Abstract: Making use of an integral systems analysis computer code, a severe accident sequence induced by large-break loss-of-coolant accident (LB-LOCA) at cold leg, with external reactor vessel cooling (ERVC) measure, was calculated for 1 000 MWe nuclear power plant. The validity of the measure for mitigating core’s melting progression, keeping the integrity of the vessel, and also the effect factors of the measure, were analyzed. The results indicate that ERVC, with enough mass of water， appropriate velocity of water injecting and water level in cavity, can provide adequate cooling capacity for vessel lower head, and the integrity of vessel can be retained.