EVALUATION OF FISSION-GAS RELEASE IN HIGH-BURNUP LIGHT WATER-REACTOR FUEL-RODS

Research to define the behavior of Zircaloy-clad light water reactor (LWR) UO2 fuel irradiated to high burnup levels was conducted as part of the High Burnup Effects Program (HBEP). The HBEP was a 12-yr program that ultimately acquired, characterized, irradiated, and examined after irradiation 82 LWR fuel rods ranging in rod-average fuel burn up from 22 to 69 MWd/kg M with a peak pellet burnup of 83 MWd/kg M. A principal emphasis of the HBEP was to evaluate the effect of high burnup on fission gas release. It was confirmed that fission gas release remained as dependent on design and irradiation history parameters at high burnup levels as at low to moderate burnup levels. One observed high-burnup effect was the development of a burnup-dependent microstructure at the fuel pellet surface when pellet-edge burnup exceeded 65 MWd/kg M. This low-temperature ''rim region '' was characterized by a loss of optically definable grain structure, a high volume of porosity, and diffusion of fission gas from the UO2 matrix to the porosity. Although the rim region has the potential for enhanced fission gas release, it is concluded that no significant enhancement of rod-average fission gas release at high burnup levels was observed for the examined fuel rods.