| Research Abstract |
The lattice parameters of ThO_2, U O_2, Np O_2 and PuO_2 samples were expressed in terms of the third order polynomial equation of the absolute temperature. The linear thermal expansion coefficients(alpha) of the actinide dioxides calculated at 1200 K were in reverse relation to their melting points. At room temperature, on the other hand, the alpha value of UO_2 was found to be higher than those of NpO_2 and PuO_2, probably due to lower Debye temperature of UO_2.
The lattice parameters of Np_yU_<1-y>O_<2+x> (y=0, 0.1, 0.3, 0.5, 0.7, 1.0), Np_yPu_<1-y>O_<2+x> (y=0, 0.05, 0.1, 0.2, 0.5) and (Pu_yU_<1-y>)O_<2 > (y=O, 0.02, 0.03, 0.05, 0.08, 0.10) solid solutions were expressed as the third order polynomial equation and smoothly increased with increasing the absolute temperature without any phase transitions. The specific heat capacities of their solid solutions were calculated from the thermodynamic relation C_p = (beta V)/( kappa_3 gamma) using measured thermal expansion data and estimat
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ed mechanical properties. The errors for heat capacities of UO_<2 > were within *5% of the experimental values. The calculated C_p values for all solid solutions in this project showed the same temperature dependence as the literature ones.
Deviation from Vegard's law had been reported at y=0.04 for stoichiometric (Pu_yU_<1-y>)O_<2.00> and at y=0.02 for stoichiometric (Th_yU_<1-y>)O_<2.00> in the literatures, However, in this study no abnormal behavior was observed for doped cation concentrations, and the lattice parameters of the stoichiometric (M_yU_<1-y>)O_<2.00> (M=Pu, Th, La) solid solutions changed linearly with increasing doped cation at small y. The valence of the doped cations was calculated from the dependence of the cation concentrations on the lattice constants by assuming a random distribution of doped cations in uranium dioxide and an ionic model where every cation and anion form a rigid sphere. By comparing the theoretical values with the experimental data, plutonium, thorium and lanthanum ions in solid solution are considered to exist as Pu^<4+> (orPu^<3+>), Th^<4+> and La^<3+>. Less
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