Problems of in-pile measurement of the nuclear fuel thermal properties

At the SCK-CEN a comparative study of the different recommended correlations for UO2 thermal conductivity and release of the fission products has been performed. Estimations of the central temperature of the fuel rod in function of its burn-up showed that differences of more than 100 K have been obtained with different formulae, even at very low burnup values. A significant uncertainty has also been found in correlations devoted to estimate the fission gas release. As a result of this comparison, two actions have been undertaken: firstly, an analysis of the underlying assumptions of the thermal conductivity formulae and the influence on them by in-pile conditions, and secondly, a comparison of the techniques used for determination of the fuel thermal conductivity. It has been found that some of the differences could be explained by the different structure of tested samples, others should be related to features of the measurement techniques used. Consequently there is a need for a more complete experimental investigation of the mentioned phenomena under conditions as close as possible to operational ones. One of the goals of this study is to investigate and design a robust measurement technique devoted to study the main thermal characteristics (fuel thermal conductivity and specific heat, fission gas pressure) of nuclear fuel rods in the BR2 material testing reactor at the SCK-CEN. Results of preliminary computer experiments performed with the code HEATING, concerned to determine the nuclear fuel thermal properties by using a temperature wave method, are presented and discussed. (UK)