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Solid-liquid phase equilibria of Np(VI) under 80%, 0.99% and 0.03% C0 2 and of U(VI) under 100% and 0.03% C0 2 were investigated in 0.1 M NaC10 4 solution of pH ranging 2.9-4.9 at 25±0.1°C. Neptunium was oxidized and stabilized in hexavalent state in the presence of ozone under various C0 2 partial pressures. Aqueous phases of Np(VI) and U(VI) were characterized by pH measurement, liquid scintillation counting, absorption spectroscopy and fluorescence spectroscopy. Solid phases were analyzed by X-ray crystallography, UV-Vis-NIR photoacoustic spectroscopy and FT-IR photoacoustic spectroscopy. Np0 2 C0 3 (s) and U0 2 C0,(s) were found as solubility limiting solid phases under 80% and 100% C0 2 , while NpO,(s) and U0 3 (s) were characterized as equilibrium solid phases under 0.03% and 0.99% C0 2 , respectively. The solubility products of the carbonates were found to be lg ^(NpOîCOj) = -14.62±0.12 and lg/f v (U0 2 C0 3 ) = -14.10±0.14, and those of the trioxides were lg/fJNpO,) = -21.73±0.17 and lgtf,"(U0 3 ) = -22.15±0.06.

Research on the plutonium rock-like oxide (ROX) fuels and their once-through burning in light water reactors has been performed to establish an option for utilizing and disposing effectively the excess plutonium. The ROX fuel is a sort of the inert matrix fuels and consists of mineral-like compounds such as yttria stabilized zirconia, spinel and corundum. A particle-dispersed fuel was devised to reduce damage by heavy fission fragments. Some preliminary results on swelling, fractional gas release and microstructure change for five ROX fuels were obtained from the irradiation test and successive post-irradiation examinations. Inherent disadvantages of the Pu-ROX fuel cores could be improved by adding 238 U or 232 Th as resonant materials, and all improved cores showed a nearly the same characteristics as the conventional UO 2 core during transient conditions. The threshold enthalpy of the ROX fuel rod failure was found to be comparable to the fresh UO 2 rod by pulse-irradiation tests simulating reactivity initiated accident conditions.

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