Microstructure evolution formed during early liquefaction stage between pre-oxidized Zircaloy 4 with SUS316 stainless steel at 1573 K

ABSTRACT

The reaction between Zircaloy and stainless steel is a key interaction in the core degradation process during a severe accident in a light water reactor because this combination leads to eutectic liquefaction. In this study, the reaction mechanism and rate-limiting process were experimentally investigated at 1573 K where the eutectic liquefaction was observed dominantly. The diffusion couple of pre-oxidized Zircaloy4 and SUS316 stainless steel were annealed for various holding times, and the interface microstructure was metallographically examined. The reaction layer consisted of five phases: the α-(Fe,Cr,Ni) phase, the metastable (Fe,Cr,Ni)₂₃Zr₆ phase, the Laves Zr(Fe,Cr,Ni)₂, α-Zr(O), and the liquid phase. The reaction layer thickness in the Fe-rich side consisting of α-(Fe,Cr,Ni) and (Fe,Cr,Ni)₂₃Zr₆ obeyed the time parabolic rate law, while the ones involving the liquid phase formation followed the saturation-type convection-controlled function. A formula in a combination of diffusion and convection process was introduced for estimation of reacted volume, which showed good agreement with the experimental results. It was newly realized that a model to provide the mass transfer coefficient regarding the convection-controlled process would be required for improvement of the core degradation model.

KEYWORDS:

• Eutectic reaction
• SUS316-Zircaloy4
• core degradation
• severe accident

Disclosure statement

This study was funded by Tokyo Electronic Power Company Holdings. M. Mizokami, M. Hirai and K. Ito are employees of Tokyo Electronic Power Company Holdings.

Additional information

Funding

This work was funded by the Tokyo Electronic Power Company Holdings and it was performed in the framework of “TEPCO Decommissioning Frontier Technology Creation Collaborative Research Base” at the Tokyo Institute of Technology. The Zry-4 plate was supplied by Nippon Nuclear Fuel Development Co. Ltd.