Heat transfer and thermal conductivity of multi-hundred watt 238PuO2 fuel spheres

The thermal conductivity of porous ²³⁸PuO₂ has been calculated from steady-state core and surface temperatures in spheres having diameters of ≈ 3 and ≈ 3.7 cm assuming linear heat transfer at the sphere surface. The spherical fuel forms had densities of 80 to 85% of T.D. for PuO₂ (T.D. = 11.41 g/cm³). Thermal conductivity in the fuel increased nearly linearly with $\text{T}{}_{\mathrm{<mtext>avg</mtext>}}\text{(}\text{K}\text{)]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ in the range 350–1100°C and thus was dominated by a gas phase present in the open and closed porosity of the fuel. For spheres heated in helium or vacuum, the gas phase had essentially three degrees of freedom. When the fuel was heated in argon saturated with water vapor, approximately 50 degrees of freedom were indicated and this was apparently largely due to the water vapor. A two-phase, series slab heat flow model adequately predicted the thermal conductivity of the ²³⁸PuO₂ phase as a function of temperature.