The High-Temperature Ex-Reactor Thermal Conductivity of Thoria and Thoria-Urania

Abstract

Thermal conductivity of a material can be determined by the product of thermal diffusivity, heat capacity, and density. Each of these properties is reviewed for ThO2 and for ThO2-UO2 solid solutions, and compared with similar data for UO2. Between room temperature and 2950 K for ThO2 and between room temperature and the temperature range 2850–2950 K for the solid solutions (Th0.92U0.08)O2 to (Th0.70U0.30)O2, thermal conductivity of fully dense (Th1-M-UM)O2, for M ≤ 0.3, can be expressed by the following fitted equations: $$\begin{array}{l} 1/{\lambda _o}\; = \;A + BT;\\ A = \;1/\left( {46.947\; - \,112.072\;M} \right);\\ B\; = \;0.0001\left( {1.597\; + \,6.736\;M\; - 21.56\;{M^2}} \right) \end{array}$$ where λo is the thermal conductivity in W/m·K and M is the mole fraction of UO2. Corrections for porosity (P) are made using the Maxwell-Eucken equation, λ/λo = (1 - P)/(1 + sP), with a constant value of 1.15 for the coefficient s. Since there is no significant electronic contribution to the heat capacity of ThO2, unlike the case for UO2, the thermal conductivity of ThO2 continues to decrease with increasing temperature until a solid-solid phase transition at 2950 K is reached, at which point heat capacity and thermal conductivity increase about 50%. Thermal conductivity decreases again above the transition temperature. ThO2-UO2 solid solutions also show evidence for phase transitions from enthalpy data and thus show increases in calculated heat capacities and thermal conductivities. The pretransion thermal conductivities of the ThO2-UO2 solid solutions are intermediate between those of thoria and urania. Experimentally determined pretransition heat capacities for the solid solutions agree with values calculated from mole averages of ThO2 and UO2.