Nuclear Technology / Volume 205 / Number 6 / June 2019 / Pages 867-880
Technical Note / dx.doi.org/10.1080/00295450.2018.1533320
Station blackout (SBO) sequences for a Westinghouse-designed three-loop pressurized water reactor (PWR) with large dry containment are investigated. Recovery of alternating-current power is considered under two separate headings in event trees of loss of off-site power: recovery of the off-site power and recovery of the emergency diesel generators (EDGs) or diesel generator 5. A reactor coolant pump seal leakage model under SBO conditions for a Westinghouse PWR with high-temperature O-rings is used. Seal failure mechanisms, including popping open, binding, and O-ring extrusion, are considered. Success criteria are established based on the results of thermal-hydraulic calculations via the MELCOR and/or RELAP-5 codes. Furthermore, the WinNUPRA software package is used for sequence quantification.
Three primary models are addressed: Basic, WOG2000, and Ultimate Response Guideline (URG). The core damage frequency (CDF) of SBO sequences for the Basic model is found to be very optimistic and underestimated. The improvement on CDF by introducing the URG model is found to be dependent primarily on the reliability of plant operators in performing the procedure URG under SBO situations.
Two sensitivity analyses on the passive shutdown seal (PSDS) are performed: WOG2000 (PSDS) and URG (PSDS). The low-failure-probability PSDS results in a >94% improvement in CDF due to SBO since the CDF contribution from long-term SBO sequences becomes negligible. Furthermore, the URG strategies are shown to have a significant impact on the reduction of CDF since seal leakage is no longer a concern in the presence of the PSDS.
The way that power recovered is distinguished from off-site power or on-site EDGs and the way that SBO CDF is assessed make the present study more realistic than general SBO models.
A key analysis of the confidence bands of the SBO CDF with the aid of the uncertainty measure is also performed to observe the induced effects by the probabilistic seal failure modes and PSDS.