Investigation of corrosion-erosion behavior of stainless steel considering SS-B4C melt

Abstract

For decommissioning of severe reactors accidents, it is necessary to understand how core construction materials, such as core support structure and vessel walls, fracture during accidents. In particular, the eutectic melting between control rods (B4C) and their cladding (stainless steel (SS)) has been a concern since the accident in Fukushima. This reaction causes SS-B4C eutectic melt. It is considered that SS-B4C melt breaks the core construction materials mainly made of SS; therefore, dissolution behavior of solid SS by SS-B4C melt should be investigated to predict the damage conditions of reactors.Previously, the authors reported that the dissolution mode of solid SS by SS-B4C melt can be divided into two categories: (1) chemical dissolution during eutectic melting between SS and B4C and (2) corrosion-erosion caused by the grain detachment from solid SS due to infiltration of the SS-B4C melt into the SS grain boundaries.Herein, immersion experiments of SS rods (Type 304 steel) into a SS-B4C melt at 1573 K with different grain sizes of SS rods were conducted in order to evaluate the corrosion-erosion behavior of SS, focusing on the infiltration phenomenon.The results from the immersion experiments demonstrated that the grain size of SS rods affects the dissolution rate of SS under static condition; a smaller grain size corresponds to a faster dissolution rate. This is due to the amount of SS-B4C melt infiltration, causing the grain detachment to be minimal for the smaller grains. Moreover, it was observed that the SS-B4C melt infiltration into SS grain boundaries can be explained by the grain boundary diffusion of the element B. Moreover, the grain boundary diffusion coefficient was estimated to be of the order of 10−5 cm2/s. These findings suggest that the fracture of core construction materials by SS-B4C melt is mainly caused by corrosion-erosion during the accident.