The high-temperature (≈800–2100°K) oxidation of nuclear reactor fuel assembly zirconium-based claddings in the air behaves in different way compared to oxidation in the steam.
First, the heat effect of the chemical reaction of oxidation in the air is approximately two times larger than in the steam.
Second, the kinetics of oxidation in the air is non-parabolic (approximately linear, that is more strong) in contrast to parabolic kinetics of oxidation in the steam.
Third, important factor is the diffusion of oxygen and nitrogen atoms through very porous (transparent) nitride layers formed in α-Zr(O) layer. It resulted in linear kinetics in small-scale tests and strong hydrogen release during reflood phase in QUENCH-16 and PARAMETER-SF4 tests. The role of nitride formation during air ingress phase seems to be extremely important especially if oxygen starvation phase is present.
Fourth, it is necessary to treat adequately the interdiffusion of oxygen and nitrogen in gas mixture containing, oxygen, nitrogen and possibly steam, hydrogen, other non-condensables. This effect influences the time moment of oxygen starvation.
Fifth, it is important to take into account the heat effect of nitridation (formation of ZrN) which is not small.
Sixth, at final phase (quenching at water reflood or slow cooling) the reoxidation of nitrides can take place. During this process. nitrogen and hydrogen (in steam atmosphere) is released and additional chemical heat generation is produced.
The development of a special model of Zr-based alloys oxidation in air considering all above-mentioned processes is currently underway, some results are presented below. A numerical scheme is realized to determine layer boundaries relocation and layer transformations in claddings.
The model is implemented to SOCRAT computer modeling code. This model is being verified on separate-effect tests as well as on integral air ingress experiments QUENCH-10, QUENCH-16 and PARAMETER-SF4.