Oxidation kinetics of austenitic stainless steels as scwr fuel cladding candidate materials in supercritical water

The oxidation kinetics of supercritical-water-cooled reactor (SCWR) fuel cladding candidate materials, e.g. 15Cr-20Ni stainless steel (1520 SS) in supercritical water at 650 and 700 °C under 24 MPa has been investigated. Characteristics of oxide layers and its relation to oxidation behaviors are also studied. The applicability of the candidate materials for the fuel cladding of SCWR from oxidation kinetics, spalling susceptibility of oxide layer, and breakdown of Cr ₂ O ₃ layer points of view has been discussed. The results indicate that the threshold condition for spalling of oxide layer is different at 650 and 700 °C. The decrease in oxidation kinetics of 1520 SS with time correspond to the change in rate-limiting process of oxidation from mass transfer through an Fe oxides to mass transfer through a Cr rich oxide layer with time. Based on the oxidation kinetics obtained in this study, 1520 SS is considered suitable for a fuel cladding of SCWR in combination with appropriate CW process. However, detailed evaluation and countermeasures for the degradation due to nodular oxidation are needed before application of tube-shaped 1520 SS in supercritical water at 700 °C. On the other hand, it is estimated that the use of that at 650 °C is acceptable because the weight gain after long-term exposure was considered to be much less than the threshold condition of the spalling.