[en] The control blade and the fuel rod installed in channel box melt in steam during severe accident of a boiling water reactor. In order to clarify the melting phenomena and relocation of the structural material in the core of the reactor, interaction and melting behavior among B₄C, 304 grade of stainless steel (SST), and Zircaloy-4 in atmosphere containing H₂/H₂O at 1473 K are investigated. The results showed that the reaction at the interface between B₄C and SST under H₂O atmosphere was slow and an oxidation was observed after 3600 s. Under H₂O/H₂ atmosphere, the concentration of B and C in the SST increased and the SST melted. Despite the atmosphere, an oxide layer formed on the surface of Zircaloy-4, and thus the reactions proceeded slowly when the Zircaloy-4 was contacted with B₄C and SST. Under H₂O atmosphere, continuous oxidation happened to SST, and SST was partially melted. Under H₂ atmosphere, the SST was also melted due to the diffusion of B and C from the B₄C. In addition, the oxidation of B₄C affected the oxidation behavior of SST and Zircaloy, and thus the oxidation and the hydroxylation of B₄C in a severe accident was discussed thermodynamically. (author)