Thermochemical reactions in partially exposed nuclear fuel

Abstract

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.