Transactions of the Korean Nuclear Society Virtual spring Meeting May 13-14, 2021 TRANOX: Mechanistic Model for Non-Isothermal Steam Oxidation of Zircaloy Cladding

Zirconium-based nuclear fuel cladding used in commercial nuclear reactor is oxidized at high temperature in Design-Based Accidents such as Loss of Coolant Accident (LOCA) and Reactivity Insertion Accident (RIA). High-temperature oxidation results in the formation of zirconium oxide (ZrO2), ZrO, α-Zr(O), and prior-β phases in descending order of the oxygen amount (Fig. 1 (a)). The innermost region (prior-β) is the basis of residual ductility and ductility of the prior-β layer is dependent on its oxygen content [1]. In that regard, predicting the thickness and oxygen content of the prior-β is important to assure an adequate level of post-accident residual ductility. In isothermal steam oxidation, resulting thickness and oxygen concentration of the prior-β are correlated with Equivalent Cladding Reacted (ECR), the total amount of oxygen uptake. Therefore, current Emergency Core Cooling System (ECCS) criteria (10 CFR 50.46) is based on ECR calculated by isothermal correlations such as Baker-Just, Cathcart-Pawel (CP), and Leistikow correlations. On the other hand, all accidents are non-isothermal transients and isothermal relations between prior-β and ECR are not valid in the accident condition. Isothermal ECR correlations can be limitedly applied to nonisothermal conditions by assuming quasi-isothermal conditions at each time step. ECR could be conserved during temperature change but the thickness of each phase is not preserved under such assumption (Fig. 1 (b)). In such a context, this study discusses the development and experimental validation of a mechanistic model for oxygen concentration distribution in Zr-alloy for both isothermal and non-isothermal conditions. Furthermore, the limited hydrogen effect on oxygen distribution was analyzed through this model.