Study of high burnup effect on steam oxidation of Zircaloy and its regulatory implications via the developement of a pre-transient oxide model of TRANOX

Abstract

A mechanisitc model that gives oxygen distribution of Zircaloy cladding with pre-transient oxide and absorbed hydrogen under steam oxidation environments has been developed. The model has been incorporated into TRANOX. The model has been validated against steam oxidation experiments with Zircaloy-4 specimens that were pre-oxidized in simulated PWR environments and past experimental data available in open literature. The high burnup effects on cladding oxidation owing to pre-transient oxide and hydrogen can be categorized into: thinning of initial Zr-matrix by the formation of pre-transient ZrO2, increasing diffusion resistance of oxygen with pre-transient ZrO2, and increasing oxygen diffusion rate with a thicker β-phase by β-stabilizing hydrogen. Relative magnitudes of these effects with temperature, pre-transient oxide thickness, and hydrogen content determine the dynamics of high burnup fuel oxidation. Mechanistically modeling the high burnup effect, the model comprehensively simulates the integrated effects of pre-transient oxide and absorbed hydrogen. This study confirms that the corrected Cathcart Pawel correlation-based Equivalent Cladding Reacted evaluation method employed by the U.S Nuclear Regulatory Commission is an effective, yet accurate, simplification of the high burnup cladding oxidation modeling for safety analysis of representative LBLOCA of current PWR systems. The application of the developed model to safety analysis revealed allowable discharge burnup limits of 59 and 76 MWd/kgU for Zircaloy-4 and Zr-Nb alloy, respectively from the view point of fuel accident safety.