Group Separation of Fission Products and Actinides with Diamides of Dipicolinic Acid

In recent years, the effort toward a closed nuclear fuel cycle has grown in importance and interest because chemically similar radionuclides (e.g., Cs and Sr) can be vitrified in a stable matrix for long term storage, while lanthanides and minor actinides can be further processed for recycling in reactors. The Universal Extraction (UNEX) is a process that has been developed for the simultaneous extraction of cesium, strontium, and actinides from acidic solutions, followed by selective stripping of radionuclides by changing aqueous phase conditions.[1] This process utilizes an organic phase consisting of 0.08 M CCD (chlorinated cobalt dicarbollide), 0.007-0.02 M PEG-400 (polyethylenglycol), and 0.02 M Ph2Bu2CMPO (diphenyl-N,N-di-n-butyl-carbamoyl¬methyl¬phosphine oxide) in the diluent FS-13 (phenyltrifluoromethyl sulfone, C6H5SO2CF3). The UNEX extraction mixture provides simultaneous extraction of Cs, Sr, actinides and lanthanides from aqueous nitric acid solutions up to ~2 Ì HNO3 and ~4 Ì total nitrates. However, the current usage of the UNEX process is limited due to the low solubility of CMPO in the organic phase. As a result, new possible extractants are being investigated as possible substitutes for a UNEX group separation process.

A large synergistic effect was observed in the extraction of trivalent metals by different dipicolinamides in the presence of CCD,[2] and therefore have been proposed as extractants instead of CMPO as the components of a modified UNEX solvent.[3] The objective of the present work is to expand our previous study on several N,N'-dialkyl- N,N'-diaryl substituted diamides of dipicolinic acid.[4] Four different diamides of different structures were studied. One of them is the alkylsubstituted N,N,N',N'- tetrabutyldiamide of dipicolinic acid (TBDPA) which contains only alkyl substituents on amidic nitrogen atom. The three other investigated diamides were ortho, meta and para isomers of N,N'-diethyl-N,N'-ditolyl-diamide of dipicolinic acid (EtTDPA).[5]

Diamides were synthesized in the procedure given Reference 6. Batch extraction experiments were performed using FS-13 as a solvent in a 1:1 aqueous:organic phase ratio by pre-equilibrating the organic phase with the corresponding concentration of nitric acid during extraction. Distribution ratios of studied actinides between aqueous solutions of nitric acid and EtTDPA dissolved in a polar solvent FS-13 (phenyltrifluoromethyl sulfone, C6H5SO2CF3) were determined in a wide range of nitric acid and added nitrate concentrations. Equilibrium concentrations of actinides were measured either radiometrically (Am-241 and Np-237 with NaI(Tl)-gamma detector, Pu-239 by LSC counting) or volumetrically (U-238 with Arsenazo(III)), and concentrations of lanthanides were analyzed by ICP-OES spectroscopy of the pre- and post-extraction aqueous phase or using Ln-radiotracers and gamma spectroscopy.

The objective of this work was a simultaneous separation of Cs, Sr, actinides and lanthanides, with selective stripping of the Cs/Sr and actinide/lanthanide products. Extractions of selected actinides (An) and lanthanides (Ln) with the studied diamides of dipicolinic acid and the mixture of diamides with CCD were investigated as a function of the ligand structure and nitric acid concentration. A great synergistic effect with chlorinated cobalt dicarbollide (CCD) on the extraction of Am and Ln with diamides was observed. As a result of numerous experiments with various complexants, the experimental conditions which provide common separation of Cs, Sr, actinides and lanthanides were found. With an appropriate stripping protocol, the modified diamide-UNEX process can be used for the extraction of Cs, Sr, An and Ln followed by selective separation.

References:

[1] V. N. Romanovskiy, I. V. Smirnov, V. A. Babain et al., Solv. Extr. Ion Exch. 19 (2001) 1

[2] M. Yu. Alyapyshev, V. A. Babain, I. V. Smirnov; Radiochemistry, Engl. Ed. 46 (2004) 270

[3] V.N. Romanovskiy, V. A. Babain, M. Yu. Alyapyshev, et al., Sep. Sci. Techn. 41 (2006) 2111

[4] V. A. Babain, M. Yu. Alyapyshev, R.N. Kisileva; Radiochim. Acta, 95, 217, 2007

[5] A. Paulenova, M. Yu. Alyapyshev, V. A. Babain, et al; Sep. Sci. Techn., 43, 2606 2008

[6] Nikitskaya, E. S., Usovskaya, V. S., Rubtsov, M. V., 1958 Russian J. General Chem. (Zh. Obsch. Khimii), 28, 161