A review of correlations of stainless steel oxidation in steam, and modeling the reaction with MELCOR

Metals oxidize in contact with steam in severe accident conditions. A lot of research has been conducted on the oxidation of zirconium, but less attention has been paid to the oxidation of stainless steel. However, it has been observed in many MELCOR simulations that steel oxidation can generate significant amounts of hydrogen. The steel oxidation correlation in the MELCOR code is from the year 1966.

Five sets of separate effect tests on stainless steel oxidation in steam above 900 °C were found in the literature. Each of the published oxidation correlations is based on a single set of experiments. In this work, a new correlation was developed by fitting the Arrhenius equation to 14 experiments from four different test facilities. The constants of the new correlation are A = 4.3×10⁷ kg_steel²/m⁴s and B = 37 800 K. These can be entered into MELCOR by changing the sensitivity coefficient 1002. The correlations were tested by modeling the separate effect tests and the Fukushima unit 1 accident with MELCOR.

None of the correlations is clearly better than the others. Generally, each correlation can reproduce well those experiments, based on which it has been developed. The MELCOR default correlation is conservative. It gives the highest oxidation rate among all the correlations, and it overestimates the oxidation in all tests, except the Bittel test, based on which it has been developed. The new correlation, developed in this study, can be considered as the best fit to all the published data, overestimating the oxidation in some experiments and underestimating it in others. In the Fukushima calculation, the new correlation gave 24 % less steel oxidation and 6 % less total in-vessel hydrogen generation than the default correlation. These are average values of seven calculations with each correlation, perturbed by small changes to see the effect of the numerical noise.