Actinide Separations - American Chemical Society

31 Removal of Americium and Curium from High-Level

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Wastes W. D. BOND and R. E. L E U Z E Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, T N 37830

A number of potential methods for removing americium and curium from high-level liquid waste have been investigated at Oak Ridge National Laboratory for their applicability to waste partitioning (1, 2, 3, 4,). Processes acceptable for americium-curium removal must give a high-degree of recovery and produce a semi-pure, concentrated product of these actinides. All separations methods investigated consisted of two general process steps. First, the trivalent actinide and lanthanide elements are separated from the other elements in the waste. In the second step, americium and curium are then separated from the lanthanide elements. Experimental studies have largely been laboratory-scale in which synthetic waste solutions and tracer levels of radioactivity were utilized. A few laboratory-scale experiments were made in hot cells on the coextraction of trivalent actinides and lanthanides. The two most promising methods investigated for co-removal of trivalent actinides and lanthanides are: 1. A solvent extraction process (4, 5) utilizing dihexyl[(diethylcarbamoyl)methyl]phosphonate (DHDECMP) as the extractant. This extractant is also called dihexyl-N,N-diethylcarbamylmethelenephosphonate. 2. The OPIX process (6, 7), which is based on an oxalate precipitation coupled with a cation exchange treatement of the supernatant liquid. Studies U, 2, 3, 4) on the separation of americium-curium from lanthanide elements indicate that both cation exchange chromatography (8, 9) and the Talspeak solvent extraction process (JJO, 11) are promising methods. Only the most recent work at Oak Ridge National Laboratory is reported in this paper. Potential chemical processes for americium-curium removal and evaluations of their feasibility have been reported previously (1_, 2, 3^, _4). The most recent experimental work carried out includes the following: 1. Hot-cell studies of the DHDECMP extraction process. 2. Feasibility studies of continuous precipitation of oxalates in the OPIX process. 3. Studies of Talspeak process flowsheets in continuous, 0-8412-0527-2/80/47-117-441$05.00/0 ©

1980 American Chemical Society

Navratil and Schulz; Actinide Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

442

ACTINIDE

SEPARATIONS

c o u n t e r c u r r e n t , m i x e r - s e t t l e r equipment and in batch e x t r a c t i o n tests. 4. E f f e c t s o f i m p u r i t i e s d e r i v e d from DHDECMP degradation on the ion exchange loading step of the c a t i o n exchange chromatography process.


H o t - C e l l Studies of the DHDECMP Solvent E x t r a c t i o n Process The e x t r a c t i o n and s t r i p p i n g of t r i v a l e n t a c t i n i d e s and l a n thanides were studied using h i g h - l e v e l l i q u i d waste (HLLW) d e r i v ed from spent LWR fuel (31,000 MWd/MT burnup and decayed f o r 4 y e a r s ) . A f t e r seven batch e x t r a c t i o n stages, the Am-Cm remaining in the waste was 99.9% of the t r i v a l e n t a c t i nides and lanthanides using the OPIX process had p r e v i o u s l y been demonstrated by Campbell (6_, 7) in s m a l l , batch t e s t s in the l a b o r a t o r y and in hot c e l l s . About 90-95% o f the t r i v a l e n t elements were removed in the p r e c i p i t a t i o n s t e p , and the small amount r e maining in the supernatant l i q u i d was removed by a c a t i o n exchange column. I t was t h e r e f o r e of i n t e r e s t to determine whether the o x a l a t e p r e c i p i t a t i o n step could be performed continuously since continuous methods a f f o r d many advantages with respect to both process scale-up and o p e r a b i l i t y . Since it was first necessary to demonstrate a continuous p r e c i p i t a t i o n concept t h a t was b a s i c a l l y sound, s t u d i e s were conducted with s y n t h e t i c wastes. Synthetic wastes were prepared to correspond c h e m i c a l l y to HLLW derived from LWR fuel having a burnup of 33,000 MWd/MT. Methods of preparation o f t h i s s y n t h e t i c s o l u t i o n were described p r e v i o u s l y (2). Elemental compositions are given in Table I. These i n i t i a l t e s t s


BOND A N D L E U Z E

Am and Cm in High-Level Wastes

443

'f t


Q LU

STAGE NUMBER Figure 1. Batch cross current extraction of Am-Cm from high-level liquid waste with 30% DHDECMP: 3M HN0 ; 25°C; organic-to-aqueous phase ratio = I. 3

STAGE NUMBER Figure 2.

Batch cross current stripping of Am-Cm from 30% DHDECMP with O.05M HNO : 25°C; organic-to-aqueous phase ratio = O.5. s


444

ACTINIDE SEPARATIONS

Table I.

S y n t h e t i c Waste S o l u t i o n Composition HN0 = 2.5 M 3

Elements


Rare earths O.205 O.398 O.196 O.660 O.143 O.0275 O.019 O.0002 O.000015 O.000005

Lanthanum Cerium Praseodymium Neodymium Samarium Europium Gadolinium Dysprosium Hoi mi urn Erbium Group VIII metals

O.344 O.0625 O.228

Ruthenium Rhodium Palladium Group I A a l k a l i

earths

Rubidium Cesium Group II

O.0535 O.390

a l k a l i n e earths

Strontium Bari urn

O.1315 O.268

Other elements Zirconium Indium Yttrium Silver Cadmium Arsenic Antimony Molybdenum Selenium Tellurium Tin


O.585 O.0002 O.0755 O.0095 O.0185 O.000015 O.002 O.55 O.008 O.0905 O.008


31.

BOND A N D L E U Z E

Am and Cm in High-Level Wastes

445

made only w i t h n o n - r a d i o a c t i v e s y n t h e t i c waste s o l u t i o n s i n d i c a t e t h a t continuous p r e c i p i t a t i o n o f t r i v a l e n t a c t i n i d e and l a n t h a n i d e o x a l a t e s appears f e a s i b l e . Important e f f e c t s t h a t can be expected by the i n t e n s e r a d i a t i o n a s s o c i a t e d w i t h h i g h - l e v e l waste are gen e r a t i o n of heat w i t h i n the o x a l a t e p r e c i p i t a t e and conversion of o x a l a t e ions to gaseous CO2 and H2O. The experimental equipment used in studying the continuous o x a l a t e p r e c i p i t a t i o n and the separation o f the p r e c i p i t a t e from the l i q u i d is d e p i c t e d s c h e m a t i c a l l y in Figure 3. The equipment allowed f o r options o f f i l t e r i n g or s e t t l i n g the p r e c i p i t a t e and the use o f e i t h e r one or two s t i r r e d tank r e a c t o r s . The f o l l o w i n g v a r i a b l e s were s t u d i e d : 1. O x a l i c a c i d c o n c e n t r a t i o n (O.2 to O.3 M). 2. Temperature (25 to 50°C). 3. Degree o f mixing (125 to 250 rpm). 4. Residence time (15 to 40 m i n . ) . The r o t a t i o n a l speeds of the s i x - b l a d e d s t i r r e r s t h a t were used corresponded to power inputs of O.02 and O.18 w a t t / l i t e r a t speeds of 125 and 250 rpm, r e s p e c t i v e l y . Concentrations of o x a l i c a c i d during p r e c i p i t a t i o n and c r y s t a l growth in the s t i r r e d tank r e a c t o r s were v a r i e d by changing the flow r a t i o o f o x a l i c a c i d - t o waste s o l u t i o n w h i l e m a i n t a i n i n g the n i t r i c a c i d c o n c e n t r a t i o n constant at O.9 M. P e r m i s s i b l e n i t r i c a c i d c o n c e n t r a t i o n s f o r the OPIX process (4, 6, 7) are O.5 to about 1.0 M HN0 . Y i e l d s o f p r e c i p i t a t e were determined on the b a s i s of praseodymium recovery. Tracer P r ( h a l f - l i f e = 19.2 day, 1.6-MeV γ-ray) was used to measure y i e l d s . The best o p e r a t i n g c o n d i t i o n s over the range o f v a r i a b l e s in v e s t i g a t e d were (12): 1. Two s t i r r e d tank r e a c t o r s in s e r i e s . 2. An o x a l i c a c i d c o n c e n t r a t i o n of O.3 M. 3. Temperature o f 25°C. 4. Residence time o f 40 min. S t i r r e r speeds were not s i g n i f i c a n t in the first s t i r r e d tank r e a c t o r , but the highest speed (250 rpm, O.18 w a t t / l i t e r ) gave s l i g h t l y b e t t e r performance in the second r e a c t o r under some of the t e s t c o n d i t i o n s (12.). C o l l e c t i o n of the p r e c i p i t a t e by the g r a v i t y s e t t l e r was not n e a r l y as e f f e c t i v e as the s e r i e s of 12, 5, and l - y - d i a m e t e r M i l l i p o r e f i l t e r s . T y p i c a l r e s u l t s obtained using two s t i r r e d tank r e a c t o r s in s e r i e s are shown in Table II. Since a s i g n i f i c a n t f r a c t i o n o f the p r e c i p i t a t e p a r t i c l e s is C L CJ tO 03 CU

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Actinide Separations - American Chemical Society

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