Mixed analytical model to estimate the anisotropic effective thermal conductivity and in-reactor performance of metallic microcell UO2

Abstract

A mixed analytical model is utilized to estimate the effective thermal conductivity of microcell UO2 pellets. The proposed model consists of a mixture of parallel and series structural models that collectively take into account the unique cell geometry of the anisotropic metal network and the corresponding fractional contribution to the heat flow. This relatively simple structure model can explain the anisotropy of the thermal conductivity of UO2-Mo microcells, as observed in both experimental tests and numerical calculations. The physics-based mixed analytical model allows us to accommodate the effects of the UO2 burnup and wall structure changes on the effective thermal conductivity of microcell UO2 pellets. To validate the model's ability to predict the burnup dependence, the reduced fuel centerline temperature in a UO2 -5vol% Cr microcell under irradiation was estimated at selected burnup levels and compared to the temperature evolution observed in a Halden reactor test, in which the sample pellets were irradiated up to 16MWd/kgU. The comparison demonstrated that the mixed analytical model suitably predicts the fuel temperature changes in irradiated UO2 -5vol% Cr microcells. The model proposed here is a preliminary type based on limited experimental data, and the parameters of the model could be modified further with the accumulation of more measurement data.