Estimation of Risk Reduction from Improved PORV Reliability in PWRs (Pressurized Water Reactors).

摘要

An analysis was performed to explore the risk reduction potential of improving the pressurizer PORV and block valve reliability for two representative PWR plants, Indian Point 3, and Oconee 3. Attention was focused upon particular transient scenarios including steam generator tube rupture (SGTR), a stuck-open PORV, cooldown to cold shutdown, and the use of PORV in low-temperature overpressurization (LTOP) protection. The feasibility of using the PORV as a high point vent to supplement the function of Reactor Vessel Head Vent System (RVHVS) was also studied. With the exception of LTOP, the core-melt frequencies attributable to PORV or block valve failures were found to be relatively insignificant, only a very small fraction of the total core-melt frequency due to internal events. For the case of LTOP, the core melt frequency and associated risk appear to be small for Indian Point 3 and Oconee 3 since the vessels have not had a substantial fraction of their estimated lifetime irradiation. The results of a conservative estimation of health effect, however, indicate that the public risk from LTOP events may become more significant late in plant life when the aging effects of the vessel contribute to increased vulnerability. These effects are being studied in more depth in the NRC's studies for Generic Issue Number 94.