Development and characteristics of the rim region in high burnup UO 2 fuel pellets

Abstract

Research to define the behavior of Zircaloy-clad light-water reactor fuel irradiated to high burnup levels was conducted as part of the High Burnup Effects Program. One observed microstructural change related to irradiation to high burnup levels (up to 83 MWd/kgM pellet-average) is the development of a well-defined, unique microstructural region at the fuel pellet edge (rim). This rim region is characterized by the loss of optically-definable grain structure, increased porosity, and the depletion of matrix fission gas. The rim region holds the potential for significant localized increases in the athermal release of fission gases at high burnup levels, but the contribution of the rim release to the fractional release of the total fission gas produced in the rod is small.