Radiative heat transfer in porous uranium dioxide

Abstract

Due to the low thermal conductivity and high emissivity of UO2, it has been suggested that radiative heat transfer may play a significant role in the transport of heat through the pores of UO2 used as a nuclear fuel. This possibility was computationally investigated and the contribution of radiative heat transfer within pores to the overall heat transport in porous UO2 was quantified. A repeating unit cell was developed to model approximately a porous UO2 fuel system, and the heat transfer through unit cells representing a wide variety of fuel conditions was calculated using an original finite element computer program. Conduction through the solid fuel matrix as well as the pore gas, and radiative exchange at the pore surface was incorporated in the heat transfer model. A variety of pore compositions were investigated; porosity, pore size, shape and orientation, temperature, and temperature gradient were parametrically varied to determine their particular influence on the radiative transfer mechanism. Calculations were made in which the pore surface radiation was both modeled and neglected. The difference between these two calculations yielded the integral contribution of the radiative heat transfer mechanism to the overall heat transport. The results of these calculations indicate that the radiative component of heat transfer within pores is small for conditions representative of light water reactor fuel, typically less than 1% of the total heat transport The radiative component is much larger, however, for conditions present in liquid metal fast breeder reactor fuel. During the restructuring of this fuel type early in life, the radiative heat transfer mode was shown to contribute as much as 10-20% of the total heat transport in the hottest regions of the fuel.