The heat capacity and enthalpy of condensed UO2: Critical review and assessment

Having established the role of the heat capacity, Cp(T), of condensed UO2 in various FBR accident scenarios, e.g. HCDA and PAHR, and having noted the unsatisfactory state of present knowledge concerning this basic thermophysical property of the fuel, all existing enthalpy and heat capacity data are collated and assessed, and certain recommendations made. The conventional method of obtaining Cp(T) by analytical differentiation of some adopted fit to this enthalpy data is then critically examined. The attendant problems are illustrated both for solid UO2, where the contribution to Cp(T) from the weak, sigmoidal, enthalpy structure (which is just discernible in the data of Hein and Flagella) is missed and for molten UO2, where not even the direction of the trend of Cp(T) with T can be definitively established, resulting, upon extrapolation to 5000 K, in Cp values which can differ by as much as 60 J mol-1 K-1. Some recent progress towards a more acceptable, ''model-independent'' approach, known as quasi-local linear regression (QLLR), is then reviewed and applied to enthalpy data of UO2 on both sides of its melting point, Tm. In the case of solid UO2, a pronounced heat capacity peak, extending over about 100 K and centred on 2610 K, is revealed, whose magnitude and location is very similar to that found in other fluorite structured materials near 0.8Tm wherein it indicates a (Bredig) transition to a state characterised by giant ionic conductivities. Whilst it is impossible to establish any definite T-dependence for the Cp(QLLR) values in molten UO2, the tendency to slightly decrease appears to marginally outweigh the converse, in qualitative accord with the dependence advocated by Hoch and Vernardakis. (orig./WL)