steel pressure tube, BMI-1683 (1964). [8] A,D.Rossin and R.J. Armani, Neutron dosimetry, vol. I (IAEA, Vienna, 1963) p. 516. [9] M.S.Wechsler, Fundamental aspects of radiation effects on diffusion controlled reactions in alloys, ASTM--STP 341 (1963) p. 86. [ 10] D.R.Harries, D.J.Barton and S.B.Wright, ibid. p. 276. [11] ANL-6739, Reactor development monthly progress report for May 1963.
COMPARISON OF NEUTRON EMBRITTLEMENT OF STEEL IN DIFFERENT REACTOR S P E C T R A * ** A. D. ROSSIN
Argonne National Laboratory, Argonne, Illinois, USA Received 20 October 1964
A procedure has been developed for measuring and reporting neutron exposure for radiation damage studies. The neutron spectrum shape is calculated and its magnitude determined by the reaction F e ~ (n,p)Mn 54. A unit, called RDU, is used for reporting exposure. To test this procedure, steel specimens were irradiated in three very different reactor spectra. The exposures were expressed in RDU's and the resulting embrittlement compared. Satisfactory agreement was obtained. The procedure proved to be easy to use. A new method for reducing a limited amount of irradiation data was devised to handle irradiations done in a facility where the neutron flux varied appreciably from place to place.
1. INTRODUCTION
2. M E A S U R E M E N T OF EXPOSURE
This study is an a t t e m p t to v e r i f y a m e t h o d f or m e a s u r i n g n eu tr o n e x p o s u r e in r a d i a t i o n d a m a g e e x p e r i m e n t s . E m b r i t t l e m e n t of c a r b o n s t e e l by n e u t r o n i r r a d i a t i o n has been studied e x t e n s i v e l y , but the e x p e r i m e n t a l data g e n e r a l l y do not s a t i s f y the r e a c t o r d e s i g n e r b e c a u s e he cannot apply the e x p o s u r e r e p o r t e d to his own needs. T h e r e is no need to look f o r a s o - c a l l e d " n e u t r o n s p e c t r u m e f f e c t " . A p p a r e n t d i f f e r e n c e s of r e s u l t s in d i f f e r e n t s p e c t r a a r e the r e s u l t of i n a d e q u a c i e s in d o s i m e t r y . T h e r e is a need f o r a dose unit, and a method of m e a s u r i n g and r e p o r t i n g n eu t r o n d o s e , that will w o r k in any n e u t r o n s p e c t r u m . Such a unit is the RDU. Its d e v e l o p m e n t and u s e a r e d e s c r i b e d i n A N L - 6 8 2 6 [1]. This unit t a k e s into account the effect of n e u t r o n s of di f f eren t e n e r g y , and g i v e s an unambiguous va l ue of e x p o s u r e f o r any i r r a d i a t i o n . To t e s t this method the e x p o s u r e r a t e s w e r e m e a s u r e d in t h r e e d i f f e r e n t r e a c t o r s y s t e m s , s a m p l e s w e r e i r r a d i a t e d in each, and the r e s u l t s a r e c o m p a r e d in t h i s p a p e r . Since the s a m e e f f e c t was m e a s u r e d in each set, the s l o p e s of c u r v e s of d a m a g e v e r s u s e x p o s u r e should be the s a m e . If all e x p o s u r e s a r e m e a s u r e d in RDU, the data f r o m al l t h r e e s y s t e m s should coincide.
A c c u r a t e d o s i m e t r y depends on the knowledge of the magnitude and shape of the n e u t r o n s p e c t r u m . S p e c t r a l s h a p e s have been d e t e r m i n e d by m e a n s of d e t a i l e d n eu t r o n t r a n s p o r t t h e o r y c a l c u l a t i o n s . The e n t i r e f a s t p a r t of the s p e c t r u m is divided into 20 n a r r o w e n e r g y i n t e r v a l s to c o v e r the r a n g e f r o m 0.067 to 10 MeV, which is b e l i e v e d to account f o r m o s t of the f a s t n e u t r o n d am ag e. The magnitude of the s p e c t r u m is d e t e r m i n e d by a c t i v a t i o n of f o i l s , and r e s p o n s e s of s e v e r a l d i f f e r e n t f o i l s p r o v i d e a rough ch eck on the c a l c u l a t e d s p e c t r u m shape. The RDU (Radiation D a m a g e Unit) is p r e s e n t ed as a function of e n e r g y in table 1 and it is d e fined such that 1018 n e u t r o n s , f o r e x a m p l e , h a v ing t h e i r e n e r g i e s d i s t r i b u t e d as in the f i s s i o n s p e c t r u m , d e l i v e r 1018 RDU to an i r o n - b a s e t a r get m a t e r i a l . In other w o r d s , in an i r r a d i a t i o n w h e r e the r e p o r t e d e x p o s u r e is 1018 RDU, the r e s u l t i n g d a m a g e would be the s a m e as that i n c u r r e d by an i r r a d i a t i o n of 1 0 1 8 f i s s i o n n e u t r o n s . The a c c u r a c y depends on the a c c u r a c y of the e n e r g y - d e p e n d e n t m o d e l u s e d f o r developing the RDU. This m e t h o d f o r m e a s u r i n g n e u t r o n e x p o s u r e w as d e v e l o p e d [2] to o v e r c o m e the s h o r t c o m i n g s of the f a m i l i a r "> 1 MeV" notation. The f i r s t is that t h e r e is no d i r e c t way to m e a s u r e the n u m b e r of n e u t r o n s above 1 MeV. The m o s t c o n v e nient d et ect i n g f o i l s have si g n i f i can t c r o s s s e c tion v a l u e s only above 3 MeV or so. C o n s e q u e n t -
* Work performed under the auspices of the U.S. Atomic Energy Commission. ** Accepted by T. A. Jaeger.
NEUTRON EMBR1TTLEMENT OF STEEL
ly, a s p e c t r u m shape has to be a s s u m e d . T h e f i s s i o n s p e c t r u m is often chosen, even though a n u n d e g r a d e d f i s s i o n s p e c t r u m n e v e r e x i s t s in a r e a c t o r . Actually only a s m a l l f r a c t i o n of the n e u t r o n s p e c t r u m is above 3 MeV and the shape of the r e s t of the s p e c t r u m is v e r y dependent on the g e o m e t r y and m a t e r i a l s in the r e a c t o r . Thus, the r e p o r t e d n u m b e r of n e u t r o n s above 1 MeV is always an a p p r o x i m a t i o n , the a c c u r a c y being different for different r e a c t o r s , or between a c o r e i r r a d i a t i o n f a c i l i t y and p r e s s u r e v e s s e l wall. Secondly, the choice of 1 MeV as a r e f e r e n c e e n e r g y is p u r e l y a r b i t r a r y . If it takes about 25 eV to knock an atom out of its lattice site, a n e u t r o n with a few h u n d r e d eV is capable of d i s p l a c i n g an i r o n atom. It is not c o r r e c t , t h e r e fore, to neglect the vast n u m b e r s of b o m b a r d i n g n e u t r o n s with 100000 eV and m o r e ' The f a s t e r the b o m b a r d i n g n e u t r o n s , the higher the e n e r g y of the p r i m a r y k n o c k - o n atom is likely to be. So it is not quite p r o p e r to a s s i g n the s a m e effect i v e n e s s to a 100000 eY n e u t r o n as to one with 20 t i m e s as much energy. The RDU method takes these f a c t o r s into account, and a l s o allows for the d i f f e r e n c e s in the e l a s t i c s c a t t e r i n g c r o s s s e c t i o n of i r o n with n e u t r o n energy. The n e u t r o n s p e c t r u m can be tabulated (see s e c t i o n 4) to show the n u m b e r of n e u t r o n s / c m 2sec in each e n e r g y i n t e r v a l at the n o r m a l o p e r ating power level of the r e a c t o r . Multiplying the flux in each i n t e r v a l by the RDU value for that e n e r g y , and adding, gives the RDU e x p o s u r e r a t e . It is u s u a l l y convenient to r e d u c e the expos u r e r a t e to RDU p e r M W - s e c of r e a c t o r o p e r a tion. Then the i n t e g r a t e d e x p o s u r e of a s a m p l e is e a s i l y obtained by m u l t i p l y i n g this e x p o s u r e r a t e by the M W - s e c of r e a c t o r operation while the s a m p l e was in the r e a c t o r .
3. EXPERIMENTAL FACILITIES
3.1. Irradiation The t h r e e r e a c t o r f a c i l i t i e s chosen differ enough to give s p e c t r a with s u b s t a n t i a l l y d i f f e r ent c h a r a c t e r i s t i c s . These d i f f e r e n c e s demand that c a r e be taken in d o s i m e t r y p r o c e d u r e s in o r d e r to obtain an a c c u r a t e value of the m a g n i tude of the fast n e u t r o n flux. The r e a c t o r s used were the C P ° 5 at Argonne, I l l i n o i s , and the E B R - I , at NRTS, Idaho. T h e s e r e a c t o r s a r e a m ply d e s c r i b e d in the l i t e r a t u r e [3,4], and in a p r e v i o u s p a p e r [5] the f e a t u r e s significant to this study a r e d e s c r i b e d in detail. C P - 5 is a heavy water m o d e r a t e d and cooled
77
t a n k - t y p e r e s e a r c h r e a c t o r . The fuel e l e m e n t c o n s i s t s of t h r e e c o n c e n t r i c t u b e s of a l u m i n i u m that contain the u r a n i u m fuel, and D 2 0 flows b e tween and a r o u n d the tubes. The s p e c i m e n s r e f e r r e d to as " C P - 5 fuel" w e r e tightly held in a l u m i n i u m f i x t u r e s to a s s u r e heat t r a n s f e r and i r r a d i a t e d in the c e n t r a l t h i m b l e of a fuel e l e ment. The m a j o r p o r t i o n of the s p e c t r u m c o m e s f r o m fast n e u t r o n s f r o m f i s s i o n s in the s u r r o u n d i n g fuel tubes. S p e c i m e n t e m p e r a t u r e was about 65°C. The set called " C P - 5 dummy" was i r r a d i a t e d in identical c a p s u l e s in a s i m i l a r a r r a n g e m e n t of a l u m i n i u m t u b e s , but these tubes contained no u r a n i u m . The d u m m y e l e m e n t is located in one of the outer r i n g of p o s s i b l e fuel e l e m e n t l o c a t i o n s in C P - 5 and f o r m s p a r t of the r e f l e c t o r r e gion for the r e a c t o r . F a s t n e u t r o n s in this l o c a tion a r e due to the leakage f r o m the core. The t h e r m a l flux in this r e f l e c t o r r e g i o n is a bit higher than in the c e n t e r of the core. With the fast flux in the C P - 5 d u m m y only about ~ of that in the C P - 5 fuel location, the high t h e r m a l flux t h e r e c a u s e s m o r e p r o b l e m s in d o s i m e t r y . S p e c i m e n t e m p e r a t u r e was 60oc. E B R - I Mark III was a compact NaK cooled fast r e a c t o r . Its s m a l l core was made of u r a n i u m alloy r o d s clad with Z i r c a l o y - 2 . Steel s p e c i m e n s w e r e placed in hollow tubes and four of t h e s e tubes were s u b s t i t u t e d for four of the fuel rods. Holes in the tubes allowed NaK to cool the s p e c i m e n s directly. S p e c i m e n t e m p e r a t u r e was about 140°C. While this t e m p e r a t u r e was about 80oc above that of the C P - 5 s p e c i m e n s , it is g e n e r a l l y a g r e e d that the r a t e of damage r e c o v e r y in s t e e l does not begin to i n c r e a s e s u b s t a n t i a l l y until the t e m p e r a t u r e r e a c h e s 200oc or higher. The E B R - I s p e c t r u m is fast, but s i g n i f i c a n t l y degraded f r o m a f i s s i o n s p e c t r u m . T h e r e a r e no t h e r m a l n e u t r o n s at all due to the heavy a b s o r p t i o n of the c o r e m a t e r i a l s and the complete a b s e n c e of light moderator elements. 3.2. Testing The m a t e r i a l and the impact s p e c i m e n s used in this study a r e d e s c r i b e d in detail in ref. [5]. The s t e e l w~ts A212B f r o m a documented heat f u r n i s h e d by U.S. Steel C o r p o r a t i o n to ASTM C o m m i t t e e E l 0 . The s u b s i z e Izod impact s p e c i m e n s w e r e notched r o u n d s a p p r o x i m a t e l y 0.5 c m (0.200 in.) d i a m e t e r , 9.5 cm (3~ in.) long and have t h r e e notches m a c h i n e d c i r c u m f e r e n t i a l l y . All s p e c i m e n s w e r e taken f r o m plate such that t h e i r long d i m e n s i o n was p a r a l l e l to the final rolling direction. S p e c i m e n s w e r e b r o k e n with a Baldwin p e n d u -
78
A. D. ROSSIN Table 1 Neutron spectra and cross sections. Fast neutron flux (n/cm2-sec) a) Group
EL
(MeV)
~Mn54
(barns)
CP-5 fuel VT 10 4.6 MW
RDU
CP-5 dummy VT 22 4.6 MW 0.147 × 1 0 1 1 0.358 0.806 0.980 1.234
EBR-I C 17 midplane 1.2 MW 0.406 x 1012 1.096 3.106 4.718 7.025
0.650 0.600 0.530 0.375
1.223 1.213 1.121 1.643
2.865
0.132x1012 0.346 0.946 1.417 0 . 1 5 0 1.601 2.074
6 7 8 9 10
2.231 1.738 1.353 1.054 0.821
0.075 0.030 0.005 0 0
1.416 1.164 0.991 0.736 0.609
2.867 3.017 3.238 3.269 3.276
2.126 1.934 2.045 2.040 1.989
8.849 10.264 12.026 12.193 12.821
11 12 13 14 15
0.639 0.498 0.388 0.302 0.235
0 0 0 0 0
0.749 0.475 0.647 0.358 0.291
3.488 3.216 2.448 3.237 2.950
2.841 2.757 1.857 3.334 3.512
15.413 16.254 14.551 13.271 11.990
16 17 18 19 20
0.183 0.143 0.111 0.086 0.067
0 0 0 0 0
0.240 0.234 0.156 0.170 0.120
2.647 2.292 2.547 2.108 2.226
3.544 3.679 3.870 4.039 4.178
1 2 3 4
7.788 6.065 4.724 3.679
5
RDU/sec Allowance for neutrons < 0.067 MeV RDU/sec RDU/MW-sec Mn54 act/sec-1024 atoms
10.252 7.949 6.774 4.531 3.834 32.406x 1012 25.463 x 1011 100.87 x 1012 +3%
+7%
+1%
33.38 x l012 27.245 x 1011~ 101.88
x 1012
7.26 x 1012
5.922 × 1011
84.90 x 1012
4.26 x 1011
3.30 x 1010
5.35 x 1012
a) Calculated numbers l u r e - t y p e i m p a c t m a c h i n e in a h i g h - l e v e l hot c e l l at Argonne. Cooling f o r t e s t s below a m b ie n t w a s obtained by holding the s p e c i m e n in a f i x t u r e and i m m e r s i n g it in liquid nitrogen. T h i s f i x t u r e w a s used to hold the s p e c i m e n s f o r i m p a c t t e s t i n g . It was equipped with a t h e r m o c o u p l e , and s p e c i m e n t e m p e r a t u r e s w e r e e s t a b l i s h e d by c o r r e l a t i o n s with a s p e c i a l i n s t r u m e n t e d s p e c i m e n . T h i s f i x t u r e has r e s i s t a n c e h e a t e r s imbedded in it, and w a s a l s o u s e d to heat s p e c i m e n s f o r h i g h e r t e m p e r a t u r e i m p a c t t e s t s . A f t e r b r e a k i n g at the f i r s t notch the s p e c i m e n was r a i s e d in the f i x t u r e so that the second notch w a s in p o s i t i o n f o r b r e a k ing, etc. In no c a s e w a s the t e m p e r a t u r e f o r a t e s t l e s s than that of the p r e c e d i n g t e s t on a given s p e c i m e n b a r . 4. DOSIMETRY Fast neutron spectra were calculated for each
f a c i l i t y and the a c t i v a t i o n r a t e s of v a r i o u s f o i l s w e r e m e a s u r e d in e a c h location. In a r e a c t o r the m a t e r i a l s p r e s e n t d e t e r m i n e the shape of the f a s t s p e c t r u m at any location, but b e c a u s e the m e a n f r e e path of t h e s e n e u t r o n s is of the o r d e r of c e n t i m e t e r s , the s p e c t r u m shape does not v a r y d r a s t i c a l l y through c a p s u l e s of the s i z e u s e d in t h e s e e x p e r i m e n t s . The d o s i m e t r y p r o c e d u r e s w e r e d e s c r i b e d in a p a p e r g i v en at the IAEA C o n f e r e n c e on N e u t r o n D o s i m e t r y in D e c e m b e r 1962 [6] and p l o t s of the d i f f e r e n t s p e c t r a a r e shown. Sulfur a c t i v a t i o n r a t e s w e r e u s e d in that p a p e r to e s t a b l i s h the magnitude of the f l u x e s , and the d e g r e e of a g r e e ment obtained f r o m i r o n , nickel, and U238 f o i l s g i v e s confidence that the c o m p u t a t i o n s a r e r e a sonably a c c u r a t e . Although the c r o s s s e c t i o n f o r Fe54(n,p)Mn54 as a d e t a i l e d function of n e u t r o n e n e r g y is not well known, its v a l u e s w e r e e s t i m a t e d b a s e d on its r a t i o to n i c k e l [7] and sulfur [6]
NEUTRON EMBRITTLEMENT OF STEEL a c t i v a t i o n in the E B R - I , and its shape was a s s u m e d to be s i m i l a r to that of the Ni58(n,p)Co58 r e a c t i o n . The m u l t i g r o u p a c t i v a t i o n c r o s s s e c tions u s e d in this study give the following v a l u e s when i n t e g r a t e d o v e r the f i s s i o n s p e c t r u m : S32(n,p)P 32 Ni58(n, p)Co58 Fe54(n,p)Mn 54 RDU
0.065 b a r n s 0.116 b a r n s 0.09! b a r n s 1.0
Although the a b s o l u t e c r o s s s e c t i o n v a l u e s m a y s o m e d a y be changed when new c r o s s s e c t i o n s a r e m e a s u r e d , the Mn 54 r e s u l t s w e r e u s e d c o n s i s t e n t l y to c o r r e l a t e all e x p e r i m e n t s d e s c r i b e d in t h i s p a p e r . The s p e c i m e n s i r r a d i a t e d in the C P - 5 d u m m y had a v e r y high r e l a t i v e l e v e l of Fe 59 a c t i v i t y f r o m t h e r m a l n e u t r o n c a p t u r e in Fe58. The' Mn 54 was s e p a r a t e d c h e m i c a l l y f r o m the r e m a i n i n g i r o n to p e r m i t a c c u r a t e counting [7]. The p r o c e d u r e was shown to be q u a n t i t a t i v e by p a s s i n g E B R - I m a t e r i a l containing no F e 59 a c t i v i t y t h r o u g h the ion exchange c o l u m n [8]. S p e c t r a f o r the t h r e e r e a c t o r f a c i l i t i e s at t h e i r n o r m a l o p e r a t i n g p o w e r l e v e l a r e given in t a bl e 1. C a l c u l a t e d s p e c t r a l shapes w e r e n o r m a l i z e d to a g r e e with the m e a s u r e d Mn54 a c t i vation r a t e . The c a l c u l a t e d a c t i v a t i o n r a t e is obtained u s i n g the c r o s s s e c t i o n s l i s t e d in table 1 and the r e s u l t s a r e shown at the bottom. The c o n t r i b u t i o n s to the RDU r a t e f o r a ll e n e r g y g r o u p s a r e s u m m e d to give the e x p o s u r e r a t e . T h e s e v a l u e s a r e given at the bottom of the t a b l e , both f o r the n o m i n a l r e a c t o r power l e v e l and the R D U / M W - s e c . Although the o r i g i n a l c a l c ula ti o n s only went down to 0.067 MeV, l a t e r c a l c u l a t i o n s extended the r a n g e . The contribution due to f a s t n e u t r o n s below 0.067 MeV i s added to e a c h column.
79
F o r t y t e s t s w e r e used to e s t a b l i s h the b a s e c u r v e for unirradiated material. CP-5 dummy irradiations covered nominal p e r i o d s of 4, 6, 10, 18, and 30 days. The s a m e p e r i o d s w e r e used f o r C P - 5 fuel i r r a d i a t i o n s in addition to 1 and 2 day e x p o s u r e s . The t r a n s i t i o n t e m p e r a t u r e shift was m e a s u r e d at the a r b i t r a r i l y c h o s e n l e v e l of 2 f t - l b (2.72 n t - m ) . Other c h o i c e s of i m p a c t e n e r g y l e v e l s gave v e r y s i m i l a r r e s u l t s . Fig. 1 shows the r e s u l t s f o r the d u m m y and fuel positions. The a b s c i s s a is in M W - s e c of r e a c t o r o p e r a t i o n ; no dose m e a s u r e m e n t s a r e involved. Note that the flux l e v e l f o r the fuel position is at l e a s t an o r d e r of m a g n i tude higher than that of the dummy. The R D U / M W - s e c r a t e s at the bottom of table 1 can be ap p l i ed d i r e c t l y to the c u r v e s to c o n v e r t the i n t e g r a t e d e x p o s u r e to RDU f o r c o m p a r i s o n . I o
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facilities. 5. R E S U L T S 5.1. C P - 5 irradiations F o r e a c h d e s i r e d l e v e l of i r r a d i a t i o n one c a p sule containing four s p e c i m e n s was used. T h e s e f u r n i s h e d 12 i m p a c t t e s t v a l u e s f o r e a c h l e v e l , s i n c e c h e c k s f o r Mn 54 a c t i v i t y at e a c h notch showed the f a s t flux v a r i a t i o n withirr a c a p s u l e in the C P - 5 fuel p o s i t i o n to be ± 2%. C h e m i c a l s o p a r a t i o n was r e q u i r e d to a n a ly z e the C P - 5 d u m m y s a m p l e s but the s t a n d a r d d e v i a t io n w a s s t i l l only ~10%. T w e l v e i m p a c t t e s t s w e r e sufficient to g e n e r ate a b r i t t l e - d u c t i l e t r a n s i t i o n c u r v e . H o w e v e r , in a few s e t s the s c a t t e r of data points c a u s e d u n c e r t a i n t y in the t r a n s i t i o n t e m p e r a t u r e shift.
5.2. E B R - I irradiations The c o r e of the E B R - I is so s m a l l (it can be a p p r o x i m a t e d by a s p h e r e 10 c m in r ad i u s) that ax i al d i f f e r e n c e s in the neutron flux cannot be ignored. E v e n in a single s p e c i m e n t h e r e is about a 10% drop in e x p o s u r e r a t e b et w een the notch c l o s e s t to the c o r e midplane and the next one, only 2.5 c m away. An additional drop of 15 to 20% o c c u r s b et w een the second and t h i r d notches. F o u r fuel r o d l o c a t i o n s w e r e c o n v e r t e d to i r r a d i a t i o n t h i m b l e s and t h e r e a r e d i f f e r e n c e s in flux l e v e l between them as well. C a l c u l a t i o n s indicate s o m e d i f f e r e n c e s in s p e c t r a as the d i s t an ce f r o m the c e n t e r of the c o r e i n c r e a s e s , but they a r e much l e s s i m p o r t a n t than the d i f f e r e n c e s in magnitude.
80
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v a l u e s w e r e p l o t t ed ag ai n st e x p o s u r e , the r e s u l t s f o r al l t e s t s at a given t e m p e r a t u r e f o r m e d a s t r a i g h t line on a s e m i - l o g plot. Some t y p i c a l s e t s a r e shown in fig. 2 * With such a set of s t r a i g h t l i n e s , it b e c o m e s p o s s i b l e to i n t e r p o l a t e to find the i m p a c t e n e r g y at a given t e s t t e m p e r a t u r e f o r any v al u e of e x posture. The i m p a c t e n e r g i e s f o r a given v a l u e of exposure form familiar S-shaped brittle-ductile t r a n s i t i o n c u r v e s , as showr~ in fig. 3. Th e t r a h sition t e m p e r a t u r e shift as a function of e x p o s u r e can be r e a d d i r e c t l y f r o m fig. 3 *. The a r b i t r a r y c h o i c e of the 2 f t - l b c r i t e r i o n f o r m e a s u r i n g
When all of t h e s e d i f f e r e n c e s w e r e taken into account it b e c a m e c l e a r that it would not be p o s sible to find enough n o t c h e s with the s a m e e x p o s u r e f o r the c o n s t r u c t i o n of c o m p l e t e b r i t t l e ductile t r a n s i t i o n c u r v e s . P r e l i m i n a r y a t t e m p t s using n o m i n al e x p o s u r e v a l u e s showed a p p r e c i a ble s c a t t e r of i m p a c t t e s t r e s u l t s . To m a k e u s e of the a v a i l a b l e data a new method of a n a l y s i s was em p l o y ed . The flux s p e c t r u m c a l c u l a t i o n s give the r a t i o of the RDU r a t e to the Mn 54 a c t i v a t i o n r a t e at any point in the r e a c t o r . Mn 54 a c t i v i t y w a s m e a s u r e d f r o m a chip of m a t e r i a l taken f r o m notch, so a v a l u e of i n t e g r a t e d e x p o s u r e in RDU was obtained f o r e a c h notch b r o k e n in i m pact t e s t i n g . When the m e a s u r e d i m p a c t e n e r g y t
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t
, , t~,~,
tO19
1018
REACTOR
I 200
Fig. 3. Impact energy versus test temperature for various RDU values of exposure, cross-plotted from
Fig. 2. Impact energy versus fast neutron exposure in RDU at four test temperatures. EBR-I irradiations.
1
J
EXPOSURE,
RDU
Fig. 4. Transition temperature shift versus neutron exposure in RDU for three different reactor facilities.
NEUTRON EMBRITTLEMENT OF STEEL t r a n s i t i o n t e m p e r a t u r e shift was also applied to these c u r v e s .
6. DISCUSSION The p u r p o s e of this study is to c o m p a r e the damage produced in t h r e e different s y s t e m s to see if the method used for d e t e r m i n i n g flux gives the s a m e damage for the s a m e m e a s u r e d expos u r e . Each set of damage r e s u l t s is plotted a g a i n s t RDU and they all a r e shown together in fig. 4. Although the data f r o m C P - 5 f o r m e d s u r p r i s ingly s t r a i g h t l i n e s when plotted a g a i n s t the l o g a r i t h m of exposure, the c r o s s - p l o t t e d E B R - I data did not do so. (The log dependence may a c tually be f o r t u i t o u s s i n c e the impact e n e r g y t r a n sition is an e m p i r i c a l m e a s u r e m e n t of a complex p r o p e r t y . Other i n v e s t i g a t o r s have suggested a ½ or ~ power law. T h e s e data plotted on log-log paper give a slope between ½ and 3.) C a r e f u l a n a l y s i s of the impact t e s t s and the influence of the choice of c r i t e r i o n and s c a t t e r e d data on the c r o s s - p l o t t e d r e s u l t s suggest that the low and high ends of the E B R - I c u r v e a r e not too m e a n ingful. The u n c e r t a i n t y in the shape of each d u c t i l e b r i t t l e t r a n s i t i o n c u r v e is enough that shifts of l e s s than 30°C cannot be c o n s i d e r e d quantitative. At the high e x p o s u r e end the 2 f t - l b c r i t e r i o n l o s e s s i g n i f i c a n c e . Highly i r r a d i a t e d s p e c i m e n s n e v e r showed high impact e n e r g i e s even at high test t e m p e r a t u r e s and the t r a n s i t i o n c u r v e s b e come v e r y much flattened. This effect is exagg e r a t e d by the c r o s s - p l o t t e d technique, making the last two points shown for E B R - I data much too high. F o r t h e s e r e a s o n s the low and high ends of the E B R - I r e s u l t s c u r v e a r e shown dotted in fig. 4. Fig. 4 shows that the d o s i m e t r y p r o c e d u r e s outlined above give s u b s t a n t i a l a g r e e m e n t for r e s u l t s f r o m different f a c i l i t i e s . On the b a s i s of these r e s u l t s alone one cannot say that the e n e r g y - d e p e n d e n t model used for c o n s t r u c t i n g the RDU is c o r r e c t . However, it is b a s e d on logical a r g u m e n t s and gives s a t i s f a c t o r y r e s u l t s . P o s s i b l e r e a s o n s for d i s c r e p a n c i e s in the c o r r e l a t i o n include t e m p e r a t u r e , effect of dose r a t e , and c o n t r i b u t i o n s to damage by t h e r m a l neutrons. T e m p e r a t u r e . No significant d i f f e r e n c e in d e fect a n n e a l i n g r a t e is expected at these t e m p e r a t u r e s , but if one w e r e to exist, it could c a u s e E B R - I impact e n e r g i e s to a bit lower than the C P - 5 v a l u e s . On the other hand, some i n v e s t i g a -
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t o r s suggest that i n c r e a s e d atomic m o b i l i t y may c a u s e the damage to be g r e a t e r for i r r a d i a t i o n s between 150 and 200oc than at r o o m t e m p e r a ture. R a t e . If any dose r a t e effect w e r e to exist, those r e c e i v i n g the lowest dose r a t e would show the g r e a t e s t p r o p e r t y change. The r e l a t i v e dose r a t e s a r e about 1 to 12 to 37 for C P - 5 dummy, C P - 5 fuel, and E B R - I , r e s p e c t i v e l y . It is v e r y unlikely that any r e a l dose r a t e effect would be p r e s e n t at these t e m p e r a t u r e s . If it w e r e , it would cause the C P - 5 d u m m y v a l u e s to be high, a s observed. T h e r m a l n e u t r o n s . A p o s s i b l e model for c o m puting the n u m b e r of d i s p l a c e m e n t s caused by the r e c o i l n u c l e u s after t h e r m a l n e u t r o n c a p t u r e is given by W e c h s l e r [9]. H a r r i e s applied this to i r o n and p r e d i c t s about 8 d i s p l a c e m e n t s per c a p t u r e [10]. On this b a s i s a value of 0.015 RDU per unit t h e r m a l n e u t r o n flux was obtained. The t h e r m a l flux in C P - 5 fuel was m e a s u r e d to be 1.1 × 1013 n / c m 2 - s e c , thus adding about 2.8% to the c a l c u l a t e d RDU r a t e . The flux of 1.25 x 1013 n / c m 2 - s e c in the C P - 5 d u m m y a d d s 23% to the fast n e u t r o n RDU r a t e . Since t h e r e a r e no t h e r m a l n e u t r o n s in the E B R - I , this c o r r e c t i o n is only significant in the C P - 5 d u m m y data. Moving the C P - 5 d u m m y line 23% f u r t h e r to the r i g h t (dashed line in fig. 4) i m p r o v e s the c o r r e l a t i o n of r e sults. The r e s u l t s indicate that if the t e m p e r a t u r e and r a t e v a r i a b l e s were r e a l l y e l i m i n a t e d in the e x p e r i m e n t s , a b e t t e r c o r r e l a t i o n might be obtainable. If t h e r m a l n e u t r o n s a r e m o r e i m p o r t a n t than suggested above, the d i s c r e p a n c y could be p a r t i a l l y explained. It is also p o s s i b l e that the model chosen to compute the RDU values as a function of energy could be improved. T h e r e is c e r t a i n l y r o o m for i m p r o v e m e n t in methods of c a l c u l a t i o n , c r o s s s e c t i o n s , and r a d i o a n a l y s i s t e c h n i q u e s , which should i m p r o v e the c o r r e l a tion. F i n a l l y , it is admitted that the impact test used in this work is not well s t a n d a r d i z e d . Being totally e m p i r i c a l it cannot be used to provide a good c o r r e l a t i o n with other t e s t s or p r a c t i c a l design p r o b l e m s . S t a n d a r d i z e d charpy impact t e s t s , while also e m p i r i c a l , would have produced data with l e s s s c a t t e r .
7. CONCLUSIONS Although the l i n e s for the t h r e e sets of r e sults do not c o r r e l a t e p e r f e c t l y , it is believed that the method is sound and p r o d u c e s dependable
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r e s u l t s . The c o n s i s t e n c y of m e a s u r e d Mn 54 a c tivation r a t e s among v a r i o u s s p e c i m e n s shows the great potential a v a i l a b l e i n the use of this r e a c t i o n for d o s i m e t r y . With m o r e e x p e r i e n c e it is b e l i e v e d that the RDU method can e l i m i n a t e the wide degree of u n c e r t a i n t y that now e x i s t s in r a d i a t i o n damage studies. One vital need is indicated by this i n v e s t i g a tion. The n u c l e a r i n d u s t r y would be well s e r v e d ff the c r o s s s e c t i o n for the r e a c t i o n Fe54(n,p) Mn 54 w e r e well known as a function of n e u t r o n energy. It is a difficult r e a c t i o n to m e a s u r e , but its potential u s e f u l n e s s j u s t i f i e s the effort r e qiJired. An attempt was made to evaluate the u n c e r t a i n t y of exposure r e s u l t s u s i n g the v a r i o u s t h r e s h o l d c r o s s section v a l u e s found in the l i t e r a t u r e for Mn 54 a c t i v a t i o n u s i n g c a l c u l a t e d and f i s s i o n s p e c t r a [11]. The u n c e r t a i n t y r a n g e for a single piece of data was well in e x c e s s of 2500/0. The u s e of r e a l i s t i c r e a c t o r s p e c t r a r a t h e r than the f i s s i o n s p e c t r u m can be e x t r e m e l y i m p o r t a n t in p r e d i c t i n g damage to r e a c t o r p r e s s u r e v e s s e l s . R e c e n t advances i n computing make s p e c t r a l c a l c u l a t i o n s r e l a t i v e l y e a s y for the r e a c t o r p h y s i c i s t , and the computation of RDU r a t e s r e q u i r e s but m i n u t e s . It can even be obt a i n e d a u t o m a t i c a l l y with the s p e c t r u m c a l c u l a tion. The amount of effort involved is not much g r e a t e r than if t h r e s h o l d c r o s s s e c t i o n s a r e used, but the r e s u l t s a r e much m o r e dependable.
But m o s t i m p o r t a n t is the logic of the p r o c e d u r e , allowing i m p r o v e m e n t m a c c u r a c y a s b e t t e r n u m b e r s b e c o m e available without changing the p r o c e d u r e itself. RE FERENCES [1] A.D.Rossin, Dosimetry for radiation damage studies, ANL-6826 (1964). [2] A.D.Rossin, Nucl. Sci. Eng. 9 (1961) 137. [3] W.H.McCorkle and H.C.Stevens, The proposed change of operating power level and fuel assembly design for the Argonne Research Reactor CP-5, ANL-5553 (1956). [4] R.E.Rice et al., EBR-I Mark HI design report, ANL-5636 (1958). [5] A.D.Rossin, Significance of neutron spectrum on radiation effects studies, ASTM-STP 341 (1963) p. 115. [6] A.D. Rossin and R. J. Armani, Neutron dosimetry, vol. II (IAEA, Vienna, 1963) p. 293. [7] M.Kangilaski, F.Shober and J.Gates, Effect of simulated in-reactor annealing of an irradiated type 347 stainless steel pressure tube, BMI-1683 (1964). [8] A,D.Rossin and R.J. Armani, Neutron dosimetry, vol. I (IAEA, Vienna, 1963) p. 516. [9] M.S.Wechsler, Fundamental aspects of radiation effects on diffusion controlled reactions in alloys, ASTM--STP 341 (1963) p. 86. [ 10] D.R.Harries, D.J.Barton and S.B.Wright, ibid. p. 276. [11] ANL-6739, Reactor development monthly progress report for May 1963.