Characterization of solidified melt among materials of UO fuel and BC control blade

Takano, Masahide ; Nishi, Tsuyoshi; Shirasu, Noriko  

To predict phase relationships in the solidified core melt of Fukushima Daiichi Nuclear Power Plants, the solidified melt samples among core materials were prepared by arc melting. Phases and compositions in the samples were determined by X-ray diffraction, microscopy and elemental analysis. The only oxide phase formed is (U,Zr)O. The stable metallic phases are Fe-Cr-Ni alloy and FeZr-type (Fe,Cr,Ni)(Zr,U) intermetallic. The borides, ZrB and (Fe,Cr,Ni)B, are solidified in the metallic part. Annealing at 1773 K under an oxidizing atmosphere resulted in the oxidation of uranium and zirconium in the alloy and ZrB, instead the (Fe,Cr,Ni)B and Fe-Cr-Ni alloy became dominant. The metallic zirconium content in the melt is found to be a key factor that determines the phase relationships. As a basic mechanical property, the microhardness of each phase was measured. The borides showed notably higher hardness than any other oxide and metallic phases.