Simulation of the Long Term-Cooling Behaviour After a PWR Loss of Coolant Accident Using RELAP5/MOD2

Abstract

RELAPS/MOD2 was used to model a 4-loop PWR behaviour during long-term cooling following a large break in the upper surf ace of a cold leg. This work was undertaken to assess the possibility of core uncovery associated with long-term cooling. The analyses performed exhibited two phenomena which could be beneficial to long-term cooling and could be induced by operator action. The first is natural circulation water flow from the upper plenum via the control rod guide tubes and upper head to the downcomer. The second is the potential for loop seal clearance when the water level. in the hot legs and loop seals reaches the hot steam generator's tube sheet and flashes to steam.