Simulation of air oxidation during a reactor accident sequence: Part 1 – Phenomenology and model development

Exposure of nuclear fuel rods to air can lead to accelerated oxidation of the cladding, since the effect of nitrogen degrades the oxide layer which hence becomes a less effective barrier to the transport of oxygen to the metallic surface, resulting faster oxidation kinetics. The oxide layer typically becomes porous and can breakaway, a process known as breakaway oxidation. Exposure to air is most likely after a prior period of oxidation in steam. A new model has been developed which captures the initially protective effect of the oxide layer and transition to breakaway. The first stage of assessment was performed using detailed time-resolved data from separate-effect tests performed under controlled isothermal conditions over a range of temperatures and gas compositions. Following implementation into a new version of RELAP5/SCDAPSIM, a second stage of assessment is carried out, namely simulation of an independent integral air ingress transient experiment. This is the subject of Part 2 of this paper. The modelling approach allows extension to alternative cladding alloys such as those recently being deployed in reactor cores.

► A new oxidation model for steam and air with N2 as a catalyst has been developed at PSI.
► The model captures the initially protective effect of the oxide layer and transition to breakaway.
► The first stage of assessment was performed using detailed time-resolved data from separate-effect tests.
► The tests were performed under controlled isothermal conditions over a range of temperatures and gas compositions.
► The modelling approach allows extension to alternative cladding alloys.