The loss of secondary heat sink accident caused by loss of all feed water for a PWR with medium pressure safety injection system (MPSI) design is analyzed in order to support the development of one of the emergency operating procedures (EOPs), “response to loss of secondary heat sink”. The bleed and feed (B-F) measure to mitigate the accident is analyzed in detail. For the bleed and feed measure, the safety injection flow is established only after the reactor coolant system (RCS) pressure is lower than the pump head of MPSI, which needs the open of the pressurizer (PRZ) safety valves to depressurize the RCS ahead. To prevent the core uncovery or minimize the fuel cladding temperature during the bleed and feed process, the time and the number of PRZ safety valves needed to be opened are investigated. A best estimated plant model with RELAP5 code is applied for the sequences calculation. The result shows that the core water level and fuel cladding temperature will be benefited from an earlier open of all safety valves.

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