Comparison of thermomechanical properties of (Ux,Th1-x)O2, (Ux,Pu1-x)O2 and (Pux,Th1-x)O2 systems

Abstract

The thermomechanical properties of mixed actinide oxide and its dependence on actinide composition are of significant interest for the safety in reactor design and operation. In this study, the elastic properties of (Ux,Th1-x)O2, (Ux,Pu1-x)O2 and (Pux,Th1-x)O2 solid solutions for 50-50 cation composition and thermal conductivity of (Pux,Th1-x)O2 system, for x = 0.3 and 0.5, have been investigated in the temperature range from 300 to 1500 K using Molecular dynamics (MD) simulations. Compared to pure oxides, reduced thermal conductivity is reported for (Pux,Th1-x)O2 mixture due to non-uniform cation sublattice. The degree of reduction in conductivity is the largest in (Pux,Th1-x)O2 than that of previously published (Ux,Th1-x)O2 and (Ux,Pu1-x)O2. This is attributed to the largest mismatch in lattice parameter between PuO2 and ThO2. In general, the elastic properties, for all pure and mixed oxide fuels, linearly decrease with the increase in temperature, and this is commonly observed also in other studies. The rate of change in elastic modulus (ε) with temperature follows the trend, for pure oxides: εPuO2>εUO2>εThO2 and for mixed oxides: εUPuO2>εUThO2>εPuThO2. In most cases, for (Ux,Th1-x)O2 and (Ux,Pu1-x)O2, the elastic modulus is similar to the linear interpolation between the corresponding pure oxides. However, for (Pux,Th1-x)O2, significant deviation from the linearity is observed and that is justified by differences in oxygen mobility in the mixed oxide fuels. In addition, for all oxides, except Pu0.5Th0.5O2, our results predict εB>εY>εG where B, Y and G being the bulk elastic properties. A calculation of elastic modulus at 0 K for all oxides is presented.