EXTRACTION AND RETENTION OF PLUTONIUM WITH gamma-IRRADIATED 30% TRIALKYLPHOSPHINE OXIDE-KEROSENE SOLUTION

The partitioning of long-lived alpha-emitting actinides, present in high-level radioactive waste (HLW) solutions from PUREX reprocessing of spent nuclear fuels, has been carried out using a commercially available, reasonably inexpensive, trialkylphosphine oxide (TRPO), Cyanex®-923 alone or its mixture with tri-nbutylphosphate (TBP) in dodecane. The studies have been carried out from nitric acid as well as from three types of synthetic HLW solutions. With 30% TRPO alone, third-phase formation was observed when HLW in 3.0 M HNO3 was taken as such. With the mixture of TRPO and TBP, it has been shown that the HLW need not be diluted 10 times, but only the acidity has to be reduced from ~3 to ~l M (without substantial dilution) to achieve an efficient separation of the minor actinides. The results thus obtained using TRPO with an increase in the total number of carbon atoms in the amide from 14 to 22. The LOG values of DBHA (C. No. 14), DiBHA (C. No. 14), DBOA (C. No. 16) were lower compared to that of DHHA (C. No. 18), DBDA (C. No. 20), DOHA (C. No. 22), which have relatively longer carbon chain length. Further it was observed that introduction of branching at the nitrogen side increases the LOC value as compared to the straight chain analogue, as in the case of DiOBA and DiBHA. In the latter case, however, this effect is not very pronounced perhaps due to the smaller carbon chain length. Introduction of branching at the position alpha to the carbonyl group was found to sharply decrease the LOC value due to steric hindrance as can be seen in the case of DOBA and DOiBA. Further, by varying the carbon chain length on both acyl and nitrogen sides, keeping the total carbon number invariant, it was found that the LOC increases considerably on increasing the chain length on the acyl side. On the contrary, the LOC for nitric acid extraction was not influenced significantly by the structure of the amide. Nevertheless, the distribution ratio at the point of dissolution of the third phase was found to decrease with introduction of branching on either end of the amide and more so, if branching was introduced on the acyl end of the amide. sulfone has been developed for this process. A UNEX flowsheet consisting of a single solvent extraction cycle has been developed as a part of a collaborative effort between the Khiopin Radium Institute (KRI) and the Idaho National Engineering and Environmental Laboratory (INEEL). This flowsheet has been demonstrated with actual acidic radioactive tank waste at the INEEL using 24 stages of 2-cm diameter centrifugal contactors installed in a shielded cell facility. The activities of (137)Cs, (90)Sr, and the actinides were reduced to levels at which a grout waste form would meet NRC Class A LLW requirements. The extraction of (99)Tc and several nonradioactive metals by the UNEX solvent has also been evaluated. has been compared with those already reported while using dihexylN,N-V-diethylcarbamoylmethylphosphonate, diisodecyl phosphoric acid, and octyl(phenyl)A^-diisobutylcarbamoylmethylphosphine oxide (CMPO). The mixture of 0.2 M CMPO and 1.2 M TBP in dodecane (TRUEX solvent) seems to be a superior extractant for the partitioning of minor actinides from HLW solutions. with an increase in the total number of carbon atoms in the amide from 14 to 22. The LOG values of DBHA (C. No. 14), DiBHA (C. No. 14), DBOA (C. No. 16) were lower compared to that of DHHA (C. No. 18), DBDA (C. No. 20), DOHA (C. No. 22), which have relatively longer carbon chain length. Further it was observed that introduction of branching at the nitrogen side increases the LOC value as compared to the straight chain analogue, as in the case of DiOBA and DiBHA. In the latter case, however, this effect is not very pronounced perhaps due to the smaller carbon chain length. Introduction of branching at the position alpha to the carbonyl group was found to sharply decrease the LOC value due to steric hindrance as can be seen in the case of DOBA and DOiBA. Further, by varying the carbon chain length on both acyl and nitrogen sides, keeping the total carbon number invariant, it was found that the LOC increases considerably on increasing the chain length on the acyl side. On the contrary, the LOC for nitric acid extraction was not influenced significantly by the structure of the amide. Nevertheless, the distribution ratio at the point of dissolution of the third phase was found to decrease with introduction of branching on either end of the amide and more so, if branching was introduced on the acyl end of the amide. sulfone has been developed for this process. A UNEX flowsheet consisting of a single solvent extraction cycle has been developed as a part of a collaborative effort between the Khiopin Radium Institute (KRI) and the Idaho National Engineering and Environmental Laboratory (INEEL). This flowsheet has been demonstrated with actual acidic radioactive tank waste at the INEEL using 24 stages of 2-cm diameter centrifugal contactors installed in a shielded cell facility. The activities of (137)Cs, (90)Sr, and the actinides were reduced to levels at which a grout waste form would meet NRC Class A LLW requirements. The extraction of (99)Tc and several nonradioactive metals by the UNEX solvent has also been evaluated.