A scoping investigation on debris bed formation with high-temperature melt simulant Fe-Sn

Abstract

In hypothetical severe accidents of Nordic boiling water reactors (BWRs), the core melt (corium) has the potential to relocate to the lower head, leading to the failure of the reactor pressure vessel (RPV). Consequently, the corium, a molten mixture of either UO2-ZrO2 or Fe-Zr, is anticipated to undergo fuel coolant interactions (FCI) in a deep-water pool within the reactor cavity which is supposed to be flooded during severe accidents. The characteristics of the resulting debris bed play an important role in corium coolability that is paramount to accident stabilization and termination. This study is dedicated to characterizing the debris bed resulting from the FCI using Fe-Sn as the simulant of Fe-Zr. Compared with other simulants in our previous studies, the Fe-Sn alloy has a a higher melting point of 1130 °C and the same element of Fe as in prototypical Fe-Zr. The Fe-Sn is melted in a clay graphite crucible with glass coating layer to minimize contamination of molten materials by graphite. Argon containing 1.26 % hydrogen is employed as cover gas to avoid the oxidation of Fe-Sn at high temperature. The process of debris bed formation, including melt jet breakup, debris fragmentation, and fragments’ sedimentation on the pool bottom, is meticulously recorded using high-speed cameras. The porosity of the debris bed is determined in the post-test analyses using a three-dimensional laser scanner and a graduated water tank. The laser scanner data are also used to reconstruct the geometric appearance of the final debris bed. The debris particulates are sieved to obtain their size distribution. The chemical interaction between water and melt is examined by the SEM analysis. The experimental results show that the debris bed of Fe-Sn has significant differences from those of low melting point simulants (Sn-Bi, Sn, Zn) in our previous studies.