Improvements in Thorium-Uranium Separation in the Acid-Thorex

26 Improvements in Thorium-Uranium Separation in the

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Acid-Thorex Process G L E N E . BENEDICT General Atomic Company, San Diego, CA 92138

The Acid-Thorex process has been used in recent years to recover U from neutron i r r a d i a t e d t h o r i a targets.(1-4) This process uses n - t r i b u t y l - p h o s p h a t e (TBP) in normal p a r a f f i n hydrocarbon (NPH) as the extractant and the r e l a t i v e uranium and thorium solubilities in each phase are adjusted by control of the nitric acid concentration. The Acid-Thorex process is the primary candidate for use in proposed aqueous thorium fuel c y c l e s . In t h i s process, uranium is separated from thorium through e x p l o i t a t i o n o f the d i f f e r e n c e in e q u i l i b r i u m d i s t r i b u t i o n s since no usable valence change is a v a i l a b l e to a i d in t h i s s e p a r a t i o n . This report describes some of the flowsheet development work done in the General Atomic Company p i l o t plant pulse column equipment in support of the High Temperature Gas Cooled Reactor (HTGR) Fuel Recycle Development Program. Data are presented showing the b e n e f i c i a l e f f e c t of adding low concentrations of f l u o r i d e ion to the thorium p a r t i t i o n i n g s o l u t i o n . These data a l s o show the r e s u l t s o f t e s t s where dibutylphosphate (DBP) was added to simul a t e solvent degradation and cold zirconium and Zr t r a c e r to simulate f i s s i o n product zirconium. F l u o r i d e a d d i t i o n not only improves the thorium-uranium s e p a r a t i o n , but a l s o minimizes the p r e c i p i t a t i o n o f thorium dibutylphosphate in the uranium s t r i p p i n g column which has been a major problem in processing thorium based nuclear fuel materials using t h i s process. (3,4) 233

95

Experimental Flowsheet t e s t i n g and data c o l l e c t i o n were performed using the General Atomic Company solvent e x t r a c t i o n p i l o t plant equipment shown in Figure 1. Included in t h i s equipment are several 5.1 to 7-6 cm (2 to 3 in.) diameter c y l i n d r i c a l glass pulse c o l umns, a 15.2 cm (6 in.) diameter annular pulse column, a c e n t r i fugal contactor (Robatel Co.) and associated tanks, feed systems and c o n c e n t r a t o r s .

0-8412-0527-2/80/47-117-371$05.00/0 © 1980 American Chemical Society In Actinide Separations; Navratil, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

SEPARATIONS


ACTINIDE

Figure 1.

Solvent extraction pilot plant

In Actinide Separations; Navratil, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


26. BENEDICT

Acid Thorex Process

373

The flowsheet used in these studies is shown in Figure 2 and i l l u s t r a t e s the path of thorium through the p a r t i t i o n c y c l e . The thorium, uranium, and f i s s i o n products enter at the center o f the e x t r a c t i o n - s c r u b ( 1 A ) column and the uranium and thorium are extracted into the r i s i n g solvent with the f i s s i o n products being l a r g e l y rejected with the n i t r i c a c i d . The loaded solvent is t r a n s f e r r e d to the bottom of the p a r t i t i o n column (4.6 m (15 feet) long, about k to 5 t h e o r e t i c a l stages) where it r i s e s through the thorium p a r t i t i o n s o l u t i o n ( 1 B X ) . A change in flow rate and n i t r i c a c i d concentration allows most of the thorium and very l i t t l e uranium to be s t r i p p e d from the s o l v e n t . The f l u o r i d e a d d i t i o n s were made into the thorium p a r t i t i o n s o l u t i o n ( 1 B X ) stream. The p a r t i t i o n c y c l e columns were operated on flowsheet values and then a DBP s o l u t i o n was pumped to the e x t r a c t i o n feed point in increasing amounts. The thorium content o f the uranium product stream from the thorium p a r t i t i o n column was monitored. The runs were then repeated using increasing increments o f f l u o r i d e in the thorium p a r t i t i o n s o l u t i o n and the thorium content again monitored in the uranium product from the p a r t i t i o n column. Residual small amounts of thorium in the uranium stream a f t e r thorium p a r t i t i o n i n g were analyzed by ion exchange chromatography from a c h l o r i d e medium followed by Thorin c o l o r i m e t r y . Results and Discussion F l u o r i d e is known to separate zirconium f i s s i o n product and plutonium from solvent degradation products.(5) Since f l u o r i d e is used to speed d i s s o l u t i o n of t h o r i a in n i t r i c a c i d , and is already present in thorium solvent e x t r a c t i o n process feed s o l u t i o n s , it was the first choice as an agent to use to improve thorium-DPB s e p a r a t i o n . The r e s u l t s of adding no f l u o r i d e and 0 . 0 0 1 and 0.005 M. f l u o r i d e to the thorium p a r t i t i o n s o l u t i o n are shown in Figure 3. An improvement o f a f a c t o r o f 1 0 in thorium separation from uranium is obtained with the 0.005 M. f l u o r i d e , a concentration recommended f o r process use. This amount of f l u o r i d e increases the t o t a l f l u o r i d e concentration in the high level waste by k0%. Furthermore, t h i s amount of f l u o r i d e a d d i t i o n increased the opera b i l i t y of the downstream uranium s t r i p column by lowering the p r e c i p i t a t i o n of thorium-DBP in that column where the a c i d i t y is lower. The thorium-DBP p r e c i p i t a t i o n caused problems in the processing of thorium target elements (k) where t h i s column p e r i o d i c a l l y required cleaning to remove the s t i c k y p r e c i p i t a t e . The decrease of the thorium-DBP p r e c i p i t a t i o n is probably the more s i g n i f i c a n t r e s u l t of the f l u o r i d e a d d i t i o n . The e f f e c t of DBP on uranium c a r r y over to the solvent wash system in the ICW stream is a l s o shown in Figure 3. E a r l y solvent e x t r a c t i o n flowsheets f o r HTGR f u e l s r e c y c l e developed at the General Atomic Company contained a c o e x t r a c t i o n c o s t r i p c y c l e f o r thorium and uranium p r i o r to the p a r t i t i o n i n g


374

ACTINIDE SEPARATIONS

To s o l v e n t wash

(11) 1BSU

(16) 1CW


(5) 1AP (2) IAS

(8) 1BX

Τ

(1*0

(9) 1BXT

1CX

IS (1)

1AF 1BX 15'

(3) 1AA ·

(k)

IBS 17'

15'

1C

1A

1 AX (7) 1BXF

(10) IBS

(13) 1BU

(6) 1AW

(15) îcul

(12) 1BT to high l e v e l waste

Stream 1AF 1 AS 1AA 1 AX 1AP 1 AW 1BXF 1BX 1BXT 1BS 1BSU 1BT 1BU 1CX 1CU 1CW

Stream No.

to 2nd Th eye 1 e

Relative Flow Rate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Figure 2.

100 130 40 1000 1000 270 1180 600 600 180 180 600 1180 593 593 1180

to 2nd U cycle

Th

U

(g/l)

(g/n 35

348

— — —

— —

(30% TBP) 35 0.05

3.5 0.005

— 2.5 0.001 to 0.005 2.98

— 5.93 0.003

3

1.0 1.0 13.0

—

0.2 2.0

—

—

—

— —

(F 0.001 to 0.005 M)

— —

HN0 (M)

(30% TBP) 20 58 (See F i g . 3)

—

Trace (30% TBP)

0.2

0.1 0.5 0.02 0.01 0. 05 0.00

Acid-Thorex partition cycle flowsheet



BENEDICT

375

Acid Thorex Process

0

0.05

0.1

0.2

0.3

DBP, g/l IN ORGANIC PHASE

Figure 3. Effects of fluoridein1BX stream: (O) no F~in1BX; (A) 0.001M F~ in 1BX; (Π) 0.005M F~in1BX; (χ) 0.001M F~in1BX.

Figure 4. Measured Zr decontamina tion factors: (A) estimated DBP level in processing 180-day cooled reference fer tile particlesinpulsed column; (B) same as above with Robatel centrifugal con tactor. 95

Β

A

0.1

0.2

0,3

0Λ

DBP IN ISP (G/LI TER)

0.5

0.6



376

ACTINIDE

SEPARATIONS

cycle. Since most r a d i a t i o n damage to the solvent occurs in the first solvent e x t r a c t i o n c y c l e , most of the DBP w i l l be formed there. In a c o e x t r a c t i o n c y c l e , the thorium-DBP p r e c i p i t a t e s in the c o s t r i p column in a manner which is very d e l e t e r i o u s to p r o cess o p e r a t i o n . That f a c t , coupled with the above observations using f l u o r i d e to a i d separation of thorium and DBP, led to the recommendation that the uranium thorium p a r t i t i o n c y c l e be the first c y c l e in any Acid-Thorex flowsheet arrangement on spent thorium f u e l s . It is of i n t e r e s t to note that a d d i t i o n of 0.001 M f l u o r i d e to the e x t r a c t i o n scrub s o l u t i o n did not improve the zirconiumthorium separation s i g n i f i c a n t l y in the scrub s e c t i o n . A large improvement in zirconium-uranium separation has been observed by a d d i t i o n of f l u o r i d e to scrub streams in the Purex process. This d i f f e r e n c e is probably due to the thorium complexing the f l u o r i d e and lowering the f r e e f l u o r i d e to a level which is i n e f f e c t i v e in a l t e r i n g zirconium d i s t r i b u t i o n . It is a l s o o f i n t e r e s t to note that the e f f e c t o f DBP on zirconium separation from thorium in the Acid-Thorex system is d i f f e r e n t than zirconium separation from uranium in the Purex system.(Figure 4) The Purex data are from reference 6 and the Acid-Thorex data are from General Atomic Company p i l o t plant studies. The thorium probably forms a stronger DBP complex than does uranyl ion and, t h e r e f o r e , the amount of uncomplexed DBP a v a i l a b l e f o r r a i s i n g the e q u i l i b r i u m d i s t r i b u t i o n of zirconium would be less in the Acid-Thorex process. Conclus ions In the Acid-Thorex process, f l u o r i d e ion should be added to the thorium p a r t i t i o n i n g s o l u t i o n ( 1 B X ) to decrease thorium t r a n s f e r to the uranium s t r i p p i n g column, p a r t i c u l a r l y where highly r a d i o a c t i v e feeds are used. This f l u o r i d e ion a d d i t i o n then decreases the p r e c i p i t a t i o n of thorium-DBP in the uranium s t r i p p i n g column. A l s o , the p a r t i t i o n c y c l e should be the first c y c l e in the Acid-Thorex process to allow separation of thorium from DBP. Acknowledgments The author acknowledges the c o n t r i b u t i o n s of G. W. Reddick, R. G. Wilbourn and L. E. J o l l e y to the study reported here. A d d i t i o n a l data from t h e i r reports are contained in references 7 and

8. L i t e r a t u r e Cited 1. 2.

Rainey, R. H.; Moore, J. G., Nuc. S c i . and Eng., 1961, 10, No. 4, 367. Haas, W. O., Jr.; Smith, D. J., U.S. Atomic Energy Commission


26.

3.

BENEDICT

Acid Thorex Process

377

Report KAPL-1306, General E l e c t r i c Co., Schenectady, New York, 1956. Rathvon, J.C.; Blasewitz, A. G.; Maher, R.; Eargle, J. C., Jr.; Wible, A. E., Recovery of U from Irradiated Thoria in "Thorium Fuel Cycle-Proceedings of Second International Thorium Fuel Cycle Symposium, G a t l i n b u r g , Tennessee, May 3-6, 1966," U.S. Atomic Energy Commission Report CONF-660524, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 1968; pp. 765-824. Jackson, R. R.; Walser, R. L., Eds., U.S. Energy Research and Development A d m i n i s t r a t i o n Report ARH-2127, A t l a n t i c R i c h f i e l d Hanford Company, Richland, Washington, 1977. Swanson, J. L., U.S. Atomic Energy Commission Report BNWL1588, P a c i f i c Northwest Laboratory, Richland, Washington, 1971. Richardson, G. L., U.S. Atomic Energy Commission Report HEDL-TME-73-51, Westinghouse Hanford Company, Richland, Washington, 1973. Reddick, G. W., U.S. Energy Research and Development Administ r a t i o n Report GA-A13835, General Atomic Company, San Diego, C a l i f o r n i a , 1976. Wilbourn, R. G., U.S. Department of Energy Report GA-A15030, General Atomic Company, San Diego, C a l i f o r n i a , 1978. 233


4.

5. 6.

7.

8.

RECEIVED July 2, 1979

Work performed under U.S. Government contract number DE-AT03-76SF1053.


Improvements in Thorium-Uranium Separation in the Acid-Thorex

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