15 A Nonaqueous Reprocessing Method for Thorium-Based Fuels J. K. B A T E S , L . J. J A R D I N E , and M . K R U M P E L T Chemical Engineering Division, Argonne National Laboratory, Argonne, I L 60439
Programs have been active periodically to develop a thorium-based fuel cycle for increasing the nation's resource of fissile material. Part of the effort has been the investigation of non-aqueous reprocessing methods for separating thorium from uranium and the behavior of these elements in the presence of fission products and other impurities (1). Interest in such pyrometallurgical methods and the entire thorium fuel cycle declined in the early 1970s, primarily due to the lower breeding ratio of the Th- U cycle compared to that of the U- Pu cycle, and the consequent effort placed on developing the PuO -UO fuel cycle for fast breeder reactors. The recent advent of proliferation concerns has renewed the interest in the thorium fuel cycle and associated pyrometallurgical reprocessing. One reprocessing scheme which addresses the current problem is described below. The effect of proliferation concerns on the acceptability of a fuel cycle, and the development of aqueous Civex processes designed to meet these concerns have been outlined previously (2). 233
238
239
2
2
One new c r i t e r i o n i m p o s e d o n r e p r o c e s s i n g b y p r o l i f e r a t i o n i s s u e s i s t h a t t h e p r o c e s s s t r e a m s be d i v e r s i o n a n d p r o l i f e r a t i o n r e sistant. F o r t h o r i u m - b a s e d f u e l c y c l e s , t h i s means t h a t t h o s e products containing f i s s i l e m a t e r i a l , U or P u , must b e c o p r o c e s s e d w i t h t h o r i u m o r be d e n a t u r e d w i t h U . In a d d i t i o n , r e p r o c e s s i n g schemes must t a k e i n t o a c c o u n t s p e c i a l f e a t u r e s o f the t h o r i u m f u e l c y c l e , s u c h as t h e b u i l d u p o f P a which decays to U . The b r e e d i n g c h a i n f o r U i s as f o l l o w s : 2
3
3
2
2
3
3
9
8
2
2
3
3
2
3
3
3
3
Since P a i s a n e u t r o n p o i s o n , (3) a s i g n i f i c a n t w a i t i n g p e r i o d may b e n e c e s s a r y b e f o r e t h e r e p r o c e s s e d f u e l w o u l d b e r e t u r n e d t o the r e a c t o r . S e p a r a t i o n of the P a would a l l o w a q u i c k r e t u r n to the r e a c t o r , b u t would l e a d to a pure U p r o d u c t , and t h u s i n c r e a s e the p r o l i f e r a t i o n r i s k . Another f e a t u r e of the thorium f u e l c y c l e which a f f e c t s r e p r o c e s s i n g i s the b u i l d u p of U i n the i r r a d i a t e d f u e l . 2
3
3
2
3
3
2
2
3
3
3
2
0-8412-0527-2/80/47-117-207$05.00/0 ©
1980 A m e r i c a n C h e m i c a l Society
208
ACTINIDE SEPARATIONS
E q u i l i b r i u m concentrations of U may be r e a c h e d i n a few y e a r s d u r i n g f u e l i r r a d i a t i o n b u t e v e n 100 day i r r a d i a t i o n s may p r o d u c e 2
substantial
concentrations
of
3
2
2 3 2
U(4).The
decay
of
the
short
half
l i f e of U l e a d s to a l o n g decay c h a i n of n u c l i d e s which i n c l u d e a p e n e t r a t i n g 2 . 6 MeV gamma r a y . This w i l l l i k e l y require 2
3
2
s h i e l d i n g and remote h a n d l i n g o f r e p r o c e s s i n g and r e f a b r i c a t i o n .
the
f u e l d u r i n g a l l phases
of
A d e s i r a b l e f e a t u r e f o r the r e p r o c e s s i n g method i s t h a t i t s h o u l d be a d a p t a b l e to v a r i o u s t h o r i u m f u e l c y c l e s and to s e v e r a l f u e l t y p e s , a n d i t must b e a b l e t o p r o d u c e a p r o d u c t w i t h a h i g h enough f i s s i l e content f o r core f u e l s p e c i f i c a t i o n s . As an e x ample, a f u e l c y c l e t h a t i n c l u d e s d i f f e r e n t needs i s a s y m b i o t i c c y c l e where P u which i s produced i n e x i s t i n g l i g h t water r e 2
3
9
a c t o r s (LWR) i s c o m b i n e d w i t h t h o r i u m a n d b u r n e d i n F B R s . The i r r a d i a t e d c o r e f u e l f r o m t h e FBR w o u l d c o n t a i n p l u t o n i u m , uranium, and t h o r i u m w h i l e the b l a n k e t c o n t a i n s U and t h o r i u m . The p r o c e s s f o r t h e FBR f u e l must t h e r e f o r e meet two d e m a n d s : one p r o d u c t s t r e a m must b e s u i t a b l e f o r HTGR o r LWR f u e l s a n d t h e 2
3
3
o t h e r f o r r e t u r n t o t h e FBR c o r e . T h e a c t i n i d e s i n t h e s p e n t FBR f u e l s h o u l d t h e r e f o r e be s e p a r a t e d i n t o a u r a n i u m - t h o r i u m p o r t i o n ( f o r HTGR o r LWR) a n d a p l u t o n i u m - t h o r i u m p o r t i o n ( f o r FBR core). Process
of
Description
A c o n c e p t u a l p y r o m e t a l l u r g i c a l method f o r thorium-based f u e l s i s presented i n F i g . 1.
the It
reprocessing is responsive
to the c o n s t r a i n t s d e s c r i b e d p r e v i o u s l y , b e i n g o p e r a b l e w i t h e i t h e r o x i d e o r m e t a l a l l o y f u e l , and p r o d u c i n g p r o d u c t s t r e a m s c o n s i s t i n g o f e n r i c h e d u r a n i u m / t h o r i u m and p l u t o n i u m / t h o r i u m . The p r o c e s s c a n b e d i v i d e d i n t o t h r e e m a i n s t e p s : (1) o x i d e r e d u c t i o n o r m e t a l d i s s o l u t i o n , (2) u r a n i u m / p l u t o n i u m s e p a r a t i o n and t h o r i u m p a r t i t i o n , a n d (3) p r o d u c t r e c o v e r y . Each step i s described 1. method
below. Assuming the
f u e l to
be d e c l a d
using
compatible w i t h subsequent
Reduction.
process
steps,
must
converted
to
metallic
reduction
of
the
2Ca + M 0 Carbides
2
can be
form.
For
an o x i d e
fuel,
it this
a then
involves
be a
type
+ M + 2Ca0 converted
M = Th, U, Pu, to
oxides
and r e d u c e d
or as
FP above.
(1) Metal
a l l o y f u e l s a r e a l r e a d y i n s u i t a b l e f o r m a n d c a n be p r o c e s s e d directly. C a l c i u m i s u s e d as t h e r e d u c i n g a g e n t as t h e f r e e energy of R e a c t i o n 1 i s -6.6 k c a l / m o l e at 1 0 0 0 ° C . The o t h e r f u e l and f i s s i o n p r o d u c t o x i d e s a r e r e d u c e d more e a s i l y t h a n T h 0 2 , thus the r e d u c t i o n s h o u l d be complete once the t h o r i a i s r e d u c e d . The r e d u c t i o n p r o c e e d s v i a t h e a d d i t i o n o f c a l c i u m t o magnesium b a s e d a l l o y w h i c h i s c o v e r e d w i t h a f u s e d s a l t
a
FUEL
OXIDE
CLAD
DECLADDING
FP-1
OXIDES
SALT
Cd-Mg*
METAL FUEL
CLADDING
SALT
C
FP-2
SALT
SALT
FP-2
CaO
FP-1
Figure 1.
SALT
FP-2
DISSOLUTION
METAL
FP-1
REDUCTION
OXIDE
Ca SALT
ELECTROLYSIS
CALCIUM
COo
FP-4
Th
U
SEPARATION
Pu/U
FP-3, 4
Pu, ΤΗ, Cd-Mg
Zn-Mg
FP-3
FP-3 SALT (OXIDATION)
3
FP-3, 4 Cd-Mg
Pu.Th
(FP-3)CI
SALT
TRANSPORT
SALT
(REDUCTION)
TRANSPORT
SALT
FP-3
Zn-Mg
A conceptual flow sheet for Th-based oxide and metal fuels
Cd-Mg
Pu, U, Th
FP-3, 4
Cd-Mg RECYCLE
RETORT
U/Th
_L_
Cd-Mg RECYCLE
RETORT
Putfh
U/Th
FP-3, 4
Pu/Th
(Ή
ο
Ο
ο
>
I
ÇA
ACTINIDE SEPARATIONS
210 The s a l t
(CaCl2, CaF2).
dium f o r t h e r e d u c t i o n . the
reduction
products
It
and takes
(FP-2)
wets
the oxide
also
of
and a l s o
a buildup of FP-2.
the iodine
fully
The v a r i a b l e s
(5).
The s a l t
and needs discard
earth
during
fission
represents
the
to be b l e d o f f because
rates
by e l e c t r o l y s i s
controlling
as a me-
are not necessary
and t h e c a l c i u m
reduction
are discussed
is remore
later. 2.
step,
High s a l t
t h e CaO i s removed
cycled.
and s e r v e s
t h e CaO p r o d u c e d
up t h e a k l a k i a n d a l k a l i n e
major waste stream i n the process because
fuel
dissolves
Uranium/Plutonium/Thorium
the metals
magnesium provides
partition
alloy.
Partition.
according
Magnesium i s used
a means o f s e p a r a t i n g
After
to t h e i r
the
solubility
reduction i n the
as the f o c u s m e t a l because
uranium from
it
plutonium.
The s o l u b i l i t i e s o f u r a n i u m , p l u t o n i u m , a n d t h o r i u m i n magn e s i u m a t 6 5 0 ° C a r e 0 . 0 0 2 wt %, 55 wt %, a n d 44 wt %, r e s p e c tively. T h u s , a s s u m i n g no s o l u t e i n t e r a c t i o n , uranium i s e s s e n t i a l l y i n s o l u b l e i n magnesium, w h i l e p l u t o n i u m i s q u i t e s o l u b l e and g o o d s e p a r a t i o n may b e e f f e c t e d . While p r e c i p i t a t i o n o f an i n s o l u b l e phase from s o l u t i o n would appear t o be a s t r a i g h t f o r ward p r o c e s s , t h e b e h a v i o r o f a s o l u t e i n a g i v e n m e t a l o r a l l o y may d i f f e r f r o m i t s b e h a v i o r when i n f l u e n c e d b y t h e i n c l u s i o n o f other solutes. One e l e m e n t may i n c r e a s e o r s u p p r e s s t h e s o l u b i l i t y of another pound f o r m a t i o n .
through c o p r e c i p i t a t i o n o r i n t e r m e t a l l i c comSuch e f f e c t s must b e d e t e r m i n e d experimentally.
U n d e r p r o p o s e d p r o c e s s c o n d i t i o n s t h e amount o f t h o r i u m f a r exceeds o t h e r elements. To o b t a i n t h e n e c e s s a r y d i v i s i o n o f t h o r i u m between the minor uranium and p l u t o n i u m c o n s t i t u e n t s , o n l y a method o f c o n t r o l l i n g t h e q u a n t i t y o f t h o r i u m i n s o l u t i o n i s needed. T h i s c a n b e a c h i e v e d b y c o n t r o l l i n g t h e amount o f l i q u i d magnesium s o l v e n t . By l i m i t i n g t h e v o l u m e o f t h e l i q u i d m a g n e s i u m , t h o r i u m can be d i v i d e d i n t o a s o l u b l e and i n s o l u b l e f r a c tion. E v e n more c o n t r o l c a n b e g a i n e d magnesium. This alloy solvent retains
b y a l l o y i n g cadmium w i t h the low s o l u b i l i t y o f u r a -
nium and complete s o l u b i l i t y o f p l u t o n i u m b u t a l l o w s t h e t h o r i u m s o l u b i l i t y t o b e v a r i e d b e t w e e n 5 a n d 42 wt % ( F i g . 2 ) . I t i s a l s o c o m p a t i b l e w i t h t h e r e d u c t i o n and r e t o r t i n g operations. As a n e x a m p l e , f o r o n e m e t r i c t o n o f p l u t o n i u m / t h o r i u m (1:4) f u e l w i t h a b u r n u p o f 55 000 MWD t h e r e s u l t i n g c o r e c o m p o s i t i o n (wt %) i s T h - 7 5 . 5 , P u - 1 4 . 6 , F P - 5 . 9 , U - 3 . 7 , a n d P a - 0 . 3 ( 6 ) . T h u s produce p r o d u c t streams p l u t o n i u m / t h o r i u m (1:4) and uranium/ thorium (1:4.5) a l l o y at 600°C.
the process
requires
The n e c e s s a r y n o n p r o l i f e r a t i o n
1600 k g o f C d - 5 0 constraints
to
wt % Mg
are provided by
the h i g h r a d i o a c t i v i t y o f each p r o d u c t s t r e a m , and t h e remote h a n d l i n g requirement of each process s t e p . In a d d i t i o n , both the f i s s i l e uranium and p l u t o n i u m a r e c o p r o c e s s e d w i t h t h o r i u m . Pure p l u t o n i u m cannot be o b t a i n e d because b o t h thorium and p l u t o n i u m have a l a r g e s o l u b i l i t y i n the s o l v e n t a l l o y . Pure uranium c o u l d i n p r i n c i p l e be obtained by r e p e a t e d l y washing the uranium-
15.
BATES E T AL.
Nonaqueous
thorium p r e c i p i t a t e with practice
such
protactinium,
3.
which decays
is
Recovery.
a solid liquid
uranium/thorium product
to
i s retorted
of Th
Fuels
211
l i q u i d magnesium. 2
3
3
U ,
a source of clean
Product
settled
fresh
s e p a r a t i o n s have proven
does n o t p r o v i d e
has
Processing
After
However,
d i f f i c u l t (7).
remains
with
(8) .
the uranium/thorium is effected.
t o remove e n t r a i n e d
the
t h e uranium and
f i s s i l e material
separation
in
Finally,
precipitate
The r e m a i n i n g
solvent
and t h e
recovered.
The a l l o y i n w h i c h t h e p l u t o n i u m / t h o r i u m i s d i s s o l v e d a l s o contains the s o l u b l e r a r e earth f i s s i o n products (FP-3). A fract i o n o f these r a d i o a c t i v e products are s e l e c t i v e l y e x t r a c t e d from t h e a l l o y u s i n g a " s a l t - t r a n s p o r t " p r o c e s s (90. T h e r e m a i n d e r o f the F P - 3 f i s s i o n p r o d u c t s s t a y s w i t h t h e p l u t o n i u m / t h o r i u m stream to p r o v i d e d i v e r s i o n r e s i s t a n c e . The p l u t o n i u m / t h o r i u m r e m a i n i n g i n the o r g i n a l a l l o y i s then r e c o v e r e d by r e t o r t i n g the a l l o y . The s o l v e n t i n b o t h r e t o r t i n g s t e p s i s r e c y c l e d t o t h e b e g i n n i n g of the p r o c e s s . Process
Chemistry
The d i r e c t i o n o f e a c h f u e l c o n s t i t u e n t can be e x e r c i s e d o v e r i t d u r i n g t h e p r o c e s s
and t h e c o n t r o l w h i c h i s summarized b e l o w .
F P - 1 ( k r y p t o n , x e n o n , h y d r o g e n ) — a r e removed d u r i n g d e c l a d and r e d u c t i o n and a r e t r e a t e d as o f f g a s e s . FP-2 ( i o d i n e , cesium, rubidium, strontium, barium, europium, s a m a r i u m ) — a r e removed d u r i n g t h e r e d u c t i o n s t e p b y e x t r a c t i o n ding
into the s a l t phase. There w i l l be an a c c u m u l a t i o n o f F P - 2 f i s s i o n products i n the s a l t which i s c o n t r o l l e d by a continuous salt bleed. F o r 1000 k g o f f u e l , 2000 k g o f s a l t a r e r e q u i r e d . To k e e p a c o n s t a n t l e v e l o f 10% F P - 2 i n t h e s a l t a b l e e d o f 200 k g of s a l t / b a t c h i s n e c e s s a r y . F P - 3 ( y t t r i u m , l a n t h a n u m , c e r i u m , praseodymium, neodymium, p r o m e t h i u m , g a d o l i n i u m , t e r b i u m ) — a r e s o l u b l e (^15 w t %) i n t h e p r o c e s s a l l o y a n d a r e removed b y s e l e c t i v e e x t r a c t i o n i n t o t h e s a l t a f t e r the r e d u c t i o n . T h i s i s a s a l t - t r a n s p o r t s t e p and i s used as t h e method o f c o n t r o l l i n g t h e F P - 3 c o n c e n t r a t i o n i n t h e p r o c e s s stream and o f c o n s o l i d a t i n g the F P - 3 f o r waste h a n d l i n g . A n o m i n a l amount o f F P - 3 r e m a i n s i n t h e p l u t o n i u m / t h o r i u m s t r e a m for diversion resistance. F P - 4 ( z i r c o n i u m , n i o b i u m , molybdenum, t e c h n e t i u m , ruthenium, r h o d i u m , p a l l a d i u m , s i l v e r , cadmium, i n d i u m , t i n , a n t i m o n y ) — a r e o n l y s l i g h t l y s o l u b l e (
