RHR Alignment During Post-Trip Cooldown
A four-loop PWR has tripped from 100% power. Two hours later the RCS is at 177°C and 2.60 MPa, steam generator levels are stable, and decay heat is about 22 MW. The crew wants to place the residual heat removal (RHR) system in service to speed cooldown. Plant data: RHR pump shutoff head equals 1.55 MPa; RHR piping design pressure is 3.1 MPa; the RHR suction isolation interlock closes on RCS pressure above 2.40 MPa; the RHR relief valve is set at 2.75 MPa. The RHR heat exchanger is rated 30 MW at design conditions. The shift supervisor asks whether the alignment can proceed and, if not, what sequence will permit it. A previous shift bypassed the interlock once during a refueling outage, and someone suggests doing it again.
1. Can the RHR system be aligned now? Justify your answer using the pressure and temperature margins, and state the correct prerequisite sequence.
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No. The RCS is at 2.60 MPa, which is above the 2.40 MPa RHR suction isolation interlock setpoint and well above the RHR pump shutoff head of 1.55 MPa. If the interlock were bypassed and the suction valve opened, the low-pressure RHR piping and pump would be exposed to 2.60 MPa, the pump could not overcome RCS pressure, and the check valve would remain closed. The pump would deadhead, motor cooling would be lost, and the 2.75 MPa relief valve would be the only pressure protection. The correct sequence is to reduce RCS pressure below the RHR pump shutoff head, with margin, and reduce temperature below the RHR design temperature before alignment. Use steam dumps or atmospheric reliefs to cool at a controlled rate, and use letdown or steam dump to lower pressure. Verify RCS temperature is below about 170°C and pressure is below 1.55 MPa, confirm no voiding in the RHR suction line, then open the suction isolation valve and start the RHR pump. The 30 MW heat exchanger is adequate for 22 MW decay heat, but only after the system is properly aligned.
2. If the interlock were bypassed and the RHR relief valve lifted, describe the physical sequence and the indications the operator would see.
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Opening the RHR suction at 2.60 MPa exposes the RHR line to RCS pressure. Because the relief valve is set at 2.75 MPa, the initial pressure may be below the relief setpoint; however, any pressure transient, pump start, or thermal expansion can raise line pressure above 2.75 MPa. The relief valve then lifts and discharges RHR inventory to the relief discharge path, usually the RHR sump or quench tank. Indications include rising temperature and flow in the relief discharge line, RHR relief tailpipe temperature alarm, falling RCS pressure, falling pressurizer level, and rising sump or quench tank level. The operator should stop the RHR pump, close the RHR suction isolation valve, verify the relief valve reseats, and assess RCS inventory. The event is a small RCS inventory loss through a low-pressure path and must be treated as such.