As the important lessons learned from Fukushima-nuclear power plant accident, mitigation of failure consequences and prevention of catastrophic failure were strongly recognized against severe accidents (SA) and excessive earthquake conditions. To improve mitigation measures and accident management, clarification of failure behaviors with locations is premised under design extension conditions (DEC) such as severe accidents and earthquakes. Design extension conditions induce some different failure modes from design conditions. Furthermore, the best estimation for these failure modes is required for preparing countermeasures and management. Therefore, this study focused on identification failure modes under design extension conditions. To realize best estimation, it is prerequisite to clarify failure modes with ultimate structural strength under extreme loadings such as very high temperature, pressure and great earthquakes.

The authors attempt to clarify unclear failure mechanisms by extreme loadings under DEC using numerical simulations. In this paper, relations between failure modes and extreme loadings were investigated by the numerical simulation using the cylindrical model which is a typical structure of nuclear reactor structures (for example, Formed Head, Nozzle, Instrument Tube, Guide Tube, Support Skirt, etc.). Moreover, it was shown that failure modes change with an effect of structural discontinuities. Local failure dominates than ductile fracture at locally constraint portions where stress triaxiality becomes high.

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