Formation of Ni 3 S 2 from the reactions between Ni and seawater-derived MgSO 4 : insights from deposit analysis in Fukushima Daiichi Unit 2

In October 2023, deposits retrieved from the X6 penetration of the primary containment vessel in Unit 2 of the Fukushima Daiichi Nuclear Power Station were analyzed and found to contain Ni and S-rich particles with platinum-group elements (Ru, Rh, and Pd) Based on microstructural and compositional analyses combined with existing knowledge of the accident analysis, the origin of Ni and S was inferred to be from structural alloys and injected seawater, respectively, and suggesting high-temperature interactions between core materials and seawater-derived species. To investigate the formation mechanism of the particles, laboratory experiments were performed on the reactions between Ni and MgSO4, the main sulfate species derived from seawater through concentration process. Pelletized mixtures of Ni and MgSO4 powders, with and without Fe addition, were heated at 900°C–1100°C under controlled Ar – H2–H2O atmospheres. Post-test metallographic analysis revealed the formation of Ni, Ni – Mg – O oxides, and Ni3S2. In Fe-containing systems, Fe – Ni sulfides formed, suppressing Ni3S2 production. These results support a mechanism in which seawater-derived MgSO4 reacts with segregated Ni during core degradation, yielding Ni3S2, as observed in Unit 2 deposits. The findings provide new insights into in-vessel chemical processes and offer indirect evidence that seawater reached and interacted with the fuel debris.