Specific heat is an important parameter for the transient behavior studies where the temperature variations are linked to the variations of reactor power. Also, it is required for the calculation of thermal conductivity from thermal diffusivity. Only a very limited number of studies are available and the specific heat of irradiated fuel is not yet fully clarified. The effects of soluble fission product ele­ments added to fresh UO2 were quantified, for instance, by Verall and Lucuta,33 Matsui et al.,34 and Takahashi and Asou.35 No large burnup effect was found because the specific heat obeys the law of mixtures (Neumann-Kopp law) and because only a limited fraction of the fresh fuel heavy metal atoms change nature during irradiation.

Specific heat measurements for irradiated fuel by calorimetric techniques show an exothermic effect during the first heatup of the sample linked to the recombination of radiation damage and to fission products redistribution. The apparent specific heat is lower than for annealed samples because of the heat effect, as observed by Gomme et al?6 and Yagnik and Turnbull.37 A similar effect is observed for (U, Pu)O2 samples damaged by autoirradiation.38 This means, for instance, that during fast power increases, the temperature will increase faster than predicted using the fresh fuel-specific heat. For the intrinsic specific heat (i. e., measured on annealed
samples), no significant difference was found when compared with fresh fuel. Similar results were obtained by direct measurements of specific heat on irradiated fuels by laser flash, reported by Ronchi et al.1 for UO2 and by Sonoda et al.23 for (U, Gd)O2. Therefore, the specific heat of irradiated fuel is gen­erally assumed to be equal to that of the fresh fuel.