An Empirical model of the effects of pellet cracking on the thermal conductivity of UO2 light water reactor fuel

Light water reactor fuel pellet cracking and pellet fragment relocation into the pellet-to-cladding gap during normal operation alters both the fuel thermal conductivity and the thermal resistance of the gap. Uranium dioxide fuel pellet thermal conductivity data from a series of tests being conducted in the Power Burst Facility to evaluate the thermal performance of LWR design fuel are presented. These data indicate that the effective thermal conductivity of a cracked and relocated light water reactor fuel pellet is strongly influenced by the closing or opening of the cracks as the rod power is increased or decreased and is dependent on the initial pellet and cladding dimensions. An empirical correlation is introduced which provides a means for calculating the effective thermal conductivity of cracked and relocated fuel within helium bonded fuel rods. The method also provides a means for estimating the relocated hot pellet-to-cladding gap width as a function of rod power. (orig.)