Depressurization setpoint

The large LOCA is defined as a pipe break followed by a decrease in the pressure down to the depressurization setpoint (23.5 MPa) even if the RCPs and the pressure control system are assumed to operate. In the present design and break flow... 

Cooldown to approximately 350 F is accomplished by releasing steam from the secondary side of the steam generators. An inadvertent SIAS or safety injection tank discharge is precluded during a controlled cooldown by manually decreasing the SIAS setpoint, and depressurizing and isolating the safety injection tanks. 

Since the RCS coolant inventory was continually being removed during this period, the core eventually began to uncover at 2430 s. The core exit steam temperature reached 922 K at 4570 s, and the PORV was latched open in accordance with the intentional depressurization strategy. This occurred soon after the SRVs ceased operation, but before the RCS pressure had dropped below the PORV closure setpoint. Once the PORV was latched open, the RCS pressure decreased to the core flood tank (CFT) setpoint pressure at 10,120 s. The time for RCS pressure reduction was over five times longer than was predicted for Surry. The PORV setpoint pressure at Oconee and other B W plants is lower than other plant types at 15.65 MPa. The CFT setpoint pressure of Oconee is typical of the accumulator setpoint pressure of W plants at 4.24 MPa, but higher than the CE value of 1.48 MPa. Finally, the PORV ratio for Oconee is 0.24, compared to Surry s value of 1.00. From the PORV ratio alone, it would be anticipated that Oconee would require more time than was predicted for Surry to reduce the RCS pressure to the CFT setpoint pressure. 

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