RIS 2008-15, NRC Staff Position on Crediting Mitigating Strategies Implemented in Response to Security Orders in Risk-informed Licensing Actions and in the Significance Determination Process
| ML080630025 | |
| Person / Time | |
|---|---|
| Issue date: | 06/25/2008 |
| From: | Case M J NRC/NRR/ADRA/DPR |
| To: | |
| References | |
| RIS-08-015 | |
| Download: ML080630025 (7) | |
June 25, 2008
NRC REGULATORY ISSUE SUMMARY 2008-15 NRC STAFF POSITION ON CREDITING MITIGATING STRATEGIES IMPLEMENTED IN RESPONSE TO SECURITY ORDERS IN RISK-INFORMED LICENSING ACTIONS AND IN THE SIGNIFICANCE DETERMINATION PROCESS
ADDRESSEES
All holders of operating licenses for nuclear power reactors, except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vesse
INTENT
The U.S. Nuclear Regulatory Commission (NRC) is issuing this regulatory issue summary (RIS) to inform addressees of the NRC staff position on crediting mitigating strategies implemented in response to security Orders in the risk assessments associated with risk-informed licensing actions and the significance determination process (SDP) used in the Reactor Oversight Proces This RIS does not transmit any new requirements and does not require any specific action or written respons
BACKGROUND INFORMATION
On February 25, 2002, the NRC sent immediately effective Orders to domestic nuclear power plant licensees regarding implementation of interim safeguards and security compensatory measure The Orders were the result of the September 11, 2001, terrorist attack As a result of the Order, domestic nuclear power plants have taken certain steps or plan to take steps to enhance safety and securit Some of the steps taken pursuant to the Orders have resulted in mitigating strategies that may improve safety by reducing risk from plant events whether related to security or no Risk-informed regulatory activities rely on realistic assessments of the change in ris These risk-informed assessments balance the traditional engineering approach embodied in the NRC regulation The NRC often uses probabilistic risk assessment models (PRAs) to support license amendment requests and to assess the safety significance of plant events or performance deficiencie PRA models are expected to model the "as-built, as-operated plant" to the extent necessary to provide useful and valid risk insight The NRC intends for this RIS to clarify when mitigating strategies implemented in response to security orders may be "credited" in risk assessments supporting licensing actions and SDP evaluation
SUMMARY OF ISSUE
Changes made to a nuclear power plant to enhance security and safety as a result of the February 25, 2002, Order may include procedural changes, plant modifications, compensatory measures, or other measure Some of these changes may provide a risk benefit in mitigating non-security events at a plan These include internal events while the plant is at power, external events while the plant is at power (e.g., internal fires and floods, seismic events, external fires and floods), events while the unit is shutdown, or events while the plant is transitioning between operating and shutdown mode The Commission, in its policy statement of 1995 on the use of PRA methods in nuclear regulatory activities, stated that PRA evaluations in support of regulatory decisions should be as realistic as practicabl Consistent with this policy, mitigating strategies (e.g., plant features and procedures) implemented in response to security orders may be credited in risk assessment However, if a licensee seeks to obtain such credit, the crediting of these strategies in risk assessments must be consistent with current, extant guidance on PRA quality such as found in Regulatory Guide (RG) 1.200, "An Approach for Determining the Technical Adequacy of Probabilistic Risk Assessment Results for Risk-Informed Activities." Because of the high degree of variability in mitigating strategies at nuclear power plants, this RIS clarifies some of the salient points of the existing guidanc Mitigating strategies often involve use of non-safety, portable, manual control, offsite, or non-standard system alignments to provide enhanced mitigation capabilit Capabilities such as these are generally reflected in PRA analyses rather than traditional design-basis accident analysis, such as those found in the accident analysis section of the plant Safety Analysis Report (SAR). Therefore, the staff believes that the best focus for crediting such capabilities rests in risk-informed action However, there may also be a few instances where such enhanced capabilities offer benefits to licensing-basis events (beyond design basis) where the staff has imposed performance requirements, (e.g., station blackout (SBO) and anticipated transient without scram (ATWS) mitigation). While not part of the SAR accident analysis (e.g. Chapter 15 for a typical plant), the NRC requires plants to have the capability to respond to these events and in some cases related actions may even be incorporated into the plant's Emergency Operating Procedures (EOPs). While the primary area of credit for mitigating strategies will remain risk-informed actions, the staff may allow credit for these few licensing- basis issues in assessing such topics as compliance with the SBO and ATWS rule requirement This will be determined on a case-by-case assessment of the licensee's associated justification basi This RIS considers two specific applications of risk assessment These are risk-informed licensing actions and SDP evaluation These are the main areas where NRC anticipates licensees may wish to credit mitigating strategies in risk assessment Risk-Informed Licensing Actions Currently, licensees may submit risk-informed license amendment requests using the guidance of RG 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis," and/or one of the application-specific risk- RIS 2008-15