Kazuo Minato

Irradiation experiment on ZrC-coated fuel particles for high-temperature gas-cooled reactors

The ZrC coating layer is a candidate to replace the SiC coating layer of the Triso-coated fuel particle. To compare the irradiation performance of the ZrC Triso-coated fuel particles with that of the normal Triso-coated fuel particles at high temperatures, a capsule irradiation experiment was performed, where both types of the coated fuel particles were irradiated under identical conditions. The burnup was 4.5% FIMA and the irradiation temperature was 1400 to 1650°C. The postirradiation measurement of the through-coating failure fractions of both types of coated fuel particles revealed better irradiation performance of the ZrC Triso-coated fuel particles. The optical microscopy and electron probe microanalysis on the polished cross section of the ZrC Triso-coated fuel particles revealed no interaction of palladium with the ZrC coating layer nor accumulation of palladium at the inner surface of the ZrC coating layer, whereas severe corrosion of the SiC coating layer was observed in the normal Triso-coated fuel particles. Although no corrosion of the ZrC coating layer was observed, additional evaluations need to be made of this layer’s ability to satisfactorily retain the fission product palladium.

A model to predict the ultimate failure of coated fuel particles during core heatup events

In this paper a model to predict the ultimate failure of TRISO-coated fuel particles under hypothetical core heatup events is proposed. Features of the model include the ability to treat the statistical variation of the number of coated fuel particles and to make a thermodynamic estimation of the stoichiometry of irradiated UO{sub 2} kernels and the equilibrium CO pressures. The model predictions agree well with the results of postirradiation heating tests. The thermal creep of pyrolytic carbon, however, must be taken into account to further improve the accuracy of the prediction.

Spectrophotometric study of Nd2+ ions in LiCl-KCl eutectic melt

A UV and visible spectrophotometric study was made in order to prove the existence of Nd2+ and clarify the equilibrium among Nd metal, Nd2+, and Nd3+ in LiCl-KCl eutectic melt. Spectra assigned to Nd2+ were observed for NdCl2 in (LiCl-KCl)eut. and Nd- NdCl3 in (LiCl-KCl)eut. melts. Black corrosion products were observed on the surface of the glass cells used for the measurements, where the spectra assigned to Nd2+ were observed. X-ray diffraction measurements and electron-probe micro-analyses of the corroded glass cells revealed that the corrosion products contained NdOCl.

Dissolution of uranium nitrides in LiCl-KCl eutectic melt

A combination of the nitride fuel and pyrochemical reprocessing has been chosen as a candidate for the fuel cycle of the minor actinides (MA) burner systems. One important issue is the feasibility of the recycled use of 15N-enriched nitrogen in the nitride pyrochemical process. Dissolution of spent nitride fuel with an oxidation agent has been considered to be a promising method for recovering 15N-enriched nitrogen. The behavior of the dissolution of uranium nitrides (UN, U2N3) and UNCI with CdCl2 in LiCl-KCl eutectic melt is investigated to show feasibility of the method for uranium containing nitride fuel. Most of nitrogen is recovered as N2 gas resulting from the reaction of uranium nitrides and CdCl2 above 550 °C, which is higher than that reported for lanthanide nitrides, surrogates of transplutonium nitrides, with similar conditions. Formation of intermediate compounds is observed with the temperature below 500°C. ZrN, a candidate of dilution material for MA burner fuel, is hard to be dissolved with CdCl2 in LiCl-KCl eutectic salt even at high temperature such as 800 °C.

Cyclic Voltammetry Behavior of Americium at a Liquid Cadmium Electrode in LiCl-KCl Eutectic Melts

Abstract The electrode reactions of americium at a liquid cadmium electrode were investigated by cyclic voltammetry of AmCl3-(LiCl-KCl)eut. at both 723 and 773 K in comparison with those at a molybdenum electrode. The redox peaks assigned to Am(III)/Am(0) (in Cd) were observed with the liquid Cd electrode, while the redox reactions of Am(III)/Am(II) and Am(II)/Am(0) were observed with the Mo electrode. The formal standard potential of Am(III)/Am(0) obtained with the liquid Cd electrode is more positive than those calculated for the Mo electrode at both 723 and 773 K. The potential shifts were attributed to the lowering of the activity of Am by the formation of the intermetallic compound at the interface between Cd and the molten salt. The Gibbs free energies of formation of the Am-Cd intermetallic compound, which could be AmCd6, are estimated to be -119 and -113 kJ/mol at 723 and 773 K, respectively.

Local Structure of Molten CdCl2 Systems

The local structure of molten CdCl2 was investigated by X-ray absorption fine structure (XAFS) and X-ray diffraction(XRD) analyses. The nearest Cd2+-Cl− distance decreases from 2.61 Å in the room temperature solid state to 2.47 - 2.50 Å in the molten state. The coordination number decreases from 6 in the solid to 4 in the melt. The obtained structural parameters from the XAFS and the XRD analyses suggest that a tetrahedral coordination (CdCl4)2− is predominant in molten CdCl2. The XAFS result of a molten 50%CdCl2-KCl mixture shows that the 4-fold (CdCl4)2− structure holds also in the mixture

Thermal conductivities of (Np,Am)N and (Pu,Am)N solid solutions

The thermal diffusivities and heat capacities of transuranium nitride solid solutions, (Np,Am)N and (Pu,Am)N, were measured by using a laser flash method and a drop calorimetry, respectively. The thermal conductivities of these samples were determined from the measured thermal diffusivities, heat capacities and bulk densities. The thermal conductivities of (Np,Am)N and (Pu,Am)N increased with temperature over the temperature range investigated. The increases in the thermal conductivities were probably due to the increases of electrical components. In addition, the thermal conductivities of (Np,Am)N and (Pu,Am)N decreased with increasing Am contents. It could be considered that the decreases in the thermal conductivities with increasing Am contents correspond to the lowering of electronic contribution.

Synthesis of Actinide Nitrides in Molten Cadmium

Abstract Pyrochemical processing has several advantages over conventional wet processing for the treatment of spent nitride fuel. Besides the molten salt electrorefining of nitride fuel, actinide nitride formation in and recovery from the liquid cadmium cathode is a key technology in the nitride fuel cycle with pyrochemical processing. It was found that heating a cadmium alloy containing plutonium and uranium at 973 K under a nitrogen gas stream resulted in the simultaneous nitridation of the actinides and the distillation of cadmium. Furthermore, nitride pellets were prepared from the powder recovered without any milling or granulating process. The results obtained in the present study suggested that the nitride fuel cycle with pyrochemical processing is technologically feasible for transmutation of minor actinides and advanced fast reactors.

Unique extraction behavior of americium and curium in a system of TBP and calcium nitrate hydrate melt

Extraction of Am(III) and Cm(III) between tri-n-butyl phosphate solution and molten calcium nitrate hydrate Ca(NO3)2RH2O was investigated radiochemically. In the range of water content R = 3.5-8.0, the distribution ratio was found to increase with the decrease of water activity. The dependence of the distribution ratios on the water activity in the hydrate melt changes at around log aH2O = −0.4, which corresponds to R = 5.0. The extraction behavior of Am(III) and Cm(III) was systematically discussed with the reported data of trivalent lanthanides.

The local structure of molten CdBr2

The local structure of molten CdBr2 was investigated by high temperature X-ray absorption fine structure (XAFS) analysis. The quartz cell designed for hygroscopic high temperature molten salts was successfully used in the measurement. At room temperature the nearest neighbor Cd2+-Br− distance decreased from 2.71 Å in solid state to 2.60 Å in the molten state. The coordination number decreased from 6 to 4 on melting. The obtained structural parameters showed that (CdBr4)2− is predominant in molten CdBr2.