Thermal properties of high burnup UO2 fuel rods

The report reviews the existing Halden database on high burnup UO2 with respect to fuel thermal conductivity degradation. New irradiation data have been accumulated since a correlation for conductivity degradation was proposed in 1996. These new data include fuel rods pre-irradiated in commercial light water reactors to a burnup of around 60 MWd/kgUO2 (IFA-597.2, IFA-597.3, IFA-610.1) as well as HBWR base irradiated and re instrumented fuel rods (IFA-533.2). In addition, two earlier irradiations reaching intermediate burnups are re-analysed (IFA-432, IFA-552.1). Measured temperature data from these experiments are analysed using the temperature calculation code in use at the Halden project. Using best estimate fuel rod dimensional data, the effect of burnup accumulation on fuel thermal conductivity is estimated. For each of the irradiations, a fit to measured data is sought by adjusting the parameters in the correlation for fuel thermal conductivity degradation. From the cases considered, the degradation parameter of the earlier correlation for thermal conductivity derived from IFA-562 appears to be a lower bound value within the Halden database. With respect to thermal properties, the analysed data seem to converge to a degradation parameter value a=16 (instead of the earlier a=14). An update of the Halden recommendation for the correlation of fuel thermal conductivity degradation with burnup is therefore proposed (author) (ml)