An ex-vessel steam explosion is a fuel coolant interaction process which may occur when the reactor vessel fails and the molten core pours into the water in the reactor cavity during a severe reactor accident. A strong enough steam explosion in a nuclear power plant could endanger the containment integrity and lead to a direct release of radioactive material to the environment. In this article, a nuclear island geometrical model of AP1000 nuclear power plant was established and different scenarios of ex-vessel steam explosions in AP1000 NPP were simulated by MC3D code. Since the initial parameters with large quantity of uncertainties under accident condition may have important effects on the steam explosion, some initial parameters study were performed by varying the location of the melt release (75°,45°,30°,0°), the cavity water subcooling, the triggering time for explosion calculations, the melt temperature and the break size. Results indicate that the higher the melt temperature, the longer the triggered time and the lower the coolant temperature would lead to the more severe steam explosion. Besides, when the angle of break reaches 45 degree and the diameter of the break is 0.5m, the steam explosion causes the largest damage.

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