Nitriding model for zirconium based fuel cladding in severe accident codes

A model has been developed to describe the nitriding of partially oxidized zirconium based cladding during an air ingress sequence when the reaction has become starved of oxidant (oxygen and/or steam), and the subsequent re-oxidation of nitride following of restoration of coolant. Key aspects of the model are the estimation of oxygen-stabilised alpha zirconium, α-Zr(O), formed during pre-oxidation and its reaction with the nitrogen. Nitriding of metallic Zr is much slower than α-Zr(O), and plays a comparatively minor role. The model is based on data from separate-effects tests comprised pre-oxidation, nitriding in the absence of oxidant, and re-oxidation in the absence of nitrogen, which were used to derive the kinetic parameters for the main reaction processes. Developmental assessment was performed using the test results, demonstrating favourable agreement for the main reaction signatures. Independent assessment against Integral Test data is underway.