Investigation of System Responses of the Maanshan Nuclear Power Plant to the Loss of Residual Heat Removal During Midloop Operation Using a RELAP5/MOD3 Simulation

Abstract

Because several abnormal incidents involving the loss of the residual heat removal (RHR) system during refueling and maintenance outages have occurred in pressurized water reactors, the importance of investigating the physical phenomena with respect to these events has been recognized. RHR cooling is a major means to remove core decay heat after a reactor power plant shutdown. If the RHR system is lost and an alternate means for heat removal cannot be established in time, the core will boil off, and the primary system will be pressurized, which potentially results in uncovering of the fuel rods and failure of the temporary boundaries. The objective of this paper is to simulate the Maanshan nuclear power plant (MNPP) responses to the loss of the RHR system during midloop operation under variable outage conditions. Without gravity feed, the current investigation concentrates on the effects of different liquid levels, the existence of vents, and the number of active steam generators.

Based on the simulated results, the total heat removal capability of one active steam generator and the pressurizer venting process is sufficient to remove core decay heat of 11.1 MW, which corresponds to the power level 3 days after plant shutdown, in the event that RHR cooling fails during midloop operation. The primary system will be stabilized, and the pressure throughout the transient will not exceed the design pressure of the nozzle dams or the temporary seals. The heat removal capability of the pressurizer vent plays a crucial role in system pressurization during loss of RHR and in the severity of this event, as shown by the calculated results of the open and closed MNPP, respectively. If only one or two active steam generators serve as an alternate cooling means, the increased pressure will exceed the design criteria of the temporary low-pressure boundaries. Then, for the closed conditions of MNPP, the loss-of-RHR event during midloop operation has the potential to induce another loss-of-coolant accident and to cause more serious consequences.