38 Separation of Actinides from High Active Waste by Means of Counter Current Ion Migration B. A. BILAL, F. HERRMANN, K. METSCHER, B. MÜHLIG, CH. REICHMUTH, and B. SCHWARZ Nuclear Chemistry Division, Hahn-Meitner-Institut für Kernforschung, Berlin, Federal Republic of Germany, D-1000 Berlin 39 The use of nuclear power as energy source is determined by the safe handling and deposition of the nuclear waste. High active waste solutions must be transformed into stable solid form which is suitable for final disposal. The separation of the actinides from the waste before its solidification (e.g. vitrification) is advantageous (or may be even necessary) from two points of view: 1. The isolation of the long-lived and highly dangerous transuranium elements reduces the control time over the waste from the scale of millions to that of hundreds of years, since about 300 years are 10 x the half life time of the longest lived fission product Sr-90. The potential danger is reduced in the same scale. 2. No long-time experience exists about the stability and leaching of glasses containing the high α-active actinides. Most processes are developed for separation of an individual actinide or certain groups of actinides (e.g. U, Np and Pu by TBP extraction) from the waste. These processes are usually based on liquid/liquid
e x t r a c t i o n and ion exchange t e c h n i q u e . More o r less complicated processes l e a d t o the s e p a r a t i o n of only one t r a n s u r a n i u m element. I n these p r o c e s s e s t h e r a d i o l y t i c d e s t r u c t i o n o f t h e o r g a n i c s o l v e n t o r t h e i o n ex changer causes some d i f f i c u l t i e s . Using t h e counter c u r r e n t ion migration process (Jh-6) , all t h e a c t i n i d e s in t h e waste a r e s e p a r a t e d as group o r i n d i v i d u a l in one stage y i e l d i n g a h i g h decon t a m i n a t i o n f a c t o r (β*1θ6). The s e p a r a t i o n t a k e s p l a c e in aqueous medium, which is l e s s s e n s i t i v e t o t h e r a d i a t i o n e f f e c t s than o r g a n i c s u b s t a n c e s . The r a d i a t i o n p r o d u c t s o f t h e c a r b o x y l i c a c i d s o l u t i o n (mainly a c e t i c 0-8412-0527-2/80/47-117-561$05.00/0 © 1980 American Chemical Society
562
ACTINIDE SEPARATIONS
a c i d ) used as a c o u n t e r c u r r e n t l i q u i d a r e n e a r l y chem i c a l i n d i f f e r e n t . The s e p a r a t e d a c t i n i d e s a r e o f h i g h p u r i t y and a r e s u i t a b l e f o r r e c y c l i n g in r e a c t o r s ( 7 ) . That means t h e i r c o n c e n t r a t i o n in t h e environment r e mains n e a r l y s t a t i o n a r y in c i r c u l a r p r o c e s s .
P r i n c i p l e o f t h e Counter C u r r e n t
Ion M i g r a t i o n
In t h i s s e p a r a t i o n p r o c e s s a f l o w o f a s u i t a b l e s o l v e n t is d i r e c t e d a g a i n s t t h e i o n i c m i x t u r e m i g r a t i n g in the e l e c t r i c field. I f t h e f l o w r a t e is e q u a l t o the average m i g r a t i o n v e l o c i t y o f t h e m i x t u r e , t h i s r e mains s t a t i o n a r y in t h e s e p a r a t i o n column, w h i l e t h e f a s t e r moving components m i g r a t e a g a i n s t t h e stream, and t h e slower components a r e g r a d u a l l y f l u s h e d back. In t h i s way, it is p o s s i b l e , i f n e c e s s a r y , t o l e t t h e m i x t u r e m i g r a t e over a l o n g d i s t a n c e r e l a t i v e t o t h e s o l v e n t and so t o o b t a i n a v e r y h i g h s e p a r a t i o n f a c t o r . In most c a s e s , n o n - i s o t o p i c i o n s a r e s e p a r a t e d in v e r y h i g h p u r i t y . I n some r a r e c a s e s , in which t h e r e l a t i v e d i f f e r e n c e o f t h e m o b i l i t y is l e s s than 1% (e.g. L a n t h a n i d e s and some a c t i n i d e s ) pure components a r e o n l y o b t a i n e d i f t h e m o b i l i t y d i f f e r e n c e is i n c r e a s e d by a d d i t i o n a l e f f e c t s , such as d i f f e r e n t c o m p l e x a t i o n degree o f t h e components by means o f a s u i t a b l e l i g a n d . F i g . 1 shows a column type f o r d i s c o n t i n u o u s o r batchwise s e p a r a t i o n p r o c e s s . A t r o u g h o f p o l y p r o p y l e n e o r another s u i t a b l e m a t e r i a l is d i v i d e d v e r t i c a l l y t o t h e l e n g t h a x i s ( s e p a r a t i o n d i r e c t i o n ) by means o f diaphragms o f p o l y p r o p y l e n e gauze t o p r e v e n t t h e t h e r mal c o n v e c t i o n in t h i s d i r e c t i o n . The diaphragms a r e welded on p o l y p r o p y l e n e frames and a r e f i t t e d l i k e s l i d e s i n t o p l a t e s o f p o l y p r o p y l e n e through which t h e c o o l i n g p i p e s p a s s . The c o u n t e r c u r r e n t l i q u i d streams w i t h a c o n s t a n t r a t e from t h e cathode t o t h e anode i f c a t i o n s a r e t o be s e p a r a t e d and v i c e v e r s a . H o l d i n g t h e c u r r e n t s t r e n g t h and f l o w r a t e cons t a n t , a s e l f s t a b i l i z a t i o n mechanism l e a d s t o t h e e s t a b l i s h m e n t o f a s t a t i o n a r y and c o n s t a n t c o n c e n t r a t i o n p r o f i l e o f t h e i o n i c m i x t u r e as a whole a l o n g t h e column. W i t h i n t h i s p r o f i l e t h e s e p a r a t i o n o f t h e v a r i o u s components g r a d u a l l y t a k e s p l a c e . The p r o c e e d i n g f r a c t i o n a t i o n o f t h e i n i t i a l homogenous m i x t u r e l e a d s on t h e o t h e r hand t o t h e e s t a b l i s h m e n t o f i n c r e a s i n g c o n c e n t r a t i o n g r a d i e n t o f each component g i v i n g r i s e t o o p p o s i t e d i f f u s i o n t r a n s p o r t , c a u s i n g r e m i x i n g . The s e p a r a t i o n p r o c e s s is f i n i s h e d when t h e two t r a n s p o r t s
38.
BiLAL E T A L .
Counter-Current Ion
Migration
563
become e q u a l . The s e p a r a t i o n time is not o n l y a f u n c t i o n of the m o b i l i t y d i f f e r e n c e of the components, but a l s o of the c o n s t r u c t i o n s p e c i f i c a t i o n of the s e p a r a t i o n c o l u m n . F i n a l l y , the components are o b t a i n e d in zones which o v e r l a p s l i g h t l y due t o d i f f u s i o n . The con c e n t r a t i o n of each i o n in i t s pure zone is a f u n c t i o n of i t s t r a n s p o r t number, of the c u r r e n t s t r e n g t h and of the f l o w r a t e . The column shown in f i g . 1 is a l s o s u i t a b l e f o r c o n t i n u o u s s e p a r a t i o n of the m i x t u r e , but o n l y in two f r a c t i o n s l i k e the s e p a r a t i o n of the a c t i n i d e s as a group from the n u c l e a r waste.
P r o c e s s i n g of the
High A c t i v e
Waste
(HAW)
The HAW o f spent f u e l r e p r o c e s s i n g c o n s i s t s of a n i t r i c a c i d (Φ4Μ) s o l u t i o n of about 30 d i f f e r e n t i o n s , namely the a c t i n i d e s U, Np, Pu, Am and Cm, the f i s s i o n p r o d u c t s Cs, Rb, Sr, Ba, Mo, Nb, Zr, Te, Tc, Ru, Rh, Pd, Ag, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Dy and the c o r r o s i o n p r o d u c t s Mn, Cr, N i and Fe. T a b l e I p r e s e n t s the c o n c e n t r a t i o n of t h e s e components in a waste s o l u t i o n of r e p r o c e s s i n g of f u e l elements of l i g h t water r e a c t o r s , s u p p o s i n g a burnup of 33000 Mwd/ ton, a s p e c i f i c power of 30 Mw per ton Uranium (3,3% U-235), a c o o l i n g time of 1a and a waste volume o f 500 l i t r e / t o n f u e l . The l a s t column of t a b l e I c o n t a i n s the c h e m i c a l form of the components in the waste so lution. F i g . 2 shows the f l o w sheet of the HAW processing. The use o f the waste s o l u t i o n d i r e c t l y f o r s e p a r a t i o n l e a d s t o the d i s s i p a t i o n of avery l a r g e p a r t of the e l e c t r i c energy f o r the e l e c t r o l y t i c t r a n s p o r t of the hydrogen and n i t r a t e i o n s . The c o n c e n t r a t i o n of the n i t r i c a c i d is t h e r e f o r e reduced t o about O.05 M through d e n i t r a t i o n by means of f o r m i c a c i d . To p r e v e n t the hy d r o l y s i s and the p r e c i p i t a t i o n of some components, p a r t i c u l a r l y Pu, a c e t i c a c i d is added as complexing agent t o the s o l u t i o n b e f o r e d e n i t r a t i o n . The elements Nb, Te and Mo p a r t i a l l y p r e c i p i t a t e as o x i d e s . Se, Ag and Pd p a r t i a l l y p r e c i p i t a t e as m e t a l s . The r e s t of Mo, r e m a i n i n g in s o l u t i o n , is p r e s e n t as p o l y m o l y b d a t e which does not p r e c i p i t a t e Zr as Z r - p o l y m o l y b d a t e , s i n c e the pH of the s o l u t i o n is s t i l l about 1.2. Due t o the n i t r i c o x i d e s d e v e l o p e d d u r i n g the d e n i t r a t i o n the Pu is c o n v e r t e d t o Pu(IV) t h a t forms s t a b l e p o s i t i v e charged a c e t a t o - f o r m a t o - c o m p l e x e s , i f the c o n c e n t r a t i o n
564
ACTINIDE SEPARATIONS
Figure 1. Column for discontinuous or batchwise separation: 1, outside walls; 2, pump; 3, plates for dividing the columnincompartments; 4, counter current liquid; 5, cooling liquid; 6, frame with diaphragm.
CH C00H
waste solution
H
.
2
3
denitration ρΗ«1·2
Nb.Te.Se.Mo.Ag.Pd.Rd
Mo,Tc
1. S e p a r a t i o n step
(Cs,Rb)/Ba/FeNi/Zr/Sr/MnCr/RE/Cm/Am/Pu/Ru/Np/U l
(
2.Separation s t e p
oxalate
precipitation
thermal decomposition to oxide
Figure 2.
Flowsheet of the HAW processing
38.
BILAL ET AL.
Counter-Current Ion Migration
565
a n c
Table I . Concentration * c h e m i c a l form o f t h e components in t h e waste s o l u t i o n . element
c [g/i]
ion
form
U
O.84
uoi
Np
1 .52
NpOj
Pu
O.09
+
Pu
4 +
e l e C [g/l] ment
ion
4 +
Zr
5.30
Zr
Tc
1 .70
TCO4
Ru
1 .48
Rh
O.50
Pd
O.90 P d ( I V ) - n i t r a to-complexes
Ru(III)-NOnitrato-nitro-complexes
Am
O.36
Am
3+
Cm
O.06
Cm
3 +
Cs
2.60
Cs
+
Rb
O.44
Rb
+
Sr
1 .06
Sr
2 +
Ag
O.12
Ag
Ba
1.28
Ba
2 +
Mn
1.1
Mn
2 +
Y
O.5
Y
Cr
O.01
Cr
3 +
Ni
O.004
Ni
2 +
Fe
O.05
Fe
3 +
3 +
Rare Earths (RE)
11 .40
Mo
3.20
M0O2
Nb
O.03
NbOj
Se
O.005
SeO§"
Te
O.40
TeO§"
RE
3 +
form
Rh(III)
+
2+
o f b o t h t h e a c e t i c and t h e f o r m i c a c i d is about O.5 M. The s t a b i l i t y o f t h e Pu complexes over s e v e r a l days was examined p r e v i o u s l y by means o f a b s o r p t i o n s p e c t r o s c o p y . N e i t h e r a s h i f t o f t h e a b s o r p t i o n band a t λ = 457 nm nor new bands were found. I f t h e r e is no i n t e r e s t t o g a i n t h e elements Pd and Rh s e p a r a t e l y , they a r e p r e c i p i t a t e d in t h e d e n i t r a t i o n s t e p by means o f hydrogen gas which is a c t i v a t e d on p a s s i n g a p l a t e o f s i n t e r g l a s s on which Pd was a l r e a d y p r e c i p i t a t e d . Ru remains in t h e s o l u t i o n as R u - N O - n i t r a t o - n i t r o - c o m p l e x e s . Tc is p r e s e n t as t h e p e r t e c h n i t a t e i o n . A l l o t h e r compo nents a r e p r e s e n t in t h e d e n i t r a t e d s o l u t i o n as s i m p l e o r p a r t i a l l y complexed c a t i o n s . A f t e r f i l t r a t i o n t h e multicomponent m i x t u r e is then s e p a r a t e d in a column shown in f i g . 1 . A m i x t u r e o f a c e t i c a c i d (O.5M) and n i t r i c acid(O.05M) is used as c o u n t e r c u r r e n t l i q u i d (cathode — > a n o d e ) .
566
ACTINIDE SEPARATIONS
Discontinuous
Separation
One l i t r e o f waste s o l u t i o n c o n t a i n i n g the components a t the c o n c e n t r a t i o n g i v e n in t a b l e I is p r o c e s s e d a c c o r d i n g t o f i g u r e 2. A f t e r about 1h o p e r a t i n g time Mo and Tc are c o m p l e t e l y washed out o f the column. A f t e r f u r t h e r 20h the maximum s e p a r a t i o n degree o f the zones (cathode — • a n o d e ) Rb, Cs/Ba/Fe, Ni/Zr/Sr/Mn, Cr/RE/Cm/Am/Pu/Ru/Np/U is o b t a i n e d ( s . f i g . 3 . I ) . The c o u n t e r c u r r e n t o v e r f l o w is h i g h l y decontaminated from all components and is t h e r e f o r e r e c y c l e d ( s . f i g . 1 ) . The o v e r l a p p i n g ranges of the a c t i n i d e s zones are sucked out o f the first column and s e p a r a t e d in a s e cond one t h a t has such a s m a l l c r o s s s e c t i o n t o g e t the l a r g e s t s p r e a d i n g of the pure zones, b u t a s m a l l o v e r l a p p i n g range. In a r u n n i n g b a t c h w i s e s e p a r a t i o n , both columns a r e o p e r a t e d s i m u l t a n e o u s l y . The pure zones of the a c t i n i d e s a r e then e x t r a c t e d out o f the column and p r e c i p i t a t e d as o x a l a t e . The o v e r l a p p i n g zones are mixed a g a i n t o the f e e d s o l u t i o n of the first s e p a r a t i o n s t e p . More than 98% of the a c t i n i d e s a r e s e p a r a t e d in the two s t e p s and a r e decontaminated from the f i s s i o n and c o r r o s i o n p r o d u c t s as w e l l as from each o t h e r by the f a c t o r > 1 0 . The r a r e e a r t h s are d e c o n t a minated from each o t h e r by the f a c t o r % 1 0 . The decont a m i n a t i o n f a c t o r of the o t h e r f i s s i o n p r o d u c t s is about >10 . 6
3
6
S e p a r a r a t i o n Data l . s t e p : Length o f the column. (L)=200 cm, f r e e c r o s s s e c t i o n (q)=5 cm , flow r a t e (V) = 40 ml/h, h i g h v o l t a g e (U)=2000 V, c u r r e n t s t r e n g t h (I)=O.2 A, s e p a r a t i o n time (t)=21h. 2 . s t e p : L=200 cm, q=O.5cm , V~=4 ml/h, U=2000 V, 1=O.02 A, t=21h. 2
2
Continuous
Separation
F i g . 4 shows s c h e m a t i c a l l y the arrangements used f o r c o n t i n u o u s s e p a r a t i o n of the a c t i n i d e s as a group from the waste. A s e p a r a t i o n column (a)(L=100 cm) is c o n n e c t e d t o the v e s s e l ( b ) c o n t a i n i n g the waste s o l u t i o n . O p p o s i t e t o it t h e r e is a second column(c) (L=10 cm) h a v i n g the same c r o s s s e c t i o n . The c o u n t e r c u r r e n t l i q u i d (O.5M a c e t i c a c i d + O.05M n i t r i c a c i d ) streams from the cathode t o the anode. The column (c) c o n t a i n s a zone of an i o n of h i g h m o b i l i t y (e.g. K, Cs, Rb) t o p r o t e c t the components in (b) a g a i n s t cathode c o n t a c t . In the column (a) the a c t i n i d e s as a group, Ru, Tc and Mo a r e s e p a r a t e d from the o t h e r components.
38.
BiLAL E T A L .
Counter-Current Ion Migration
567
I Figure 3. Stationary distribution of the separated componentsina discontinuous process: I,firststep; II, second step.
Figure 4. Equipment for continuous separation
568
ACTINIDE SEPARATIONS
Ru is q u a n t i t a t i v e l y o x i d i z e d t o the v o l a t i l e R U O 4 on p a s s i n g the anode chamber. The o v e r f l o w p a s s e s an a n i o n exchanger (d) t o h o l d back Tc and Mo. The pure a c t i n i d e s are p r e c i p i t a t e d as o x a l a t e which decomposites t h e r m a l l y t o the o x i d e s . The d e c o n t a m i n a t i o n factor a c t i n i d e s / f i s s i o n - and c o r r o s i o n p r o d u c t s is £106. The s e p a r a t i o n p r o c e s s s t a r t s as f o l l o w s : F i r s t a d i s c o n t i n u o u s s e p a r a t i o n is c a r r i e d out in the column (a) u s i n g the same v a l u e s of I/q and V as mentioned above. These v a l u e s a r e then c o r r e c t e d t o a l l o w a con t i n u o u s s e p a r a t i o n t a k i n g p l a c e between the REE- and Cm/Am-zones, where the c o n d i t i o n
b E
u
' REE
>
v
>bE
u
- Cm/Am
( 1 )
must be f u l f i l l e d . Ε is the e l e c t r i c field s t r e n g t h a t the c o n t a c t p o i n t between the column (a) and the v e s s e l (b) . I t s v a l u e is g i v e n by the r e l a t i o n Ε = I/q*
,
(2)
where is the s t a t i o n a r y c o n d u c t i v i t y o f the s o l u t i o n in the v e s s e l ( b ) . The m o b i l i t y u is d e t e r m i n e d from measurement of £ o f the c o r r e s p o n d i n g component in i t s pure zone o b t a i n e d by means o f d i s c o n t i n u o u s s e p a r a t i o n and c a l c u l a t i o n a c c o r d i n g t o u
L
=
V #/I
(3)
2 S e p a r a t i o n Data q (of (a) and (c) = 5 cm , U = 1200 V, I = O.22 A, V = 25 ml/h, M ( s e p a r a t e d a c t i n i d e s [mole/d]) = O.05. E s t i m a t i o n o f Energy n e c e s s a r y
for
Separation
A c c o r d i n g t o a l i n e a r e x t r a p o l a t i o n o f the s e p a r a t i o n d a t a , about 10"^ o f the energy g a i n e d p e r one t o n o f f u e l in l i g h t water r e a c t o r s (s.above) must be used f o r the p r o c e s s i n g o f i t s h i g h a c t i v e waste. T h i s seems t o be h i g h . On the o t h e r hand, t h i s method makes it p o s s i b l e t o s e p a r a t e all the a c t i n i d e s s i m u l t a n e o u s l y w i t h a h i g h d e c o n t a m i n a t i o n f a c t o r . I t is easy t o ope r a t e the s e p a r a t i o n equipment a u t o m a t i c a l l y and so t o have low p e r s o n n e l c o s t .
nique
However, the development of the l a r g e s c a l e t e c h and the c o n s t r u c t i o n of s e p a r a t i o n columns from
38.
BILAL
ET
Counter-Current Ion Migration
AL.
materials more resistant to question.
Literature
to radiation
569
are s t i l l
open
Cited
1.
Brewer,A.K., Madorsky,S.L., Science 1946 104, 156
2.
Martin,Η.,
Ζ.Naturforsch.
3.
ClusiusK.,
Ramirez,E.R.,
and Westhaver,J.W., 1949
4a, 28
Helv.Chim.Acta
1953 36,
1160
4.
Wagener,K., Ζ.Elektrochem., Chem. 1960 64, 922
5.
Wagener,K., "Report of the Hahn-Meitner-Institut für Kernforschung Berlin", Rept. HMI-B 44 (1965)
6.
Bilal,B.A.,
7.
Cunningham,G.W., IAEA-CN
RECEIVED
May
14,
Ber.Bunseges.Physik.
Chem.Ing.Techn.
1979.
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36/561
