Tomozo Koyama

Separation of Actinide Elements by Solvent Extraction Using Centrifugal Contactors in the NEXT Process

Using the advanced aqueous reprocessing system named NEXT process, actinides recovery was attempted by both a simplified solvent extraction process using TBP as an extractant for U, Pu and Np co-recovery and the SETFICS process for Am and Cm recovery from the raffinate. In U, Pu and Np co-recovery experiments a single cycle flow sheet was used under high nitric acid concentration in the feed solution or scrubbing solution. High nitric acid concentration in the feed solution aided Np oxidation not only in the feed solution, but also at the extraction section. This oxidation reaction accomplished Np extraction by TBP with U and Pu. Most of Np could be recovered into the product solution. In the SETFICS process, a TRUEX solvent of 0.2 mol/dm3 CMPO and 1.4 mol/dm3 TBP in n-dodecane was employed instead of 0.2 mol/dm3 CMPO and 1.0 mol/dm3 TBP in n-dodecane in order to increase the loading of metals. Instead of sodium nitrate, hydroxylamine nitrate was applied to this experimental flow sheet in accordance with a “salt-free” concept. The counter current experiment succeeded with the Am and Cm product. On the high-loading flow sheet, compared with the previous flow sheet, the flow of the aqueous effluents and spent solvent were expected to decrease by about one half. Two solvent extraction experiments for actinides recovery demonstrated the utility of the flow sheet of these processes in the NEXT process.

Batch Crystallization of Uranyl Nitrate

Batch crystallization of uranyl nitrate is carried out in order to obtain fundamental data required for the development of reprocessing involving crystallization. Particular attention is paid to the development of a method for predicting the concentrations of uranium and nitric acid in the mother liquor and the amount of uranyl nitrate crystals produced. Initial concentrations of uranyl nitrate and nitric acid are 500–600 g/l and 4–6 mol/l, respectively, corresponding to the condition of a dissolver solution of spent fuel. Steady-state mass balance equations including the correlation equation for the equilibrium solubility of uranium nitrate are applied to the prediction. The calculated concentrations of uranium and nitric acid are in close agreement with the experimental ones. The recovery of uranium is accurately predicted by the calculated concentrations, with an error of less than 10%.

Uranium Crystallization Test with Dissolver Solution of Irradiated Fuel

The crystallization process has been developed as a part of the advanced aqueous process, NEXT (New Extraction System for TRU recovery) for fast reactor (FR) cycle. In this process, a large part of U is separated from dissolver solution by crystallization as UO2(NO3)2.6H2O. The U crystallization test was carried out with real dissolver solution of irradiated FR fuel to investigate the influence of cooling rate on the crystal size and the behavior of fission product (FP) compared with that of Pu(IV). In regard to the influence of the cooling rate, it was confirmed that the crystal size was smaller as the cooling rate is faster. Although it was expectable that the decontamination performance was improved by diminishing the specific surface of the crystals, it was suggested that a large crystal produced by crystallization was not always high purity. Concerning the behavior of FPs, Eu behaved similarly to Pu(IV). Cs accompanied with U into the crystals under the condition in this test.

Flowsheet Study of U-Pu Co-Crystallization Reprocessing System

A U-Pu co-crystallization reprocessing system is proposed for light water reactor fuels and its flowsheet study is carried out. This reprocessing system is based on experimental evidence indicating that Pu(VI) in a nitric acid solution is co-crystallized with uranyl nitrate, whereas it is not crystallized when uranyl nitrate is not present in the solution. The system consists of five steps: dissolution of spent fuel, Pu oxidation, U-Pu co-crystallization, re-dissolution of the crystals, and U re-crystallization. The proposed system does not require the use of organic solvents, resulting in a relative increase in safety and cost-effectiveness. The system requires recycling of the mother liquor from the U-Pu co-crystallization step to recover almost the entire amount of U and Pu at this step. The appropriate recycling ratio is determined, such that satisfactory decontamination is achieved. A consistent ratio of Pu to U in the mother liquor from the U re-crystallization is maintained by regulating the temperature, suggesting that the quality of the liquor, which can be a source of mixed oxide fuels, can be controlled despite differences in the composition of the spent fuel. The size of a plant utilizing the proposed system is estimated to be about 30% less than that of the PUREX system.

Experimental Study on U-Pu Cocrystallization Reprocessing Process

A new reprocessing system with a 2-stage crystallization process has been developed. In the first stage of the system, U and Pu are recovered from dissolver solution by U-Pu cocrystallization. Laboratory-scale experiments were carried out with U and Pu mixed and irradiated fuel dissolver solutions to obtain fundamental data on the U-Pu cocrystallization process. Pu(VI) was cocrystallized with U, but crystallization yields of Pu were lower than those of U. FPs were separated from U and Pu by cocrystallization, and decontamination factors of Cs and Eu to U in crystals were over 100.

Distribution Coefficients of U(VI), Nitric Acid and FP Elements in Extractions from Concentrated Aqueous Solutions of Nitrates by 30% Tri-n-butylphosphate Solution

The distribution coefficients of uranium, nitric acid and 8 elements simulating fission products were obtained from the concentrated aqueous solutions of nitrates, whose nitrate ion concentrations were 4–15 mol dm−3. The relationships among the distribution coefficient of uranium, the nitrate ion concentration in the aqueous phase, and the concentration of tri-n-butylphosphate uncombined with nitrates in the organic phase were obtained from the data.