PNNL

Abstract

The evolution of the thermal conductivity of research reactor fuel during in-pile irradiation plays a significant role for the performance of the fuel element. To correctly simulate the heat fluxes and temperatures in the fuel meat during normal reactor operation and also off-normal scenarios, it is crucial to investigate the change in thermal properties, especially on the thermal conductivity, depending on the fission density and temperature. Two different U-Mo fuel types, dispersion and monolithic design, have to be considered. The thermal conductivity of fresh dispersion fuel lies between 40 and 80 W/mK depending on the particle shape, matrix material composition and U-loading. In contrast, the thermal conductivity of non-irradiated monolithic fuel is already below 15 W/mK due to the lack of the highly conductive aluminum matrix. The thermal conductivity is a composition of the thermal diffusivity, specific heat capacity and density. The specific heat capacity does not change significantly during irradiation. The density decreases slightly due to the formation of fission gas bubbles. The thermal diffusivity is strongly dependent on the materials structure. During irradiation this structure is destroyed by the fission products and fission gas bubbles. This decreases the thermal diffusivity of the irradiated material drastically down to 20% of the fresh material and has therefore the highest impact on the change of the thermal conductivity during irradiation. Therefore, during irradiation, the thermal conductivity of the dispersion fuel strongly deceases down to about 15% of the original value. The measurement of a dispersion segment from AFIP-1 with a burn-up of 6·1021f/cc yielded a thermal conductivity of 7 W/mK. The low thermal conductivity of monolithic fuel further decreases as well with increasing burn-up. At a burn-up of 3·1021f/cc, the thermal conductivity is already below 10 W/mK.