Shunji Homma

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%.

Fluorination Reaction of Uranium Dioxide by Fluorine

Kinetics of the fluorination reaction of uranium dioxide is studied using un-reacted core model with shrinking particles. The model includes the film mass transfer of fluorine gas and its diffusion in the particle. The rate constants of the model are determined by fitting the experimental data for 370–450°C. The model successfully represents the fluorination in this temperature range. The rate control step is identified by examining the rate constants of the model for 300–1,800°C. For temperature range up to 900°C, the fluorination reaction is rate controlling. For over 900°C, both mechanisms of the mass transfer of fluorine and the fluorination reaction control the rate of the fluorination. With further increase of the temperature, however, the fluorination reaction becomes so fast that the mass transfer of fluorine eventually controls the rate of the fluorination.

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.

Neptunium concentration profiles in the Purex process

Numerical simulations of neptunium concentration profiles in the co-decontamination step of the Purex process is carried out in order to understand neptunium extraction behavior and to propose a flow sheet for neptunium recovery from the reprocessing process. A simulation result for a flow sheet containing Pu(VI) in the feed solution shows that Pu(VI) plays a role of scavenger of nitrous acid. The rate constant of neptunium reduction with n-butyraldehyde is evaluated by using the process data. The determined rate constant is available for the numerical simulation of neptunium concentration profiles in the reprocessing process.

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.

An approach for evaluating equilibrium and rate constants for the reaction of Np(V) with nitric acid in the Purex process using process data

A new approach for evaluating the apparent equilibrium and rate constants for the reaction of Np(v) with nitric acid by using the process data of multistage countercurrent solvent extraction experiments is proposed. The numerical simulation of neptunium extraction in the co-decontamination process of the Purex process is carried out on the assumption of the rate equation. The apparent equilibrium and rate constants are determined by the nonlinear least-squares method with experimental data for multistage countercurrent solvent extraction with a numerical simulation code. The determined apparent equilibrium and rate constants are 1.25 to 1.97 {times} 10{sup {minus}2} min {sup {minus}1} and 6.04 to 11.5 {times} 10{sup {minus}4} ({ell}/mol){sup 3/2}, respectively. These values are consistent with those obtained in other kinetic studies.

Development of Crystallizer for Advanced Aqueous Reprocessing Process

Crystallization is one of the remarkable technologies for future fuel reprocessing process that has safety and economical advantages. Japan Atomic Energy Agency (JAEA) (former Japan Nuclear Cycle Development Institute), Mitsubishi Material Corporation and Saitama University have been developing the crystallization process. In previous study, we carried out experimental studies with uranium, MOX and spent fuel conditions, and flowsheet analysis was considered. [1, 2, 3] In association with these studies, an innovative continuous crystallizer and its system was developed to ensure high process performance. From the design study, an annular type continuous crystallizer was selected as the most promising design, and performance was confirmed by small-scale test and engineering scale demonstration at uranium crystallization conditions. In this paper, the design study and the demonstration test results are described.Copyright

Flowsheet Analysis of U-Pu Co-Crystallization Process as a New Reprocessing System

A new fuel reprocessing system by U-Pu co-crystallization process is proposed and examined by flowsheet analysis. This reprocessing system is based on the fact that hexavalent plutonium in nitric acid solution is co-crystallized with uranyl nitrate, whereas it is not crystallized when uranyl nitrate does not exist in the solution. The system consists of five steps: dissolution of spent fuel, plutonium oxidation, U-Pu co-crystallization as a co-decontamination, re-dissolution of the crystals, and U re-crystallization as a U-Pu separation. The system requires a recycling of the mother liquor from the U-Pu co-crystallization step and the appropriate recycle ratio is determined by flowsheet analysis such that the satisfactory decontamination is achieved. Further flowsheet study using four different compositions of LWR spent fuels demonstrates that the constant ratio of plutonium to uranium in mother liquor from the re-crystallization step is achieved for every composition by controlling the temperature. It is also demonstrated by comparing to the Purex process that the size of the plant based on the proposed system is significantly reduced.Copyright

Numerical Investigations of Drop Solidification by a Front-Tracking Method

We present a front-tracking/finite difference method for simulation of drop solidification, where the melt is confined by its own surface tension. The problem includes temporal evolution of three interfaces, i.e. solid–liquid, solid–air, and liquid–air, that are explicitly tracked under the assumption of axisymmetry. The solid–liquid interface is propagated with a normal velocity that is calculated from the normal temperature gradient across the front and the latent heat. The liquid–air front is advected by the velocity interpolated from nearest bulk fluid flow velocities. Method validation is carried out by comparing computational results with exact solutions for two-dimensional Stefan problems, and with related experiments. We then use the method to investigate a drop solidifying on a cold plate in which there exists volume expansion due to density difference between the solid and liquid phases. Effects of the tri-junction in terms of growth angles on the solidification process are also investigated. Computational results show that a decrease in the density ratio of solid to liquid or an increase in the growth angle results in an increase in the height of the solidified drop. In addition, reducing the gravitational effect also increases the drop height after solidification.Copyright

Application of Data Reconciliation and Gross Error Detection to Nuclear Material Accounting for Spent Fuel Reprocessing

Data reconciliation and gross error detection are applied to nuclear material accounting of spent fuel reprocessing in order to demonstrate their usefulness for verification activities in a safeguards system. In a simple flowsheet of the Purex process including dissolution, co-decontamination, and uranium-plutonium partition processes, the measurements of the steady-state mass flow rate, which include artificial random errors imitating measurement errors and/or process disturbances, are reconciled by a least-squares method with linear constraints of mass balance equations. The reconciled measurements satisfy the mass balance equations and their random errors are reduced. By use of another set of measurements, which includes both random and systematic errors, the gross error detection with χ2 test statistic is tested in order to find the systematic errors, which can correspond to the failure of measurement devices and the loss of nuclear materials. The systematic error is successfully detected with a given confidence limit.