4.0 Summary of Major DCISC Review Topics, 18th Annual Report - July 1, 2007 thru June 30, 2008
4.8 Risk Assessment and Management
4.8.1 Overview and Previous Activities
PG&E has developed in-house capability to perform risk assessments and periodically updates its Probabilistic Risk Assessment (PRA) to incorporate changes in plant configuration and, if appropriate, operations. The PRA Group has been updating the original 1995 risk assessment which included the sum of internal, seismic, fire and shutdown risks. The 1995 core damage frequency (CDF) was 1.12x10-4 per year, and the revised risk is lower at 9.72x10-5 per year – a 13 decrease. Much of the reduction is due to implementation of the Maintenance Rule and resultant increased equipment reliability and PRA modeling improvements. The reduced overall risk permits more flexibility in scheduling on-line maintenance. The NRC criteria are based on a 1.0x10-6 per year risk threshold for on-line maintenance, and DCPP will have more room within that threshold to perform additional maintenance on line.
PG&E controls its risk from on-line maintenance procedurally. For On-Line Maintenance the PRA Group prepares a Risk Profile on a weekly, monthly and fuel cycle basis. The PRA Group works very closely with personnel performing the On-Line Maintenance risk assessment, and the program has been working well.
The On-Line Maintenance (OLM) model has been used by Operations and Maintenance as an on-line planning tool for various operations and maintenance activities. DCPP is now using ORAM Sentinel (Outage Risk and Management) instead of OLM. Using ORAM, the PRA Group has increased allowable outage times (AOTs) for the Auxiliary Feedwater Pump and identified more sources of water. Similarly, AOTs have been increased for the EDGs, startup power, and CCWPs. AOTs have decreased for the SI, Charging and RHR pumps and have decreased significantly for the SSPS, which represents the highest-risk AOT.
The DCISC reviewed the following item during its previous reporting period:
- Outage Risk Analyzer – Maintenance (ORAM)
Although the DCISC did not review Probabilistic Risk Assessment (PRA) per se during the 2006-2007 reporting period, the Program has appeared satisfactory in previous reporting periods. DCPP’s use of PRA-based Outage Risk Analyzer – Maintenance (ORAM) appeared appropriate. The DCISC will continue to review PRA including the model updates and upgrades as they are completed.
4.8.2 Current Period Activities
The DCISC did not review Probabilistic Risk Assessment (PRA) items per se during the current reporting period but did review the following related item:
Status of DCPP Initiative on the New American Nuclear Society (ANS) Methodology Standard for Fire PRA
The DCISC Fact-finding Team met with Amir Afzali, Supervisor PRA Group, at its February 27-28, 2008 Fact-finding Meeting (Volume II, Exhibit D.7, Section 3.3) who is leading the group at DCPP that is developing an up-to-date fire Probabilistic Risk Assessment (PRA), the objective of which is to analyze probabilistically the probabilities and consequences of potential severe fires. The fire PRA is still only partly completed, and will not be ready for another half-year or so. The group’s work is using as guidance both the new American Nuclear Society Standard 58.23, for fire-PRA methodology, and also the recently completed NRC/EPRI joint report published as NUREG/CR-6850 (Reference 6.2). The ANS standard contains requirements on “what to do” to perform a high-quality fire PRA, and the NRC/EPRI NUREG report contains guidance on “how to do it”.
A recent change in NRC requirements for fire protection allows an operating nuclear plant like DCPP to implement another new standard, National Fire Protection Association standard NFPA 805. This methodology relies on a large number of performance-based approaches to meeting the intent of the NRC’s fire protection regulations. It will replace elements of the traditional NRC fire protection regulations, which permit only a few performance-based approaches. Most of the industry is in the process of implementing 805, and DCPP is also.
Thus two major challenges are confronting DCPP in this area simultaneously: implementing NFPA 805 and performing an up-to-date fire PRA using ANS 58.23 and NUREG/CR-6850. Mr. Afzali is leading DCPP’s analysis work in both -- and the work is going on all at the same time,
The benefit to DCPP from this work is two-fold: first, producing a high-quality fire PRA will enable DCPP to understand the risk significance of various fire issues, enabling the plant to deal with any fire events or possible fire vulnerabilities in the context of the actual risk they might pose to the plant. Second, the fire PRA will enable DCPP to implement the more modern NFPA 805 as an alternative to the current NRC fire regulations, thereby allowing the plant to modify certain fire maintenance and fire-upgrade activities using performance-based logic, which should enhance DCPP’s fire protection while focusing resources better.
DCPP will have a peer review of the DCPP Fire PRA by the end of 2008. It will likely take about 6 years for all Fire PRAs to be completed, but the NRC is urging completion sooner.
4.8.3 Conclusions and Recommendations:
- Conclusion:
- Although the DCISC did not review the overall Probabilistic Risk Assessment (PRA) during the 2007-2008 reporting period, the Program has appeared satisfactory in previous reporting periods. The DCPP Probabilistic Risk Assessment (PRA) team that is currently developing an up-to-date fire PRA is recognized as one of the leading groups in the industry, and its recent work, including having been the first plant selected for an outside peer-review, to pilot the peer review process, demonstrates its stature. DCPP has also been in the forefront, with a few other nuclear-power-plant colleagues at other plants, in helping the whole industry to upgrade its capabilities in fire PRA. This work should substantially help DCPP to maintain a strong position vis-à-vis the fire-protection aspect of the plant’s overall safety. When the fire PRA is complete, toward the end of 2008, the DCISC will request a briefing on the PRA’s findings and any resulting safety insights at a Fact-finding and/or public meeting.
- Recommendations:
- None