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Characterization of solidified melt among materials of UO2 fuel and B4C control blade
Authors:Masahide Takano  Tsuyoshi Nishi  Noriko Shirasu
Affiliation:1. Fukushima Project Team, Nuclear Science Research Institute, Japan Atomic Energy Agency, 2-4 Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japantakano.masahide@jaea.go.jp;3. Fukushima Project Team, Nuclear Science Research Institute, Japan Atomic Energy Agency, 2-4 Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan;4. Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, 2-4 Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
Abstract:To predict the fundamental phase relationships in the solidified core melt of the Fukushima Daiichi Nuclear Power Plant, solidified melt samples of the various core materials [B4C, stainless steel, Zr, ZrO2, (U,Zr)O2] were prepared by arc melting. Phases and compositions in the samples were determined by means of X-ray diffraction, microscopy, and elemental analysis. With various compositions, the only oxide phase formed was (U,Zr)O2. After annealing, the stable metallic phases were an Fe-Cr-Ni alloy and an Fe2Zr-type (Fe,Cr,Ni)2(Zr,U) intermetallic compound. The borides, ZrB2 and Fe2B-type (Fe,Cr,Ni)2B, were solidified in the metallic part. Annealing at 1773 K under an oxidizing atmosphere (Ar-0.1%O2) resulted in the oxidation of U and Zr in the alloy and in ZrB2, and consequently the (Fe,Cr,Ni)2B and Fe-Cr-Ni alloy became dominant in the metallic part. The experimental phase relationships in the metallic part agreed reasonably with the thermodynamic evaluation of equilibrium phases in a simplified B4C–Fe–Zr system. The metallic Zr content in the melt was found to be a key factor in determining the phase relationships. As a basic mechanical property, the microhardness of each phase was measured. The borides, especially ZrB2, showed notably higher hardness than any other oxide or metallic phases.
Keywords:Fukushima Daiichi Nuclear Power Plant  boiling water reactor  severe accident  core melt  boron carbide  control rod  mechanical property
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