Air oxidation of Zircaloy, Part 2: New model for Zry-4 oxidation

Abstract

The accident in Fukushima brought up new issues in the area of safety of nuclear reactors. Among others, Spent Fuel Pool accidents gained new focus. The computer codes applicable for safety analyses of Nuclear Power Plants have limited verification and validation in this area and their applicability remains still to be proven. An important phenomenon occurring during loss of water in SFP is air oxidation of Zircaloy cladding material. Mathematical modeling of this phenomenon in computer codes has been under development during the last years. This document presents a review of models for air oxidation of Zircaloy, including: up to date models available in open literature, as well as models available in computer codes: ASTEC, MELCOR, and SPECTRA. The models were tested by performing simulations of a number air oxidation experiments from ANL, KIT, and IRSN.As a result of this work, a recommended set of correlations, applicable for wide range of temperatures, including pre- and post-breakaway reaction, has been selected. For the pre-breakaway (parabolic) regime the correlation of Benjamin et al. (Sandia National Laboratories, Albuquerque, NM, 1979) was selected for the low temperatures and a new correlation has been proposed for the high temperatures. For the post-breakaway (linear) regime, Boase and Vandergraaf (Nucl. Technol., 1977;32:60–71) were selected for the low temperatures and a new correlation has been proposed for the high temperatures. Furthermore, a new model for the breakaway transition has been proposed. The correlation set is applicable for Zircaloy-4, for practically the entire temperature range. The recommended set provides an improved accuracy of results compared to previous models. The current model is applicable to any situation where air oxidation of Zircaloy may occur, for example loss of water in a Spent Fuel Pool.The model recommended in this paper is applicable only for Zircaloy-4. Concerning two other Zr-based alloys, Zirlo™ and M5®, reaction rate coefficients exist in literature, however the existing models are not sufficient to calculate oxidation of these materials with satisfactory accuracy. Breakaway models, specific for these materials, do not exist and should be developed.