Laboratory Measurement of the Heat Capacity of Urania up to 8000 K: I. Experiment

The heat capacity C_p of UO₂ was measured in a laboratory experiment where sintered 0.5-to 1-mm-diam microspheres were heated by four tetrahedrally oriented laser beams in an inert-gas-filled autoclave at pressures up to ∼1000 bar. The sample, suspended by a tungsten needle, was heated to 8000 K during pulses of a few milliseconds duration. The experimental technique, the instrumentation, and the analytical method used to deduce C_p from the experimental pulse-heating curves are described. Between the melting point T_m and ∼4000 K, the heat capacity decreases to a value close to that given by the Neumann-Kopp rule for a triatomic, harmonic lattice, i.e., 9R. Near 5000 K, however, the heat capacity again increases, and it appears to saturate at a value ∼30% higher by 8000 K. The new results are compared with published C_p values for molten UO₂ (and other relevant materials) and are briefly discussed in light of the established temperature dependence of C_p at T < T_m and the high-energy electronic structure of UO₂.