Abstract
The temperature of zirconium alloy cladding on the postulated spent nuclear fuel pool complete loss of coolant accident is abruptly increased at a certain time and the cladding is almost fully oxidized to weak ZrO2 in the air. This abrupt temperature escalation phenomenon induced by the air-oxidation breakaway is called a zirconium fire. Although an air-oxidation breakaway kinetic model correlated between time and temperature has been implemented in the MELCOR code, it is likely to bring about unexpected large errors because of many limitations of model derivation. This study suggests an improved time–temperature correlated kinetic model using the Johnson–Mehl equation. It is based on that the air-oxidation breakaway is initiated by the phase transformation from the tetragonal to monoclinic ZrO2 at the oxide–metal interface in the cladding. This new model equation is also evaluated with the Zry-4 air-oxidation literature data. This equation resulted in the almost similar air-oxidation breakaway timing to the actual experimental data at 800 °C. However, at 1000 °C, it showed an error of about 8 min. This could be inferred from the influence of the ZrN phase change due to the nitrogen existing in air.
Highlights
A new kinetic model was proposed by considering the possible cause of the air-oxidation breakaway phenomenon in order to improve the weaknesses of the existing model
It has been known that the air-oxidation breakaway is initiated by the phase transformation from the tetragonal to monoclinic ZrO2 at the oxide–metal interface in the cladding
The new model considers no effect of nitrogen on the initiation of air-oxidation breakaway
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 9 × 9 boiling water reactor (BWR) spent fuel assembly tests were performed in a postulated complete LOCA [14] From both SFP complete LOCA experiments, it was observed that zirconium alloy cladding temperature was abruptly increased at a certain point and the cladding was almost fully oxidized. Since the current model is an empirical equation derived by introducing the concept of a lifetime function through log–log scale fitting using limited airoxidation experimental data, it has an uncertainty in which a tiny change (± 3%) of model parameters induced a large difference in breakaway timing in comparison to the experimental data of the onset of zirconium fire From this sensitivity analysis, we demonstrated that the current air-oxidation breakaway model should be improved by adopting the new formula of the model rather than previous log–log scale curve fitting. We suggest a new model equation that can more accurately predict the air-oxidation breakaway timing based on the Johnson–Mehl model
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