Experimental investigation of nonisothermal interaction between Fe-Zr melt and stainless steel forming "metallic debris" in Fukushima Daiichi Nuclear Power Station

In the Fukushima Daiichi Nuclear Power Station Unit 2, the formation of a metallic pool, mainly comprising Fe and Zr, has been proposed as a mechanism contributing to the failure of the reactor pressure vessel. This study focuses on material interactions during the early core degradation that led to metallic pool formation in the late phase of the in-vessel degradation process. It investigates the nonisothermal reaction between the Fe-Zr melt and stainless steel (SS), hypothesizing that metallic debris could have formed during the relocation of the melt along the SS structure to the lower region. Initially, two compositions, Fe-87Zr and Fe-15Zr (at%), were heated to the liquidus temperature of 1723 K, dropped onto SS at lower temperatures, and the metallographic structure of the reaction products was examined. The formation of intermetallic compounds such as M23Zr6, M2Zr, and MZr2 (M = Fe, Cr, Ni) was confirmed, with varying Ni concentrations in M23Zr6 depending on the Zr concentration of the melt. Subsequently, the Fe-87Zr melt at temperatures ranging from 1723 to 1873 K was dropped onto oxidized SS to evaluate the influence of the oxide layer on degradation. The oxide layer provided some protection to the degradation of SS; however, the Zr-rich melt corroded the FeCr2O4 oxide layer, 20 mu m thick, above 1723 K, and severe degradation of SS was observed at 1873 K. In contrast, the Fe-rich melt did not react with the oxide layer due to poor wettability. This study confirmed that the liquidus temperatures of all intermetallic compounds were below 2000 K, and the metallic debris could be a source of the "metallic pool formation" predicted by recent severe accident analysis.