Yuko Kani

Solubility of Lanthanide Fluorides in Nitric Acid Solution in the Dissolution Process of FLUOREX Reprocessing System

A new reprocessing technology, FLUOREX, is proposed for the thermal reactor cycle. In the dissolution process of FLUOREX, it needs to estimate the concentration of fluoride ion in the solution to avoid formation of insoluble plutonium precipitate. We measured the solubility products of lanthanide fluorides, LaF3, CeF3, and NdF3 in nitric acid solution and evaluate the concentration of fluoride ion in the dissolution process. Solubilities were measured by both of dissolution method and precipitation method. The solubility products are about 101 to 102 times larger than those in water. This largeness may be caused by higher ionic strength of nitric acid solution. The newly obtained solubility products for the lanthanides are useful to calculate the maximum concentration of plutonium ions in the FLUOREX dissolution process, although the further study for the solubility products for plutonium tetrafluoride is needed in the nitric acid solution.

Radiolytic decomposition of organic C-14 released from TRU waste

It has been found that metallic TRU waste releases considerable portions of C-14 in the form of organic molecules such as lower molecular weight organic acids, alcohols and aldehydes. Due to the low sorption ability of organic C-14, it is important to clarify the long-term behavior of organic forms under waste disposal conditions. From investigations on radiolytic decomposition of organic carbon molecules into inorganic carbonic acid, it is expected that radiation from TRU waste will decompose organic C-14 into inorganic carbonic acid that has higher adsorption ability into the engineering barriers. Hence we have studied the decomposition behavior of organic C-14 by gamma irradiation experiments under simulated disposal conditions. The results showed that organic C-14 reacted with OH radicals formed by radiolysis of water, to produce inorganic carbonic acid. We introduced the concept of “decomposition efficiency” which expresses the percentage of OH radicals consumed for the decomposition reaction of organic molecules in order to analyze the experimental results. We estimated the effect of radiolytic decomposition on the concentration of organic C-14 in the simulated conditions of the TRU disposal system using the decomposition efficiency, and found that the concentration of organic C-14 in the waste package will be lowered when the decomposition of organic C-14 by radiolysis was taken into account, in comparison with the concentration of organic C-14 without radiolysis. Our prediction suggested that some amount of organic C-14 can be expected to be transformed into the inorganic form in the waste package in an actual system.Copyright

Radiolytic hydrogen evolution in a closed vessel

ABSTRACT Radiolytic hydrogen gas evolution under the liquid-gas two-phase condition has been studied using a closed vessel and γ -rays from a Co-60 source to develop an evaluation method for the H2 evolution amount during transfer and storage of radioactive materials. An experiment was conducted using a closed vessel in which air and aerated pure water were present at room temperature. Several vessels were irradiated once with ɣ-rays at 5.2 × 103 Gyh–1. It was found that apparent G-values of H2 production, calculated with a pressure increase of the closed vessel, became smaller for the cases of higher ratio of gas phase volume to liquid phase volume due to the recombination reaction of radiolytic H2 with O2 and H2O2. Also, equilibrium H2 partial pressure became 10 times higher than the expected value using Henry’s law. These behaviours were explained by the developed model, which includes the liquid-gas distribution ratio of radiolytic H2, the equation of state for H2 in the gas phase, and the effective volume of liquid phase relevant to the liquid-gas distribution under the irradiation conditions. The effective volume of liquid phase was determined by considering the extent of the recombination reaction of radiolytic H2 during mass transfer from the liquid phase to the gas phase.

Experimental study on elemental behaviors in fluorination of nuclear spent fuel with flame reactor

Our proposed spent nuclear fuel reprocessing technology named FLUOREX is a hybrid system based on reprocessing technologies of fluorination and solvent extraction for light water reactor fuel. In the current research, we experimentally clarified solid–gas transfer behaviours of the fluorides in the FLUOREX process and identified the volatile and non-volatile compounds in the fluorination. We carried out a fluorination experiment for simulated spent nuclear fuel and solid separation from the UF6 gas stream. The distribution ratios of fission product elements in the experimental apparatus were evaluated. Molybdenum, Te, Nb, and Ru were volatilized by fluorination and they accompanied the UF6 gas. However, 22.9% of the Ru and 3.4% of the Nb were retained as solids in the experimental apparatus, contrary to the fact that their partial pressures in the experiment were lower than their vapor pressures. Rubidium, Sr, Zr, Ce, and Nd were completely recovered as solid fluorides, and these results agreed with the prediction based on boiling points of their fluorides. Antimony was completely recovered as a solid; nevertheless, the boiling point of antimony pentafluoride was lower than the process temperature, and that was attributed to the formation of a non-volatile antimony oxyfluoride.