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

Abstract

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.