4.0 Summary of Major Review Topics, DCISC 15th Annual Report - July 1, 2004 thru June 30, 2005
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.
During the previous recent reporting periods, The DCISC reviewed the following risk-related items:
- DCPP PRA Update
- ORAM-Sentinel Risk-Based Decision Tool
- PRA Shutdown Analysis, Fire Analysis & Human Reliability Analysis
- Steam Generator Risk Assessment
Overall, in the previous period PG&E’s risk assessment and risk management programs appeared to be effective in supporting safe plant operation. The PRA Group has become pro-active and effective in supporting station decisions with risk-based analyses.
4.8.2 Current Period Activities
The DCISC reviewed one Probabilistic Risk Assessment (PRA) item (Volume II, Exhibit D.5, Section 3.5) during the current reporting period as described below.
Update on Probabilistic Risk Assessment (PRA) Program
The DCISC Fact-finding Team met with several members of the Probabilistic Risk Assessment (PRA) Group in December 2004 for an update and to review future plans.
The Group consists of five positions; however, there are currently four including a new member. The head of the Group is leaving for an opportunity with NRC. Turnover has been a problem. During 2004, the PRA Group performed the following major tasks:
- Staff Training and Development – continuing training in the latest models and techniques as well as training new members of the Group.
- Shutdown Model Development – the PRA Shutdown Model provides informed decision-making for outage-based activities. Outage risk is dominated by human action. The Shutdown Analysis complements the existing Operating PRA, which has been in effect for a number of years, and is being upgraded.
- Base Operating Model Upgrade - The Base Operating Model upgrade is being developed to support the fire protection program and incorporate more AC/DC power variables. It is more detailed and more realistic than the Base Model. The Fire Model is more rigorous based on NRC and EPRI guidelines.
- Plant Support - Plant support included analyses supporting License Amendment Requests to NRC, Steam Generator risk assessment to support continued operation without mid-cycle tube inspections, Outage Safety Plan risk support, and on-line maintenance equipment outage risk support.
For 2005 the Group plans the following:
- PRA Operating Model Update – update the model to reflect “as-built” plant configuration. The PRA Group reviewed all procedure and design changes [as they have been implemented] and will now incorporate them into the model.
- Continue with Model Upgrade (Fire Model) - (see above 2004 work activities).
- Finish Shutdown Model Development – the Human Reliability Analysis is about half complete, and the model needs to be benchmarked.
- Support MSPI (NEI 99-02) – replacement of NRC’s Mitigating Systems Performance Indicators with indicators combining system unavailability with system reliability. Completion is expected in early 2006.
- Continue with Staff Training and Development
- Continue with Plant Support
Long-term plans include
- New PRA Quality Standards coming from NRC (Regulatory Guide 1.200)
- System, Structure & Component (SSC) Categorization by NRC (Regulatory Guide 1.201)
- Switch to the Safety Monitor Program from the existing ORAM-Sentinel Program for on-line maintenance equipment outage risk determinations
- Streamline Documentation for System Files, Event Tree Calculation Files, and Maintenance
The DCISC reviewed the Outage 2R12 ORAM Safety Analysis for quantifying the RCS boiling risk. The charts presented the relative boiling risk for activities involving/affecting plant safety systems. The analysis defined risk into the following four areas:
- Green – all equipment for the specified safety function is available (or not required by plant conditions).
- Yellow – some equipment for the specified safety function is not available; however, key safety functions are fully supported.
- Orange – the specified safety function is satisfied, but desired redundancy of equipment is not available.
- Red – the specified safety function is jeopardized and possibly compromised.
The overall status during 2R12 was Yellow, and individual safety functions were generally either Green or Yellow during various operations and equipment line-ups. For a few hours during the 2R12 core offloaded window the Spent Fuel Pool (SFP)/Support function risk was Orange. This was caused by two inoperable Component Cooling Water (CCW) trains (CCW 2-2 and CCW 2-3). With only one train of support cooling for the SFP, ORAM gives an orange risk color. CCW 2-2 was inoperable due to Bus G maintenance. CCW 2-3 (bus H) was considered inoperable because of maintenance on battery cell 57 (battery SD23 which supports Bus H). During maintenance, the cells on SD23 being replaced were jumpered allowing the battery to supply power to the bus. Despite being considered inoperable because of the jumpered battery, the battery and the bus were functional and CCW 2-3 was functional. The ORAM evaluation conservatively considered Bus H non-functional.
The DCISC will review the Shutdown PRA, Fire PRA Upgrade, and Human Reliability Analysis assessments when completed during the next reporting period.
The Probabilistic Risk Assessment (PRA) Program appeared satisfactory in performing the proper risk analyses and operational support functions for DCPP. The PRA Group is below full staff due to personnel turnovers, and it is not known how this will affect on-going work. The DCISC will continue to review PRA including the model updates and upgrades as they are completed.
4.8.3 Conclusions and Recommendations
- Conclusion:
- The Probabilistic Risk Assessment (PRA) Program appeared satisfactory in performing the proper risk analyses and operational support functions for DCPP. The PRA Group is below full staff due to personnel turnovers, and it is not known how this will affect on-going work. The DCISC will continue to review PRA including the model updates and upgrades as they are completed.
- Recommendations:
- None