Modeling of the rim effect on thermal behavior of high-burnup UO2 fuel

A model for rim porosity that takes into account the effect of overpressuri zation on rim pores is proposed for high-burnup UO2 fuel. It is based on th e assumption that all the fission gases produced are retained in rim pores, and the threshold pellet average burnup required for the formation of the rim region is 40 MWd/kg U. In addition, a thermal conductivity correlation is proposed that uses the rim porosity model developed. This correlation fo r the rim region considers both degradation of thermal conductivity with bu rnup across the fuel pellet and additional degradation at the pellet rim du e to very high porosity. To calculate the temperature profile across the fu el pellet where the rim region is formed, the present correlation for the r im region is combined with the HALDEN, MATPRO, and SIMFUEL correlations for thermal conductivity for the fuel interior region where the rim feature do es not exist. Application of the present correlation to the measured HALDEN fuel centerline temperature (Nuclear Energy Agency public database IFA-562 ) shows that good agreement between the calculated and measured fuel center line temperature is obtained when the present correlation is combined with HALDEN thermal conductivity. On the other hand, when it is combined with SI MFUEL thermal conductivity, which does not consider the effect on thermal c onductivity of fission gases and other volatile fission products, lower cen terline temperature is obtained due to the characteristics of the SIMFUEL.