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Thermochemical reactions in partially exposed nuclear fuel

Oxidation of fuel-cladding during a hypothetical severe accident in light water reactors is theoretically investigated in an attempt to realistically simulate the time dependent heat generation in a partially uncovered reactor core. Two modes of oxidation, heterogeneous and homogeneous, are generally considered. Heterogeneous oxidation refers to chemical reactions with steam at the outer cladding surface in which a dense oxide layer is formed that physically hinders the oxidant from reaching the reacting metal. That mode of oxidation may not fully explain the kinetics of cladding oxidation following severe accident, where thermochemical reactions take place in pores and cracks inside the metal. A theoretical model is, therefore, developed to quantitatively understand the homogeneous oxidation of degraded fuel cladding. A dimensionless cracks’ concentration parameter, , is developed in order to study the effect of cracks on the ignition temperature of zirconium. A critical parameter, max  , is identified above which runaway ignition conditions are expected. Steam temperature and decay power are the main conditions leading to cladding heterogeneous ignition. Cracks density could be important for evaluating homogeneous oxidation of degraded cladding. INTRODUCTION Thermo-chemical reactions between fuel cladding and a hot steam environment have to be considered for realistically estimating the thermal-hydraulic behaviour of a nuclear reactor under severe accident conditions. Recent interest in zirconium oxidation has concentrated on the exothermic nature of the reaction and the hypothetical possibility of reaching runaway conditions during a reactor transient. The chemistry and kinetics of the oxidation of zirconium at moderate temperature has been a subject of extensive studies for the past 50 years [1,2]. It seems to be qualitatively agreed-upon that at temperatures below ca 800 o C oxidation may be considered as a heterogeneous surface phenomenon in which a dense oxide layer is formed that physically hinders the oxidant from reaching the reacting metal. As the oxide layer thickness increases, the rate of additional oxide layer growth varies inversely with the layer thickness (parabolic law) for a given temperature [2,3,4]. For safety evaluation, a predetermined maximum temperature and oxidized layer thickness, based on the parabolic law, are defined to guarantee cladding integrity during design-base accident conditions. Data are typically based on out-of-pile oxidation experiments using small specimens of cladding material [4,5,6]. Major mechanistic severe accident codes [7,8] generally contain empirical models and correlations for conservative estimation of cladding integrity and excess hydrogen generation during quenching following hypothetical severe accident.