Hajime Kinoshita

Thermodynamic modelling and phase stability assessment of MO2−X oxides with a fluorite structure

Abstract Thermodynamic modelling was performed for O–Th and O–Np systems to assess the phase stability of the MO 2− X phase together with the data in the literature for O–Pu, O–U, and O–Zr systems. Firstly, thermodynamic modelling was demonstrated for O–Th and O–Np systems with thermodynamic data and phase diagram information available in the literature. The assessed data reproduced the general feature of the system with respect to the phase diagrams of the both systems and the oxygen potential in the ThO 2− X and NpO 2− X phases. Secondly the phase stability of the MO 2− X phase was assessed using the obtained sets of data together with those for O–Pu, O–U, and O–Zr. The phase stability of the MO 2− X phase was discussed with respect to the deviation from the ideal solution in oxygen potential and Gibbs free energy.

Phase relation assessment of the O–Pu–Zr system by thermodynamic modelling

Abstract Thermodynamic modelling was performed to assess the phase relation in the O–Pu–Zr system. Firstly a preliminary thermodynamic modelling was attempted for the O–Pu system with thermodynamic data and phase diagram information available in the literature. The assessed data reproduced the general feature of the system with respect to phase diagram, oxygen potential and heat capacity. Secondly a possible set of phase diagrams for the O–Pu–Zr system was calculated using obtained data for the O–Pu system together with those for the O–Zr and Pu–Zr sub-systems available in the literature. Thermodynamic parameters assessed for the PuO 2 –ZrO 2 pseudobinary system gave a phase diagram with a feature very similar to those for CeO 2 –ZrO 2 and UO 2 –ZrO 2 systems. O–Pu–Zr ternary phase diagrams were also calculated in the temperature range of 773–1773 K, and phase relation in the system was discussed.

Chemical Durability of Yttria-Stabilized Zirconia for Highly Concentrated TRU Wastes

Neptunium has an extremely long half-life and will be the main toxic element in the later stage of disposal. Yttria-Stabilized Zirconia (YSZ), with a fluorite structure, samples doped with Np-237 in high concentration (20, 30, 40 mol %) were fabricated (sintered in Air or Ar, at 1773 K, for 80 hours), and their leaching test (in deionized water, at 423 K, for 84 days) was carried out. The results indicated that the obtained leaching rates were much smaller than those of the Synroc and glass waste form, and that the increase in Np content did not cause any drastic changes in the leaching rates of Zr, Y, or Np. They were also compared to the results of YSZ doped with Ce or Nd used as surrogates for actinides. The work showed that YSZ doped with Np in high concentration has an excellent chemical durability

Re-evaluation of the phase relationship between plutonium and zirconium dioxides

The phase relationship between ZrO 2 and PuO 2 was examined in a low PuO 2 content region, from 3.1 to 11.2 mol% PuO 2 , at temperatures between 1273 K and 1473 K, by high temperature X-ray diffractometry. The measurements were carried out in air. At 1273 K, the samples in this composition range consisted of two phases, monoclinic and cubic. Another phase, tetragonal, was observed at 1373 K. The low temperature monoclinic phase disappeared at 1473 K. It was confirmed that the monoclinic phase disappears around 1463 K; the disappearance temperature does not depend on the composition of the sample. It was, thus, inferred that there should be a eutectoid line in the phase diagram. Though the eutectoid point is not clear, the PuO 2 content at the point should be less than 3.1 mol%.

Identification of oxide phase and alloy phase obtained by heat treatment of calcined high level liquid waste with TiN reducing agent at 1873 K

Abstract Simulated calcined HLLW was heat treated with TiN as a reducing agent at 1873 K. The treatment resulted in the melted and separated alloy phase and oxide phase. The alloy phase contains CrFeNi alloy, FeNi alloy, CrNi alloy, RuFe alloy, Mo and Pd metals which are dissolved by Fe and does not contain Ti. The dissolution of Fe in Ru and Mo caused the melting of alloy phase at the treatment temperature. The oxide phase contains ZrTiO 4 ,Sr 0.5 NdTi 2 O 6 , BaTiO 3 , La 4 Ti 9 O 24 , SrCeO 3 and some neodymium-zirconium phases. Some phase are those used as the host phase in the ceramic waste and the other phases are resulted from the smaller amount of Ti in the method than in ceramic waste. The formation of the complex oxides caused the melting of the oxide phase at the treatment temperature.

Processing high-level liquid waste by super-high-temperature method, (IV) : Reducing reactions and alloy formation by platinum group elements, molybdenum and corrosion products taking place in simulated HLLW

An experimental study made on the reducing reactions and the formation of alloys taking place in oxide mixtures representing high-level liquid waste (HLLW) generated by the PUREX process, and in particular, on the effect brought on the metal melting temperature by the presence of corrosion products mainly Fe, Cr, Ni deriving from the vessels used in the process. It proved that heat treatment at 1,873K melted the metal phases produced from all the starting mixtures taken up. The metal phases obtained from starting mixtures of corrosion products or their oxides proved to contain alloys of Cr-Fe-Ni, Fe-Ni and Cr-Ni, and in addition Ru-Fe alloy and Mo, Pd metals when the starting mixture further included platinum metals and molybdenum oxides, which also are found in the HLLW from PUREX process. The same six alloys and metals cited above were formed in the metal phases obtained from simulated calcined HLLW used as starting mixture. The melting of Ru and Mo at 1,873 K far below their melting points was attributed to the dissolution of Fe into Ru and of Fe or Ni into Mo.

Mechanical integrity of yttria-stabilised zirconia doped with Np oxide

Mechanical properties of YSZ doped with Np oxide were studied to investigate the sufficiency to be a waste form for immobilisation of highly concentrated TRU. The study was conducted focusing on Vickers (H V ) and Knoop (H K ) hardness, Youngs modulus (E) and fracture toughness (K IC ). The results showed that YSZ is harder and more resistant to elastic deformation and crack development than such waste forms as borosilicate glass and synroc. The effects of porosity and Np content on H V , H K , E and K IC are also discussed.