Study of thermal behaviour of UO2 and (U,Gd)O2 to high burnup (IFA-515)

The gadolinia thermal conductivity rig IFA-515.10 has been irradiated in the HBWR since July 1994. The assembly burnup reached at discharge in October 2000 was 76 MWd/kgUO2. The main objective of this test has been to compare the thermal behaviour of (U,Gd)O2 to that of UO2 fuel from fresh condition to high burnups. The fuel temperature data have been analysed with respect to differences in thermal conductivity between the two fuel types and thermal conductivity degradation with burnup. The analysis has been performed using data from two pairs of rods with UO2 and (U,Gd)O2 fuel; pairs A1/A2 and B1/B2. The (U,Gd)O2 fuel shows higher temperatures than UO2 fuel at the same heat rate. The observed temperature differences at steady state correspond to a difference in thermal conductivity of 15-20% for the temperatures and powers experienced by the rods. The data are very consistent between the two rod pairs that have been analysed. The difference in thermal conductivity inferred from steady state measurements are confirmed by both scram data and noise analysis. Thermal conductivity degradation is apparent in both types of fuel as a gradual increase of temperature with burnup. The amount of degradation is of the same order for both UO2 and Gd2O3 fuel. The thermal conductivity inferred for the Gd2O3 fuel from the steady state temperature data are compared with recent laser flash measurements of thermal diffusivity on irradiated samples for selected temperatures. (Author)