Analysis on inadvertent operation of decay heat removal system in NuScale reactor

Abstract

Analysis on inadvertent operation of decay heat removal system in NuScale reactor has been carried out through numerical simulations using the RELAP5 code. The model used includes primary system, secondary system, decay heat removal system (DHRS) and water pool. Simulation begins with an opening of single DHRS train isolation valve when reactor runs at nominal power. The valve opening causes part of feed water bypasses the steam generator so heat removal from primary system reduces. This causes reactor coolant temperature to increase to set point trip of 294 K within 1236 seconds. Following the trip, another DHRS valve opens and the feed water and steam isolation valves close. Besides, the coolant temperature and flow rate drop significantly and then gradually increase to reach a new balance in natural circulation. Decay heat removal is performed by combination of steam generator and DHRS units afterward. At the later stage, the simulation shows that after 764 seconds of reactor trip, the temperature and pressure in the primary system tends to decrease which shows that the DHRS capable to remove decay heat from the primary system and maintain safe shutdown. Hence, inadvertent DHRS operation does not threaten the safety of reactor.