Besides the importance of the actinide dioxide series as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. Their surprisingly varied physical properties at low temperatures stimulate continuing interest for both theory and experiment. Recently, we have performed 17O-NMR studies for the first time on Pu and Amcontaining dioxide systems, (Pu1-xAmx)O2. For the x=0.09 sample, a temperature-dependent NMR line broadening has been observed at low temperatures. By comparing the experimental data with the results of NMR line simulations, we have estimated the effective moment of Am ions to be Peff=1.38 μB. The value suggests the 5f5 (Am4+) state of the Am ion in PuO2. For the x=1 (=AmO2) sample, on the other hand, our 17O-NMR data provide the first microscopic evidence for a phase transition at 8.5 K as a bulk property in this system. A spectrum with a triangular line shape indicates that the internal field is distributed very nearly randomly in the ordered state.

We proposed a new concept for densification of inert matrix fuels containing minor actinides. In this concept, magnesium silicates which are both a naturally-occurring material and asbestos waste were used as a sintering additive which protects public health by safely disposing of the asbestos waste. In this study, the effects of magnesium silicate additives on the densification behaviors of MgO, Mo and CeO2 were experimentally investigated. The densities of MgO and CeO2 pellets increased with only 1 wt.% additives of MgSiO3 and Mg2SiO4. The densities of Mo pellets showed little change with additives.

Oxygen potentials of (Th0.7Ce0.3)O2-x were experimentally determined by means of thermogravimetric analysis as a function of non-stoichiometry at 1173 and 1273 K. Oxygen potentials of (Th0.7Ce0.3)O2-x at each temperature increased with increase of oxygen to metal (O/M) ratio (=2-x) and steep increases of the oxygen potentials when approaching O/M ratio = 2 were observed. These characteristics are typical for non-stoichiometric fluorite-type actinide dioxides. The oxygen potentials of (Th0.7Ce0.3)O2-x were similar to those of CeOO2-x when they were plotted as a function of average Ce valence.

Thermodynamic modeling has been performed for O-Am binary system and O-Pu-Am ternary system on the basis of thermodynamic data available in the literature. The interaction parameters of the excess Gibbs energies of the fluorite structure FCCC1 phase in the O-Am and the O-Pu-Am systems were evaluated with respect to the deviation from the ideal solution in oxygen potential, respectively. The thermodynamic modeling used in this study is based on the CALculation of PHAse Diagram (CALPHAD) technique.

The molecular dynamics (MD) calculation was performed for minor actinide (MA: Np and Am)-containing mixed oxide (MOX) fuels, U0.7-xPu0.3MAxO2, in the temperature range from 300 to around 2500 K to evaluate the thermal expansion, heat capacity, and thermal conductivity. The MD results showed that the calculated heat capacity and thermal conductivity were similar in all the composition ranges, indicating that MA scarcely affected the thermal properties of the MOX fuel in the perfect crystal system.