Heat capacity measurements on unirradiated and irradiated fuel pellets

Heat capacities (Cp) of oxide fuels are essential for the evaluation of fuel temperature at normal, transient and accidental conditions of light water reactors. In this study, the effect of soluble fission products (FPs) was first examined, and then the Cp measurement technique was improved to obtain reliable data for irradiated pellets of smaller than the standard size. A differential scanning calorimeter was applied to the Cp measurements. From the measurements of standard size α-Al2O3, the inaccuracy of the apparatus was estimated to be about 4%. The Cp data of undoped and simulated FPs-doped UO2 pellets of standard size were measured in the temperature region from 325 to 1673 K. From these results, the effect of impurities on the Cp values of UO2 pellets and the presence of the Cp anomalies in the simulated FPs-doped UO2 pellets were investigated. For measurements of small size specimens, the sample-side crucible was modified. The inaccuracy of the apparatus was estimated to be 5% using a small size of the α-Al2O3 sample. This improved technique was applied to obtain the Cp data of UO2 and UO2–10 wt%Gd2O3 specimens with small sizes from 325 to 1673 K, which had been irradiated at isothermal conditions in a test reactor.