The blowdown of feedwater (FW) line breaks (FWLBs) has been successfully analyzed by using the Appendix K version of RELAP5-3D. To adequately simulate a feedwater blowdown event, one must consider the main steam system, the turbine system, the moisture separator reheaters (MSRs), the main condenser, and the condensate and FW system as all are involved in the modeling scope. The essential components of the simulation scope include the steam header, the high- and low-pressure turbines, the MSR, the FW pump (FWP) turbines, the main condenser, the condensate and booster pumps, the FW heaters of six stages, the steam extraction of seven stages, and the turbine-driven FWPs. All of the components are connected by associated piping as designed.

Regarding the FW blowdown analysis, blowdown mass and energy are the two most important parameters to be calculated. Several essential phenomena are involved in this FW blowdown event, which include critical flow at the break and the internal venturi, flashing of FW near the break, runout and coastdown of the FWPs, steam extraction to FW heaters and FWP turbines, flashing of saturated water initially stored inside the FW heater shell sides and MSR drain tanks, energy release from saturated water and system metal, and cold water transportation from the main condenser to the break. All the essential processes involved during FWLB can be well simulated by the advanced Appendix K version of RELAP5-3D. The blowdown analysis calculated by RELAP5-3D/K for the FWLB was contracted to provide a solid basis for the final safety analysis report containment design analysis for the Lungmen advanced boiling water reactor (ABWR) plant. The successful application of RELAP5 for the entire balance-of-plant simulation and associated FW blowdown analysis indicates that the advanced RELAP5 can extend its traditional reactor safety analysis to the entire power conversion system simulation and analysis.