Kuniyoshi Hoshino

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.

Adsorption of Molybdenum Hexafluoride on Magnesium Difluoride for Uranium Purification in FLUOREX Reprocessing

The nuclear fuel reprocessing method FLUOREX is a hybrid system based on fluoride volatility and using solvent extraction. Spent nuclear fuel is fluorinated, and most of the uranium is recovered as UF6 gas. UF6 contains some volatile fission product (FP) fluorides, so we considered their elimination from UF6 by adsorbing them on fluoride adsorbents. We experimentally examined the adsorption of MoF6 on MgF2 adsorbent; MoF6 is present as a volatile FP fluoride in UF6 produced by the fluorination of spent nuclear fuel. The adsorption isotherm of MoF6 adsorption on MgF2 was obtained at MoF6 pressures from 10−4 to 50 kPa. The saturated adsorption amount was 1:3 ± 0:4 mg/m2 at MoF6 pressures from 10−4 to 1 kPa. At MoF6 pressure of about 10−3 kPa, the saturated adsorption amount had no dependence on adsorption temperatures from 398 to 463 K. We deduced that MoF6 was adsorbed as a monomolecule layer on the MgF2 surface at MoF6 pressures from 10−4 to 1 kPa, and the MoF6 partial pressure in UF6 could be decreased below 1 × 10−4 kPa, which is the specific MoF6 concentration for the reenrichment process.

Transition period fuel cycle from current to next generation reactors for Japan

Long-term energy security and global warming prevention can be achieved by a sustainable electricity supply with next generation fast breeder reactors (FBRs). Current light water reactors (LWRs) will be replaced by FBRs and FBR cycle will be established in the future considering the limited amount of uranium (U) resource. The introduction of FBRs requires plutonium (Pu) recovered from LWR spent fuel. The authors propose advanced system named “Flexible Fuel Cycle Initiative (FFCI)” which can supply enough Pu and hold no surplus Pu, can respond flexibly the future technical and social uncertainties, and can achieve an economical FBR cycle. FFCI can simplify the 2nd LWR reprocessing facility for Japan (after Rokkasho Reprocessing Plant) which only carries out U removal from LWR spent fuel. Residual “Recycle Material” is, according to FBRs introduction status, immediately treated in an FBR reprocessing to fabricate FBR fuel or temporarily stored for the utilization in FBRs at necessary timing. FFCI has high flexibility by having several options for future uncertainties by the introduction of Recycle Material as a buffer material between LWR and FBR cycles.

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.

Minor Actinides-Loaded FBR Core Concept Suitable for the Introductory Period

We proposed the “Flexible Fuel Cycle Initiative” (FFCI), which has flexibility for the uncertainties like the introduction speed of FBRs. On the other hand, during the FBR introduction period, Pu from LWR spent fuel is used for startup of FBRs. But the FBR core being loaded with Pu from LWR spent fuel has larger burnup reactivity due to its larger isotopic fraction of Pu-241 than the core being loaded with Pu from the FBR multi-recycling core. The increased burnup reactivity may reduce the cycle length of the FBR. In this paper, an FBR transitional core concept to handle the issues of the FBR introductory period was investigated. Core specifications are based on the compact type sodium-cooled MOX-fueled core designed in the Japanese FBR cycle feasibility studies, because the lower Pu inventory should be better for the FBR introductory period in view of its flexibility for the required reprocessing amount of LWR spent fuel to start up the FBR. The reference specifications are selected as follows. Output is 1500MWe and the average discharge fuel burnup is about 150GWd/t. Minor Actinides (MAs) recovered from LWR spent fuels which provide Pu to startup FBR are loaded to the initial loading fuels and exchanged fuels during some cycles until equilibrium. We set a kind of MA fraction rate of the initial loading fuel with 4 as the number of the fuel exchange batches. The average of the MA fraction of the initial loading fuel assumed is 3%, and the MA fraction of the exchange fuel is set as 5%. This 5% maximum of the MA fraction is based on the irradiation results of the experimental fast reactor Joyo. The core performance including burnup characteristics and reactivity coefficient were evaluated, and we confirmed that the transitional core from the initial loading until equilibrium cycle loaded Pu from LWR spent fuels could keep the resemble performance with the FBR multi-recycling core.Copyright

Uranium Recovery From LWR Spent Fuel for the Future FR Deployment

Nuclear energy systems are necessary to assure sufficient energy resources without harming the environment. Fast reactor (FR) systems are especially important taking into account the limited uranium resources and the nuclear sustainability. As the FR system is still under development, FR deployment start-time and rate are unclear. On the other hand, it is desirable to reduce light water reactor (LWR) spent fuel due to the difficulties of storage and disposal (retrievable) site determination. Reprocessing is one of the effective methods to reduce LWR spent fuel but the recovery and long-term storage of plutonium, even with uranium, is undesirable for the aspect of proliferation resistance. The authors propose the new system named Flexible Fuel Cycle Initiative (FFCI), which recovers only uranium (∼90%) from LWR spent fuel and stores the residual material (∼5% U, ∼1% Pu, ∼4% other nuclides) for the future FR deployment. Residual material named recycle material (RM) is suitable for FR fresh fuel preparation due to its high Pu concentration and similar Pu/U ratio to FR core fuel, and for proliferation resistance due to its high concentrations of fission products (FP) and minor actinides (MA). The volume of RM is about 1/10 of that of LWR spent fuel. However RM needs sufficient heat removal, radiation shielding and criticality safety. After the FR development is finished and several years before the commercial FR deployment start-time, Pu and U will be recovered from the RM that might be stored liquid or solid state. Many well known methods can be applied for U recovery such as solvent extraction, crystallization, precipitation, electro refining, and fluoride volatilization. As recovered U has slightly higher U-235 concentration than natural U, its re-enrichment and recycling in LWRs seems to be effective for ultimate utilization of nuclear resources. In this case fluoride volatility U recovery method is most preferable because the product is UF6 that is the supply material for enrichment. Quantitative evaluations have been carried out for several fuel cycle systems including FFCI with parameters such as spent fuel amounts, facility capacity and Pu balance, which revealed the feasibility and flexibility of FFCI for LWR spent fuel reduction, high facility capacity factors and sufficient (no excess) Pu supply to FR.Copyright