Production of Transplutonium Elements in the High Flux Isotope Reactor

1 Production of Transplutonium Elements in the High Flux Isotope Reactor J. E. BIGELOW, B. L. CORBETT, L. J. KING, S. C. McGUIRE, and T. M. SIMS

Downloaded by 80.82.77.83 on October 1, 2017 | http://pubs.acs.org Publication Date: July 20, 1981 | doi: 10.1021/bk-1981-0161.ch001

Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, TN 37830

The National Transplutonium Element Production Program was e s t a b l i s h e d i n the l a t e 1950's to concentrate the " l a r g e - s c a l e " production of transplutonium elements at a c e n t r a l l o c a t i o n . These products are then d i s t r i b u t e d to researchers throughout the country upon the recommendations of a Transplutonium Program Committee which i s comprised of r e p r e s e n t a t i v e s from the major l a b o r a t o r i e s which have an i n t e r e s t i n transplutonium e l e ment research. The Oak Ridge N a t i o n a l Laboratory was s e l e c t e d as the s i t e f o r these production facilities, c o n s i s t i n g of a high f l u x reactor and an adjacent radiochemical processing p l a n t , which are capable of producing gram amounts of Cf and r e l a t e d q u a n t i t i e s of the other heavy elements (1). These manmade elements are all i n t e n s e l y r a d i o a c t i v e and can be processed s a f e l y and r e l i a b l y only i n an elaborate remote handling facility, such as the Transuranium Processing Plant (TRU). This facility and some of the processes c a r r i e d out t h e r e i n f o r recovery and p u r i f i c a t i o n of transplutonium elements are described i n other papers i n t h i s symposium (2,3,4,5). We have now made over 1000 shipments of these products to 30 d i f ferent l a b o r a t o r i e s throughout the U.S. and i n s e v e r a l f o r e i g n c o u n t r i e s , a t t e s t i n g to the success of the Program. 252

A l l of t h i s would not be p o s s i b l e without the High Flux Isotope Reactor (HFIR) to serve as a source of neutrons to carry out the transmutation of the elements. Since f i r s t reaching f u l l power (100 MW) on October 21, 1966, the HFIR has logged 4148 equivalent f u l l power days through December 31, 1979, f o r an o v e r a l l operating e f f i c i e n c y of 86%. During many years, t h i s f i g u r e has run 93% or more. The purpose of t h i s paper i s to i n d i c a t e the c a p a b i l i t i e s of the HFIR f o r transplutonium element production and p a r t i c u l a r l y to dwell on the mathematical techniques involved i n f o r e c a s t i n g the composition of i r r a d i a t e d target m a t e r i a l s . A l s o described are some of the uses to which such f o r e c a s t s are put. E a r l y work along t h i s l i n e was published by Burch, Arnold, and ChethamStrode (6), p r o v i d i n g the basis f o r design of HFIR and TRU.

0 0 9 7 - 6 1 5 6 / 8 1 / 0 1 6 1 -0003$05.00/0 © 1981 American Chemical Society

Navratil and Schulz; Transplutonium Elements—Production and Recovery ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

TRANSPLUTONIUM ELEMENTS

4 Transmutation

Reactions

Figure 1 i s a p o r t i o n of the chart of the n u c l i d e s which i n c l u d e s those n u c l i d e s which are formed by neutron i r r a d i a t i o n and decay from Pu, our o r i g i n a l s t a r t i n g m a t e r i a l . When a nucleus captures a neutron, the mass number i n c r e a s e s by 1 and the new nucleus w i l l be represented by the square to the r i g h t . This process w i l l continue producing heavier and heavier isotopes u n t i l a n u c l i d e i s formed that has a high p r o b a b i l i t y of decaying before i t can react with another neutron. I f the decay i s a beta decay, a new element i s formed ( t h i s i s r e p r e sented by a move d i a g o n a l l y upward to the l e f t ) . Continued i r r a d i a t i o n produces isotopes of t h i s new element u n t i l another beta decay produces s t i l l another element. The process t e r m i nates at " ° F m because that i s o t o p e decays by spotaneous f i s s i o n with a h a l f - l i f e of 380 ps and no b e t a - a c t i v e i s o t o p e of fermium i s formed before t h i s point i s reached. Other n a t u r a l decay processes can occur besides beta decay, such as alpha decay, e l e c t r o n capture, and isomeric t r a n s i t i o n . Neutron-induced processes, besides capture, i n c l u d e f i s s i o n and v a r i o u s s p a l l a t i o n r e a c t i o n s . With a couple of minor exceptions, the l a t t e r are not very important i n the transplutonium element region and they are not modeled i n our c a l c u l a t i o n s .


2

The High Flux Isotope

Reactor

The High Flux Isotope Reactor (HFIR) was designed to produce very intense neutron f l u x e s (>10^ n e u f m ' ^ s " ^ ) e x p r e s s l y f o r the production of transplutonium elements (_7). The v a r i o u s core components are arranged i n c o n c e n t r i c c y l i n d r i c a l regions, a l l of which have a height of about 0.6 m. The innermost region i s a f l u x trap c o n t a i n i n g the target i s l a n d . This i s surrounded by the two-piece annular f u e l assembly. A new f u e l assembly contains i n i t i a l l y 9.4 kg of U and can operate 21-23 days before replacement. Farther out from the c e n t e r l i n e are the c o n t r o l c y l i n d e r s , inner and outer, and l a s t l y the b e r y l l i u m r e f l e c t o r which i s made up of s e v e r a l annular segments to f a c i l i t a t e replacement as r e q u i r e d by r a d i a t i o n damage. The b e r y l l i u m r e f l e c t o r region i s penetrated by a number of thimbles which are very u s e f u l f o r isotope production or i r r a d i a t i o n experiments of many kinds ( 8 ) . 2 3 5

The primary coolant ( l i g h t water) i s admitted to the pressure v e s s e l at 49°C and at a pressure of 5.2 MPa. The coolant flow of 1 m /s r e s u l t s i n a temperature r i s e of 24°C and a pressure drop of 0.76 MPa as the coolant flows through the r e a c t o r , removing the 100 MW of f i s s i o n heat. The target i s l a n d contains 31 p o s i t i o n s f o r the aluminumc l a d target assemblies. As p r e s e n t l y operated, the target i n the c e n t e r l i n e p o s i t i o n i s replaced with a v e r s a t i l e h y d r a u l i c r a b b i t f a c i l i t y , which gives ready access to the p o s i t i o n of


1.

BIGELOW E T A L .

High Flux

Reactor

5

highest f l u x i n the r e a c t o r . From time to time, other t a r g e t p o s i t i o n s have been replaced by s p e c i a l experimental assemb l i e s , so that the number of target p o s i t i o n s a v a i l a b l e f o r transplutonium element production f l u c t u a t e s between 27 and 30. A target assembly i s shown i n F i g . 2 i n a cutaway view to show the i n t e r i o r f e a t u r e s . The a c t i n i d e oxide—aluminum powder blend i s pressed i n t o p e l l e t s , 35 of which are loaded i n t o an aluminum tube f i t t e d with aluminum a l l o y l i n e r s to maintain a v o i d a t each end. Plugs are welded i n t o the ends to encapsulate the r a d i o a c t i v e m a t e r i a l . The upper end plug a l s o serves as a remote handling f i x t u r e . A coolant flow shroud i s mechanically attached to the outside of the tube p r o v i d i n g each target rod with i t s own coolant channel, as w e l l as maintaining the hexagon a l l a t t i c e spacing i n the r e a c t o r . Target assemblies may be loaded with up to 10 g of ^ P u , ^ Am, or ^Cm, or any comb i n a t i o n of the above, i n c l u d i n g e q u i l i b r i u m amounts of heavier i s o t o p e s , f o r a t o t a l of 10 g of heavy metal (11.15 g of o x i d e ) . The P u targets f o r the i n i t i a l r e a c t o r loading were f a b r i cated i n a glove box f a c i l i t y (9), but the other m a t e r i a l s are a l l s u f f i c i e n t l y r a d i o a c t i v e as to r e q u i r e remote f a b r i c a t i o n i n the TRU hot c e l l s (10). 2


Isotope

2

2

2

3

2

2

Model of Flux i n the HFIR Target

Island

The neutron f l u x (the product of neutron c o n c e n t r a t i o n and v e l o c i t y ) i s a strong f u n c t i o n of neutron energy, p o s i t i o n i n the target i s l a n d , and the r e a c t o r operating c o n d i t i o n s . The means by which these v a r i a t i o n s are handled i s discussed below. A 2-group set of f l u x e s i s used f o r e s t i m a t i n g the transmutat i o n of a c t i n i d e elements. A l l neutrons having an energy l e s s than 39.9 kJ/mol (0.414 eV) are considered "thermal." A l l neutrons having energies between 39.9 kJ/mol and 9.75 MJ/mol (101 eV) were t a l l i e d and the r e s u l t d i v i d e d by 5.5, the number of l e t h a r g y u n i t s spanned by t h i s energy range. (Lethargy i s r e l a t e d to the negative logarithm of the energy.) This l a t t e r f l u x i s c a l l e d the "resonance" or "epithermal" f l u x per u n i t l e t h a r g y . The values of these two f l u x groups c a l c u l a t e d f o r the o r i g i n a l r e a c t o r neutronic design are shown on F i g . 3 as a f u n c t i o n of r a d i a l distance from the r e a c t o r c e n t e r l i n e f o r s e v e r a l d i f f e r e n t assumed target l o a d i n g s . The v e r t i c a l l i n e s represent the r a d i a l p o s i t i o n s of the various groups of target assemblies. One p a r t i c u l a r target l o a d i n g was chosen to r e p r e sent the loadings t y p i c a l l y encountered i n r e g u l a r operation and the i n t e r s e c t i o n of those curves ( f o r both groups) with the v a r i o u s target p o s i t i o n s were then designated as Standard Midplane Fluxes f o r that r i n g of t a r g e t s . The a x i a l d i s t r i b u t i o n was measured i n e a r l y experiments i n the HFIR. The data were very w e l l f i t by the usual chopped cosine d i s t r i b u t i o n with a small amount of r e f l e c t o r peaking ( F i g . 4). We g e n e r a l l y c a l c u l a t e the target compositions at


TRANSPLUTONIUM

ELEMENTS

|Fm 254J F m 2 5 5 |Fm 256_ |Fm 257 100

Fm

SF " |Es 253

Es 254]

Es Cf

β'

Cf 2 4 9 Cf 2 5 0 Cf 251 Cf 252 [tf 253

Cf254|

α \ οg,(n£)ij g . S F q,(n,fr ~ 6k 249 §îIk 2 5 0 Bk 251

SF

Bk

β'

β'

Cm247| Cm 248] ;m249 |Cm250

|Cm 242j Cm 243||Cm 244 96

[ËT255

Cm

β'

g,(n.f: g.(n.f) 31 a.(n.f) [Am 241 Am 2421[Am 2431[Am 244| Am245|[Am246|

SF


k

95

Am

94

Pu

>

α EC Λ, α« ν 'fir | P u 2 4 0 f P u 2 4 1 ]Pu 242[Pu 243|Py^244[P^245 |Pu 246 Pu 2 42 \

β'

>37(n.f)

Figure L

β'

Transuranium nuclide production paths

Figure 2.

Diagram of HFIR target assembly


g


1.

BIGELOW E T A L .

Figure 3.

High

Flux Isotope

7

Reactor

Radial flux distribution in HFIR target island

1.2 FROM TWO-DIMENSIONAL ( r , ζ ) • C o ( n , y ) C o FOILS FOUR-GROUP DIFFUSION | o' Au(n,y) AuF0ILS THEORY CALCULATION ! 1.0 I-OF T H E R M A L FLUX 5 9

6 0

9 7

ο

0.9 r»3cm -r=Ocm ΰ ' r»RADIAL \r*4.5cm DISTANCE \ O' FROM REACTOR · ω CENTER LINE

0.8 Ν

> 9 8

0.7

r

0.6

s

5

7

c

m

α

0.5 r =0 TO 4 c m / r=4cm

0.4

BOTTOM HALF** 0.3 -24 AXIAL

Figure 4.

-16

-8

0

• T O P HALF ' 8

DISTANCE FROM REACTOR HORIZONTAL

i_ 16

24

MIDPLANE(cm)

Axial flux distribution in HFIR target island



8

TRANSPLUTONIUM ELEMENTS

various places along the assembly using the f l u x appropriate to that l o c a t i o n and then a x i a l l y average by Simpson's Rule. In most r e a c t o r s operating at a constant power l e v e l , the f l u x increases with time as the f u e l i s consumed. This i s not true w i t h i n the HFIR target i s l a n d (because of i t s f l u x - t r a p design) where the neutron f l u x e s are e s s e n t i a l l y constant throughout the length of an operating c y c l e (about 23 days). Therefore, a time-average f l u x can be c a l c u l a t e d which i s pro p o r t i o n a l to the r e a c t o r thermal power. When c a l c u l a t i n g exact h i s t o r i e s of target assemblies, the power data are taken from the r e a c t o r operating l o g s . For design s t u d i e s , a constant f l u x f o r a 23-day p e r i o d i s assumed. Another p o t e n t i a l v a r i a t i o n i n the thermal f l u x , i . e . l o c a l p e r t u r b a t i o n s , i s assumed to be n e g l i g i b l e because of the r e l a t i v e l y small q u a n t i t i e s of transplutonium elements contained i n an i n d i v i d u a l target assembly. C r o s s - S e c t i o n Model Neutron cross s e c t i o n s are a measure of the p r o b a b i l i t y of neutrons i n t e r a c t i n g with a given nucleus. The r a t e at which a given r e a c t i o n occurs i s given by the product of the number of atoms of the n u c l i d e , N, i t s microscopic cross s e c t i o n , σ, and the neutron f l u x , φ. Since d i f f e r e n t kinds of i n t e r a c t i o n s are p o s s i b l e (e.g., neutron capture, s c a t t e r i n g , f i s s i o n ) , a cross s e c t i o n i s a s s o c i a t e d with each of these processes and the v a r i o u s cross s e c t i o n s are a d d i t i v e . The cross s e c t i o n s are a very strong f u n c t i o n of the i n c i d e n t energy of the neutron, and some means of f o l d i n g t h i s information i n t o the spectrum of neutrons must be u t i l i z e d . F o r t u n a t e l y , as f a r as computing transplutonium element production i n the HFIR i s concerned, we need only consider i n t e r a c t i o n s i n the thermal and epithermal energy regions. The two regions are modeled d i f f e r e n t l y . Thermal Cross S e c t i o n . In most of the n u c l i d e s of i n t e r e s t , the cross s e c t i o n i n the thermal r e g i o n v a r i e s i n v e r s e l y with the neutron v e l o c i t y , v, which i s p r o p o r t i o n a l to the square root of the neutron k i n e t i c energy. The neutron energy spectrum i n t h i s same region i s reasonably w e l l approximated by a Maxwell-Boltzman d i s t r i b u t i o n i n thermal e q u i l i b r i u m with the l i g h t - w a t e r moderator (which i s estimated to average about 54°C i n the f l u x t r a p ) . Two conventions are used here: (1) the thermal cross s e c t i o n used i n our c a l c u l a t i o n s i s the cross sec t i o n f o r i n t e r a c t i o n with neutrons having a v e l o c i t y of 2200 m/s, which corresponds to an energy of 2.41 kJ/mol (0.025 eV), the most probable energy f o r neutrons i n thermal e q u i l i b r i u m at 293.15 K. The symbol f o r t h i s cross s e c t i o n i s σ2200 thermal f l u x ( φ ^ ^ * P e d by an "equivalent 2200 m/s" f l u x (Φ2200) f l u x which, when m u l t i p l i e d by o ^ o o * y i e l d s the same r e a c t i o n r a t e as does the a c t u a l f l u x m u l t i p l i e d ;

s

w

n

i

c

n

i s

t

r e

n

a

r e s e n t

t


t

h

e

1.

BIGELOW

ET A L .

High Flux Isotope

Reactor

by the a c t u a l cross s e c t i o n s , when i n t e g r a t e d across the e n t i r e energy spectrum from 0 to 39·9 kJ/mol. I f the cross s e c t i o n t r u l y v a r i e d as 1/v and i f the neutron energy spectrum were t r u l y i n e q u i l i b r i u m at some temperature, T, the r a t i o between t h i s equivalent f l u x and the a c t u a l f l u x would be:


For the HFIR, t h i s r a t i o i s 0.839, and f o r c a l c u l a t i o n a l purposes, the thermal f l u x e s described i n the preceding s e c t i o n were m u l t i p l i e d by t h i s q u a n t i t y before use i n computing r e a c tion rates. Epithermal Cross S e c t i o n . In the region of neutron energies s l i g h t l y higher than thermal (epithermal) the modeling of cross s e c t i o n s i s q u i t e d i f f e r e n t . In t h i s region, there are sharp peaks i n the cross s e c t i o n s at c e r t a i n energies where the k i n e t i c energy plus the binding energy of the neutron i n the nucleus matches an energy s t a t e of the compound nucleus. This phenomenon i s c a l l e d resonance a b s o r p t i o n ; thus, t h i s energy r e g i o n i s f r e q u e n t l y r e f e r r e d to as the "resonance" r e g i o n . The great amount of d e t a i l i n the energy-dependent neutron c r o s s s e c t i o n data (11) makes the e v a l u a t i o n of the o v e r a l l r e a c t i o n r a t e extremely d i f f i c u l t unless some kind of o v e r a l l averaging can be accomplished. F o r t u n a t e l y , the energy dependence of the f l u x i n t h i s part of the spectrum approaches an i d e a l i z e d case which i s e x a c t l y the form r e q u i r e d to s i m p l i f y the a n a l y s i s . So the cross s e c t i o n s can be i n t e g r a t e d through the resonance region without i n v o l v i n g the f l u x e s , and then the f l u x can be i n c l u d e d l a t e r i n the form of the f l u x per u n i t l e t h a r g y . Thus, the o v e r a l l r e a c t i o n rate constant, k, f o r resonance neutrons w i l l be k =

R

I

* «frres

where RI i s the resonance i n t e g r a l of the cross s e c t i o n s and i s the resonance f l u x per u n i t l e t h a r g y . As i n d i c a t e d above, i n e v a l u a t i n g φ > the averaging was performed only between 39.9 kJ/mol and 9.75 MJ/mol since the m a j o r i t y of the i n t e r a c t i o n s i n v o l v i n g transplutonium n u c l i d e s occur i n t h i s i n t e r v a l ; f u r t h e r , i t was a sub-grouping r e a d i l y a v a i l a b l e to us from the complete r e a c t o r neutronic a n a l y s i s . res

Γ β 8

Resonance S e l f - S h i e l d i n g . At the energy corresponding to the peak of a given resonance, the absorption cross s e c t i o n can be enormous. Here, the nucleus becomes e f f e c t i v e l y a sponge, soaking up the vast m a j o r i t y of the neutrons with energies near that of the resonance. In a target of f i n i t e thickness (such as the 5-mm-diameter a c t i v e region i n the HFIR target assemblies),


9

TRANSPLUTONIUM

10

ELEMENTS

the atoms i n the outer l a y e r s of the target react s t r o n g l y with the incoming neutrons and prevent the neutrons from reaching the atoms i n the i n t e r i o r . This phenomenon i s known as resonance s e l f - s h i e l d i n g , and i s a f u n c t i o n of the atom d e n s i t y of the absorbing n u c l e i , the geometry of the region c o n t a i n i n g the absorbing n u c l e i , and the s c a t t e r i n g p r o p e r t i e s of a l l n u c l i d e s contained w i t h i n that region. The r e l a t i o n s h i p below i s v a l i d f o r a s i n g l e resonance absorption peak, but f o r a r e a l n u c l i d e possessing a multitude of resonances, i t should be regarded more as an e m p i r i c a l c o r r e c t i o n f o r resonance s e l f - s h i e l d i n g : R I

RI " γϊ+CN

eff


i s

where R I f f the e f f e c t i v e resonance i n t e g r a l , Ν i s the number of grams of the p a r t i c u l a r n u c l i d e i n one target rod, and C i s a constant i n c o r p o r a t i n g the conversion f a c t o r s of Ν i n t o atom d e n s i t y , as w e l l as the information r e l a t i n g to the target geometry and neutron s c a t t e r i n g p r o p e r t i e s . A l l of these models must now be combined to y i e l d a u s e f u l approximation f o r the r e a c t i o n rate of a n u c l i d e with the neutrons i n the HFIR. e

RI Reaction Rate - N(a) ff - Φ2200 2200 Ν

σ

+

e

^res

The constant C was i n i t i a l l y c a l c u l a t e d f o r the n u c l i d e ^ ^ P u based on the f i r s t major resonance at 259 kJ/mol (2.68 eV) (12). For some n u c l i d e s , values of C were assumed based on the peak absorption cross s e c t i o n i n the major resonance. Others were assumed based on p r o p o r t i o n a l i t y to the resonance i n t e g r a l (which can be measured e m p i r i c a l l y without knowing the d e t a i l e d energy-dependent spectrum). Then, these assumed values f o r C and a l s o o~2200 d j u s t e d by t r i a l and e r r o r procedures to produce reasonable agreement with experimentally determined tranmutation r e a c t i o n s . Table I shows values p r e s e n t l y i n use f o r the parameters 02200» ^* * ^ ^ ^ capture and f i s s i o n f o r the t r a n s u r a n i c n u c l i d e s considered i n t h i s program. Both processes occur simultaneously and each i s f i r s t order with respect to the r e a c t a n t . Thus, the rate of change of the quantity of n u c l i d e i s given by w

e

r

e

a

a n
res

7 = > + yi+Ci i

p

N

where most of the symbols were defined before, and the super s c r i p t , a, r e f e r s to the sum of neutron capture and f i s s i o n processes. Ρ i s the production term and i s e i t h e r of the form:


1.

BiGELOW E T A L .

High Flux

Isotope

11

Reactor

Table I· Neutron cross s e c t i o n parameters used to compute transmutations i n HFIR target i r r a d i a t i o n s

Nuclide

238 239^ 240* 24l£ 242^ 243* 244* D

u

u


U

u

u

u

Pu 243 / / 244. Am A

A m

2 4 4 C m

2

S

2 4 7

Cm Cm c l °c:

2 4 8

2

4

2 5

9

249 Bk Bk 249

res


Î-1

,

V if

1 4 C

process

) N

i-i i-i

the n u c l i d e i s formed from the precursor by neutron

capture.

System of Equations. In a target assembly which may c o n t a i n 20 or more n u c l i d e s i n s i g n i f i c a n t c o n c e n t r a t i o n s , a very complex system of l i n e a r d i f f e r e n t i a l equations with ( n e a r l y ) constant c o e f f i c i e n t s i s required to p r o p e r l y model the transmutation reactions. Various methods could be used to solve t h i s system of equations, but A. R. Jenkins, of the ORNL Mathematics D i v i s i o n , recommended f o r our p a r t i c u l a r type of problem that we use the a n a l y t i c s o l u t i o n to the Bateman Equations and that the cross s e c t i o n s which vary slowly as the composition changes be h e l d constant w i t h i n any one time step. The cross s e c t i o n s are then re-evaluated f o r each new time step as r e q u i r e d to maintain a r e a l i s t i c modeling. A c c o r d i n g l y , he modified the e x i s t i n g CRUNCH Code (13) to take account of the 2-group, 3-parameter cross s e c t i o n s as described above. When t h i s new program was f i r s t implemented i n 1964 (on the CDC 1604 côtoputer), a t y p i c a l run r e q u i r e d about 20 minutes. Today, on the IBM 360, Model 91, the same job would run i n a few seconds. A p p l i c a t i o n s of Computer Models P r e d i c t i o n of Target Compositions. One a p p l i c a t i o n of the computer program developed f o r mathematically modeling the transplutonium element transmutations i n a HFIR target i s that of p r e d i c t i n g the amounts of transplutonium elements which w i l l be a v a i l a b l e from a given i r r a d i a t i o n . This information i s then used i n the planning of processing campaigns. This i s a l s o the mechanism f o r v a l i d a t i n g the model by comparing c a l c u l a t e d and measured values. If s i g n i f i c a n t d i s c r e p a n c i e s a r i s e , some new values f o r parameters can be chosen and the process repeated u n t i l the c a l c u l a t e d values are acceptably c l o s e to the measured ones. Table II shows the comparison between c a l c u l a t e d and measured values f o r a recent campaign to process 13 HFIR targets. The exact i r r a d i a t i o n h i s t o r i e s were included i n the computation of each i n d i v i d u a l target assembly (with some m u l t i p l i c i t i e s ) and the r e s u l t s summed. It can be seen that agreement between c a l c u l a t e d and measured values up through mass 253 i s probably w i t h i n the range of a n a l y t i c a l u n c e r t a i n t i e s .


1.

BIGELOW E T A L .

High Flux

Isotope

13

Reactor


Q u a l i f i c a t i o n of Targets f o r I r r a d i a t i o n * A second a p p l i c a t i o n of the model i s the p r e d i c t i o n of f i s s i o n r a t e s (and hence heat f l u x e s ) f o r t a r g e t s being t r a n s f e r r e d to the r e a c t o r for irradiation. The allowable heat f l u x e s (14) were s e l e c t e d to prevent melting of aluminum at the center l i n e of the t a r g e t assembly and must not be exceeded at any time during the proposed i r r a d i a t i o n of the t a r g e t s . O p t i m i z a t i o n of I r r a d i a t i o n Times. By f a r the greatest usage of the calculâtional model has been to study the o p t i m i z a t i o n of i r r a d i a t i o n times. This i s a multi-dimensional problem of great complexity which has as i t s motivation the proper u t i l i z a t i o n of very expensive f a c i l t i e s and a very v a l u a b l e inventory of intermediate products, mainly the mixtures of curium isotopes. The problem does not lend i t s e l f to a complete s o l u t i o n ; however, various s i m p l i f y i n g approximations can be a p p l i e d to the problem to explore the i n t e r a c t i n g parameters. The f i r s t s i m p l i f i c a t i o n i s to adont C f as a y a r d s t i c k f o r p r o d u c t i v i t y . The n u c l i d e s past ^ C f i produced more or l e s s i n p r o p o r t i o n to the Cf. A l s o , i t i s the major source of p e n e t r a t i n g r a d i a t i o n so that many features of the design of TRU and some of the operating schedules were p r e d i c a t e d on the amounts of -* Cf ^ p i Various attempts have been made (6 15,16) to develop methods of maximizing the C f prod u c t i o n r a t e , u s u a l l y assuming an u n l i m i t e d supply of feed m a t e r i a l of a given composition. In the e a r l i e r years, the poor q u a l i t y of feed a v a i l a b l e put greater emphasis on t h i s approach. Since the C a l i f o r n i u m - I campaign (17) at the U.S. Department of Energy Savannah River s i t e and the consequent a v a i l a b i l i t y of a much b e t t e r q u a l i t y of feed, the emphasis i s s h i f t i n g toward e f f i c i e n t u t i l i z a t i o n of the f i n i t e inventory of curium feedstocks so as to be able to support the transplutonium e l e ment research program on a u s e f u l s c a l e f o r an extended p e r i o d of time. More d e f i n i t i o n s are necessary to attempt t h i s sort of optimization: P o t e n t i a l c a l i f o r n i u m i s a measure of the maximum amount of c a l i f o r n i u m that can be produced from a given batch of feed, taking i n t o account the f a c t that many atoms undergo f i s s i o n along the path from feed to product. The e f f i ciency of a p a r t i c u l a r i r r a d i a t i o n i s the amount of c a l i f o r n i u m produced d i v i d e d by the amount of p o t e n t i a l c a l i f o r n i u m consumed i n the i r r a d i a t i o n and subsequent p r o c e s s i n g . This e f f i c i e n c y measure takes i n t o c o n s i d e r a t i o n the d e s t r u c t i o n of the -> Cf ^ decay and neutron capture and processing l o s s e s of a l l the n u c l i d e s i n the chain. The e f f i c i e n c y defined i n t h i s manner v a r i e s with cumula t i v e i r r a d i a t i o n time as shown on F i g . 5. I n i t i a l l y zero, the e f f i c i e n c y r i s e s as more of the heavier intermediate n u c l i d e s 2 5 2

2

2

a

r

e

a

l

2 5 2

2

2

t Q

r

o

c

e

s

s

e

Production of Transplutonium Elements in the High Flux Isotope Reactor

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