Discussion About the Main Parameters Affecting the Thermal Conductivity of LWR UO2 and MOX Fuels

The thermal conductivity of irradiated nuclear fuels is usually extrapolated from the data acquired on fresh ceramics, incorporating in the formulation a burn-up dependency. However, the burn-up parameter covers several recoverable or not recoverable mechanisms. This methodology could be justified for low burn-up, but is no more applicable at high burn-up, mainly in case of recristallization. An example is the effect of the addition of plutonium in UO2. The common understanding is that the lower conductivity of the fresh MOX is a result of the differences in size and mass between the U and Pu atoms. However, this interpretation does not take into account the impact of the Pu valence, which leads to local fluctuations in stoichiometry in heterogeneous MOX, which in turn has a strong impact on the thermal conductivity. For unirradiated fuels, neglecting the effect of the stoichiometry fluctuations has no impact on the accuracy of the thermal conductivity correlations, because their parameters are adjusted to reproduce the experimental results which are available in sufficient number. However, when extrapolated to irradiated fuels, these correlations overestimate the effect of Pu because the oxygen distribution evolves differently between plutonium rich agglomerates, matrix and coating phase. The use of an appropriate model is strongly needed in interpreting experimental data, as far as the number of available characterizations is small, and their interpretation tricky. The investigation of the Pu and stoichiometry effects combines experimental results obtained on irradiated fuels at ITU and also in-pile temperature measurements conduced for EDF

STAICU Dragos;  BARON D.; 

2010-10-22

American Nuclear Society

JRC59324

https://publications.jrc.ec.europa.eu/repository/handle/JRC59324,