Cm from Lanthanides in an

Chapter 16

Downloaded by NORTH CAROLINA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: June 9, 2006 | doi: 10.1021/bk-2006-0933.ch016

Use of Cyanex-301 for Separation of Am/Cm from Lanthanides in an Advanced Nuclear Fuel Cycle Dean R. Peterman, Jack D. Law, Terry A. Todd, and Richard D. Tillotson Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415

As part of the Advanced Fuel Cycle Initiative (AFCI), technologies for advanced aqueous processing of spent L W R fuel are being developed. Included in this program is the separation of A m and Cm from the lanthanides. This separation would allow the Am/Cm to be fabricated into a target and recycled to a reactor and the lanthanides to be disposed of with the raffinates from the process. A Laboratory Directed Research and Development (LDRD) Project at the INEEL is investigating the use of the active extractant in the Cyanex-301 reagent, bis(2,4,4-trimethylpentyl)dithiophosphinic acid, as a potential method to accomplish this separation. Specifically, the extractant is being developed based on an ammonium acetate/acetic acid buffered feed to the process which can be used to strip the actinides and lanthanides from a preceding transuranic separation process. The extraction and scrub distribution coefficients of A m and Eu with this extractant, as a function of total acetate concentration and pH, have been measured. Additionally, the extraction behavior of several additional lanthanides are compared. The potential for developing a Cyanex-301 process for the separation of Am/Cm from lanthanides to support an advanced nuclear fuel cycle is discussed.

© 2006 American Chemical Society

In Separations for the Nuclear Fuel Cycle in the 21st Century; Lumetta, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction The Advanced Fuel Cycle Initiative (AFCI) is funded by the U.S. Department of Energy's Office of Nuclear Energy, Science and Technology to develop advanced separation technologies to safely and economically reduce the volume and heat generation of spent nuclear fuel requiring geologic disposition, thereby extending the capacity of the Yucca Mountain repository and delaying or avoiding the need for a second repository. To accomplish this, the A F C I is developing advanced fuel reprocessing systems to separate key radionuclides from spent light water reactor (LWR) fuel. A n advanced aqueous process that is being developed is the U R E X + process. The U R E X + process consists of a series of solvent extraction processes that separate the following radionuclides: a) Tc and U using the Uranium Extraction ( U R E X ) process, b) Cs and Sr using a cobalt dicarbollide/polyethylene glycol (CCD-PEG) process, c) Pu, Np, A m , Cm, and rare-earth fission products using the Plutonium Uranium Refining by Extraction (PUREX) based processes and/or the transuranic extraction ( T R U E X ) processes , and d) A m and Cm from the rare earths using a selective actinide extraction ( S A N E X ) process. Limited research is being performed as part of the A F C I program to support development of the U R E X + process relative to the separation of A m and Cm from the lanthanides. Separation of the A m and Cm from the lanthanides will allow the Am/Cm to be fabricated into a target and recycled to a reactor and the lanthanides to be disposed of with the final raffinate from the U R E X + process. Several extractants have been previously identified as promising technologies for the separation of Am/Cm from the lanthanides but have not been developed specifically to support the U R E X + process (1). One such technology is the use of the active extractant in the Cyanex-301 reagent - bis(2,4,4trimethylpentyl)dithiophosphinic acid (1-3). As part of a laboratory Directed Research and Development (LDRD) project at the INEEL, the use of bis(2,4,4trimethylpentyl)dithiophosphinic acid is being investigated as a potential method to accomplish this separation. One of the limitations of using the active extractant in the Cyanex-301 reagent is that efficient separation of Am/Cm from the lanthanides can only be accomplished at a pH greater than approximately 3. Many acidic waste streams, including dissolved spent nuclear fuel, which could benefit from this separation have a pH much lower than 3. However, it is possible to design the flowsheets in the U R E X + process to provide a feed stream to the Cyanex-301 process with a pH greater than 3. This can be accomplished by extracting the Am/Cm and lanthanides from the dissolved spent L W R fuel using the T R U E X process and stripping these radionuclides with an ammonium acetate/acetic acid buffered strip solution. This strip product is at a favorable p H for separation of Am/Cm


253 from the lanthanides. The separation of Am/Cm from the lanthanides in this buffered feed solution is the primary focus of the L D R D work performed at the INEEL.


Scope The primary scope of this research was to experimentally evaluate the conditions required to effectively separate Am/Cm from the lanthanides in an ammonium acetate/acetic acid buffered solution using the purified Cyanex-301 reagent. Buffered solutions ranging from 0.2 M to 1.0 M total acetate concentration and a pH ranging from 3.7 to 5.7, obtained by varying the concentration ratio of acetic acid and ammonium acetate, were used. Distribution coefficients and the resulting separation factors were determined for A m and E u . Additionally, the separation factors f o r A m from E u were determined as a function of pH in a simulant (1.0 M total acetate) containing approximately 6 g/L total lanthanides, which is a typical lanthanide concentration which is expected to be present in dissolved spent L W R fuel. Differences in the separation factors of various lanthanides (La, Ce, Pr, Nd, Sm, Eu) were also evaluated. 2 4 1

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Experimental The Cyanex-301 reagent was obtained from Cytec Industries, Canada. As received, the Cyanex-301 is a mixture of approximately 77% of the active extractant, bis(2,4,4-trimethylpentyl)dithiophosphinic acid, with the balance being synthetic by-products. Purification of the Cyanex-301 reagent was performed using the method developed by Zhu et al. (4) and detailed by Modolo and Odoj (2). This purification resulted in a >99.9% pure product. Due to degradation of the purified solvent, the Cyanex-301 was converted to the ammonium salt and refrigerated until ready for use. At this time the ammonium salt was converted to the acid form. The purified Cyanex-301 was then diluted by T B P and n-dodecane based on the results of Hill et al. (3). A solvent composition of 0.25 M Cyanex-301 and 0.37 M T B P in n-dodecane was used for all testing. A l l experiments were conducted at ambient temperature. A combination p H electrode and meter (Accumet) were used to measure the pH of the aqueous phases before and after contact with the organic solvent. The combination p H electrode was calibrated by acid-base titration in the appropriate ionic strength electrolyte solution. The measured pH of the aqueous phases before and after contact with the organic solvent agreed to within the experimental error.


254

Results

p H and Total Acetate Dependency 154

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Distribution coefficients for E u and Am, as a function of pH and total acetate concentration, are given in Figures 1 and 2, respectively. For E u with a 0.2 M total acetate concentration, distribution coefficients increase from 14 to 371 as the pH is increased from 4.2 to 5.7. At a total acetate concentration of 0.5 M , the E u distribution coefficients increase from 0.8 at a pH of 3.7 to a high of 21.4 at a pH of 4.8 then decrease to 6.9 at a pH of 5.8. At a total acetate concentration of 1.0 M, the E u distribution coefficients increase from 0.4 at a pH of 3.7 to a high of 1.4 at a p H of 4.8 then decrease to 0.22 at a p H of 5.8. These data indicate that operating at a 1.0 M total acetate concentration will minimize the extraction of Eu. Additionally, for a total acetate concentration of 1.0 M, the lowest Eu distribution coefficients are obtained at a pH >5. For total acetate concentrations of 0.2 M and 0.5 M, the measured Am distribution coefficients increase with increasing pH. However, detection limits were reached in the aqueous phase at pH values ranging from 4.8 to 5.8. These data are indicated by the open symbols in Figure 2. As a result, actual distribution coefficients for A m at these p H values are unknown. At a total acetate concentration of 1.0 M, distribution coefficients for A m increase from 72 at a pH of 3.7 to a maximum of 500 at 4.8, then decrease to 82 at a pH of 5.8. The separation factor for A m from Eu is defined as Dp^/D^. The higher the separation factor, the more efficient the resulting separation. Separation factors for A m from Eu are summarized in Table I. The separation factor for the extraction of A m from Eu by Cyanex-301 in the absence of T B P was not measured. Separation factors increase as the total acetate concentration is increased. In general, separation factors are also higher at the higher p H values. Based on these data, the best separation of A m from Eu is obtained at a total acetetate concentration of 1.0 M a n d a pH in the range of 5.2 to 5.8. 154


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Table I. Separation Factors ( D / D ) . Am

PH 3.8 4.2 4.8 5.2 5.5 5.8

0.2 MA cetate 5.9 75.6 >33.0 14.1 >28.6 13.8

0.5 MA cetate 155 139 >93 >127 >157 >185

Eu

1.0 M Acetate 171 257 353 372 383 365


255 t*

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1000

100

•

0.20 M Acetate

A

0.50 M Acetate

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1.0 M Acetate


10

'C •+-> OA 5

1 1

0.1

i

4.0

3.5

5.0

4.5

5.5

6.0

pH h4

Figure 1. Distribution coefficients of Eu as a function ofpH at total acetate concentrations of 0.2 M 0.5 M, and 1.0 M and a Cyanex-301 solvent composition of 0.25 MCyanex, 0.37 M TBP in n-dodecane. t

3

10000 T

u

-

£ B o "•£3 9

•

0.20 M Acetate 0.50 M Acetate 1.0 M Acetate A

1000 -

.*£

Q S

Cm from Lanthanides in an

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