The high-temperature heat capacity of the (Th,U)O2 and (U,Pu)O2 solid solutions
S.O. Valu (TU Delft - RST/Applied Radiation & Isotopes, European Commission Joint Research Centre, Institute for Transuranium Elements Karlsruhe)
O. Beneš (European Commission Joint Research Centre, Institute for Transuranium Elements Karlsruhe)
D Manara (European Commission Joint Research Centre, Institute for Transuranium Elements Karlsruhe)
R. J.M. Konings (European Commission Joint Research Centre, Institute for Transuranium Elements Karlsruhe, TU Delft - RST/Reactor Physics and Nuclear Materials)
M. W.D. Cooper (Los Alamos National Laboratory, Imperial College London)
R. W. Grimes (Imperial College London)
C Guéneau (CEA-Saclay)
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Abstract
The enthalpy increment data for the (Th,U)O2 and (U,Pu)O2 solid solutions are reviewed and complemented with new experimental data (400–1773 K) and many-body potential model simulations. The results of the review show that from room temperature up to about 2000 K the enthalpy data are in agreement with the additivity rule (Neumann-Kopp) in the whole composition range. Above 2000 K the effect of Oxygen Frenkel Pair (OFP) formation leads to an excess enthalpy (heat capacity) that is modeled using the enthalpy and entropy of OFP formation from the end-members. A good agreement with existing experimental work is observed, and a reasonable agreement with the results of the many-body potential model, which indicate the presence of the diffuse Bredig (superionic) transition that is not found in the experimental enthalpy increment data.