Mitsubishi Heavy Industries (MHI) is developing improved reactor cooling process in accident conditions, by using steam generator (SG) on accident in order to enhance safety of operating plants.

In case of station black out (SBO), leak from the reactor coolant pump (RCP) seal should be terminated by depressurizing the primary system. In this process, operators use the main steam relief valves (MSRVs) to reduce primary coolant temperature and pressure. However, coolant temperature in vessel head and upper plenum tends to remain higher than reactor temperature, and the coolant in these regions eventually starts boiling because of depressurization. This phenomenon inhibits smooth decrease in primary coolant temperature and pressure by forming large void region in the primary system. MHI is investigating improved process to achieve smooth decrease of primary temperature and pressure. In the process, condensation of the void generated should be appropriately achieved.

In this study, experimental tests on void condensation in the vessel head were performed using Large Scale Test Facility (LSTF). In addition, applicability of M-RELAP5, a plant transient computer code used in safety analysis for Japanese PWRs, to actual plant behavior modeling was confirmed, in terms of void generation/condensation in vessel head in accidental condition.

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