Jiro Koga

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.

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.

Mixing of solid particles of different density in a horizontal batch mixer. Measurement of axial diffusion coefficients

Abstract Axial diffusion coefficients of tracer particles were obtained by the method of impulse response. The results show that the diffusion coefficients of the particles of different densities are larger than those of identical particles. The assumption of completely reflecting barriers for the end walls is inapplicable to the mixing of heterogeneous particle systems.

Size distribution of products pulverized by a screen mill

Abstract The comminution process of brittle materials in a continuous impact mill with a screen is discussed. The process being considered to be promoted by impact stress of hammers or lining plates in the mill, the size distributions of products were calculated theoretically under the following assumptions: the equation given by Tanaka can be used to obtain the probability of fracture, the breakage function can be expressed by the form of the Gaudin-Schuhman equation, and the size distribution of particles in the mill is equal to that of the product. The calculated results agreed well in tendency with the experimental results obtained with Soma sand and Toyoura sand. It was found from the size distribution calculated that the product size distribution changed only slightly by varying feed size, open-size of the screen and opening area ratio of the screen when the mean residence time of particles was much larger than the time interval for collision of particles in the mill against the hammers and lining plates.

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

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.

Reaction Model for Fluorination of Uranium Dioxide Using Improved Unreacted Shrinking Core Model for Expanding Spherical Particles

A gas-solid reaction model is developed to represent the fluorination of uranium dioxide (UO2), which consists of a two-step reaction: the formation of a solid intermediate of uranyl fluoride (UO2F2) on thecore of unreacted UO2 and the consumption of UO2F2. The model is an extension of the unreacted shrinking core model with a shrinking spherical particle and takes into account particle expansion resulting from the density difference between UO2 and UO2F2. This model successfully represents the initial expansion of the particle by the formation of the low-density UO2F2 intermediate. The accuracy of this model is higher than that of the original model, which does not allow particle expansion.

Transition of Scaling for Pinch-Off of an Axisymmetric Liquid Ligament in Another Immiscible Liquid

Scaling for pinch-off of an axisymmetric liquid ligament in another immiscible liquid is studied computationally for intermediate Reynolds number, where both inertial and viscous forces are dominant. Interface motion near pinch-off is traced by a front tracking/finite difference method with higher resolution. When the ligament breaks up, a thin thread forms between a bulb of the tip and the rest of the ligament. Computational results show that the minimum radius of the thread decreases with time and the exponent of time shifts from 2/3 to 1. This shift corresponds to the transition from potential flow scaling law to the scaling law for viscous thread in another viscous fluid. The rate of the thinning down of the thread is in good agreement with the one obtained from experiments for very small Reynolds number range, where viscous force is dominant. Since the length scale becomes very small close to the pinch-off, the viscous effect overcomes the inertial effect and the scaling alters from the potential flow scaling law to the viscous scaling law.Copyright