A Study of the simulated corium interaction with Zr-stainless steel metallic debris

Zubekhina, B. ; Pshenichnikov, A. ; Sato, Takumi ; Takano, Masahide 

An interaction between damaged fuel or corium and metallic debris melt was previously observed in TMI-2 and ChNPP-4 accidents. Since the real samples of Fukushima Daiichi Nuclear Power Station (FDNPS) metallic debris are not yet available, experimental investigation of possible corium composition in case of the FDNPS Units 1-3 has become the main scope of the present research. To have knowledge on possible corium compositions is important from both severe accident and waste management perspectives. In this work, authors attempted to investigate an interaction of liquid metallic debris and simulated corium of 20/80 mol ZrO/UO in the form of sintered crucible. The metallic debris included two types of Zr-steel melting products formed as a result of melt relocation during a severe accident test CLADS-MADE-04 and a specially synthesized Zr - 304 steel eutectic with 83 wt. percent Zr which has the lowest melting point about 1373 K for such kinds of debris. Experimental results show that zirconium from liquefied metallic debris induced both U and O diffusion from the (U,Zr)O crucible into the liquefied metallic debris already after 1 hour of interaction at temperature about 1900 K. After cooling, uranium was captured in Fe-Zr-Ni-Cr-Mo-U-O solid solution phase. Oxygen which entered from the crucible reacted mostly with Zr, which resulted its redistribution and finished in formation of new phases, such as Fe-Cr-Ni alloy (almost as conventional steel) and Fe-Cr-Mo containing Laves phases. Both did not contain measurable amounts U and were not available in the initial samples. The obtained data may be of high importance during FDNPS fuel debris removal and treatment and hopefully can be used during planning of the nuclear waste disposal strategy.