Out-of-Reactor Studies on the Role of Graphite in Reducing Pellet-Cladding Interaction Susceptibility

摘要

One of the fuel designs under consideration incorporates a thin (approx. 6 micron) layer of graphite on the inner surface of zircaloy cladding. The purpose of the graphite is to reduce the mechanical and chemical interaction that might occur between the UO sub 2 fuel pellets and the cladding. This topical report summarizes out-of-reactor laboratory studies directed at understanding the role graphite plays in reducing PCI susceptibility. A PCI simulator was built that utilized iodine to simulate the corrosive environment in an irradiated fuel rod. The standard cladding used in all FPIP irradiation tests was remarkably failure resistant in the PCI simulator. Other cladding, intentionally selected for its greater failure susceptibility, was used for many of the tests. These tests showed that a graphite coating can improve the failure resistance of susceptible cladding, especially with water vapor present in the test atmosphere. The dominant protection mechanism appears to be physical rather than chemical, by decreasing strain localization through friction reduction. (ERA citation 08:002099)