United States Nuclear Regulatory Commission - Protecting People and the Environment

Operating Reactors Sub-Arena

The Nation's fleet of operating reactors comprises one of four sub-arenas that the staff of the U.S. Nuclear Regulatory Commission (NRC) identified in considering which areas of the reactor safety arena to target for greater use of risk information. This page summarizes the following aspects of the Operating Reactors Sub-Arena:


Make continuing, incremental improvements in rulemaking, licensing, and oversight of operating reactors, while focusing on implementing existing risk-informed and performance based activities.

This objective focuses on activities that are already in progress to risk-inform the operating reactor subarena, including completed rulemaking activities, guidance documents, and implementation of some initiatives.

The NRC will revisit and update this objective (as appropriate) once the industry has implemented the currently planned activities and feedback becomes available.

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The risk-informed initiatives currently in progress were originally selected using screening criteria similar to those presented in the RPP. Consequently, the five activities (listed below) that support the goals for this subarena satisfy the following screening criteria:

  • The risk-informed initiatives that are currently underway help to improve the effectiveness and efficiency of the NRC’s regulatory process, including improved safety and reduction of unnecessary regulatory burden.
  • Information and analytical models of operating reactors, particularly for at-power operations, exist and are fairly mature.
  • The cost-beneficial nature of several of the risk-informed initiatives is evidenced by their voluntary adoption by licensees.
  • No factors have been identified to date that would motivate changing the regulatory approach in the areas where risk-informed activities are already underway. Stakeholder feedback substantiates that there is no immediate need to initiate any new risk-informed initiatives, and that the NRC should focus on completing currently identified activities and allowing the industry time to implement those activities.
  • Goals and activities to meet the objective for this subarena will be performance-based, to the extent that they meet the following four criteria:
    1. measurable parameters to monitor performance
    2. objective criteria to assess performance
    3. flexibility to allow licensees to determine how to meet the performance criteria
    4. no immediate safety concern as a result of failure to meet the performance criteria

Risk-informed activities for operating reactors occur in five broad categories:

  • applicable regulations
  • licensing process
  • revised oversight process
  • regulatory guidance
  • risk analysis tools, methods, and data

The activities in these categories are derived from the Commission’s policy statements and guidance, and include revisions to technical requirements in the regulations; risk-informed technical specifications; a new framework for inspection, assessment, and enforcement actions; guidance on other risk-informed applications (e.g., in-service inspections); and improved standardized plant analysis risk models.

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The following goals are derived from the Commission’s policy statements and guidance, which reflect the current phase of NRC and industry development, as well as the current implementation of risk-informed activities:

  • Finish the development of current risk-informed regulations (e.g., 10 CFR 50.46a rulemaking) and associated regulatory/staff guidance.
  • Implement existing NRC risk-informed activities [e.g., risk-informed technical specifications and pilots for 10 CFR 50.69 and the National Fire Protection Association (NFPA) Standard 805].
  • Encourage the industry to implement risk-informed rules and approved/endorsed activities.
  • Continue making incremental improvements to the established licensing, rulemaking, and oversight activities.
  • Modify/update established activities to account for lessons learned.

Risk-Informed and Performance-Based Activities

The following tables specify the ongoing programs and projects that the staff of the U.S. Nuclear Regulatory Commission (NRC) has targeted for greater use of risk information in the Operating Reactors Sub-Arena within the Reactor Safety Arena.

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Rulemaking Function for Operating Reactors

Initiative, Program or Project Project Description and Major Activities Status
Sep 2013
Emergency Core Cooling System (ECCS) Requirements: Re-definition of Loss-of-Coolant Accidents (LOCAs)

Make risk-informed changes to LOCA-related technical requirements (10 CFR 50.46a).

  • Issue final rule (revised 10 CFR 50.46) to redefine large LOCA.
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Emergency Core Cooling System (ECCS) Requirements: LOCA-Loss of Offsite Power (LOOP)

Remove the requirement to consider a LOOP in conjunction with a large LOCA.

  • Complete safety evaluation of the LOCA-LOOP topical report promulgated by the Boiling-Water Reactor Owners' Group (BWROG).
  • Issue final rule to remove LOCA-LOOP requirement.
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Licensing Function for Operating Reactors

Initiative, Program or Project Project Description and Major Activities Status
Sep 2013
Revise Regulatory Guide 1.201

Guidelines for Categorizing Structures, Systems, and Components (SSCs) in Nuclear Power Plants (NPPs) According to Their Safety Significance

  • Conduct pilot application of Title 10, Section 50.69, of the Code of Federal Regulations (10 CFR 50.69).
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Update Regulatory Guide 1.174 As directed in SRM-SECY-11-0014, Regulatory Guide (RG) 1.174 and related regulatory guides (RG), (i.e., RGs 1.175, 1.177, and 1.178) were updated to clarify language describing defense-in-depth.
Risk Basis: These RGs provide guidance on the use of probabilistic risk assessment in plant-specific risk-informed decisionmaking.
The draft regulatory guides (DGs 1285, 1286, 1287, and 1288) were made available for public review and comment in 2012. Since the work under Near Term Task Force (NTTF) Recommendation 1 directly relates to defense-in-depth in the NRC's regulatory framework, the update to the RGs will be contingent upon the issuance of the SRM for the NTTF Recommendation 1 paper.
Develop risk-informed improvements to the standard technical specifications (STS) Initiative 1 - Define the preferred end-state for technical specification actions [usually hot-shutdown for pressurized-water reactors (PWRs)]. More Information
Initiative 4b - Modify the current system of fixed completion times to allow reliance on a configuration risk management program (CRMP) to determine risk-informed completion times.
Initiative 5 - Optimize surveillance frequencies.
Initiative 6 - Modify actions associated with Limiting Condition for Operation (LCO) 3.0.3 to allow a risk-informed evaluation to extend operating time prior to shutdown.
Initiative 8 - Risk-inform the scope of 10 CFR 50.36.
Risk Basis: Consistent with the Commission’s policy statement on technical specifications and the use of PRA, the NRC and the industry continue to develop more fundamental risk-informed improvements to the current system of technical specifications. Initiatives for fundamental improvements to the Standard Technical Specifications are being developed by the industry and discussed with the NRC staff in public meetings.
National Fire Protection Association (NFPA) Standard 805 Review the implementation of NFPA 805, "Performance-Based Standard for Fire Protection for Light-Water Reactor Electric Generating Plants."
Risk Basis: NFPA 805 is a performance-based standard, endorsed via 10CFR50.48[c], that critically depends on risk information in the form of Fire PRA to enable licensees to transition from existing "deterministic" fire protection programs to ones that are "risk-informed, performance-based." Fire PRA is an integral part of the new licensing basis, and includes both quantitative evaluations of base risk and changes to base risk in accordance with RG 1.174 gudelines as well as supporting qualitative considerations, such as traditional defense in depth and safety margin, also as per RG 1.174.
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Methods associated with the transition to 10 CFR 50.48c National Fire Protection Association Standard NFPA 805 Risk-Informed, Performance-Based Implementation and Improvements in Fire PRA and related activities.
Risk Basis: The (RES) tasks identified provide support for the continuing implementation of the risk informed, performance-based fire protection rule Title 10 of the Code of Federal Regulations (CFR) 50.48(c), National Fire Protection Association (NFPA) Standard 805, and other on-going fire protection activities. Research projects incorporate risk information to further the state of the art application of methods tools and data used in accordance with NUREG/CR-6850 (EPRI 1011989)
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Digital Systems Probabilistic Risk Assessment (PRA)

Development of methods, tools, and guidance for (1) including digital system models in nuclear power plant (NPP) PRAs and (2) incorporating digital systems into the NRC’s risk-informed licensing and oversight activities. Specific areas currently being pursued by RES include the quantification of software reliability (software failure quantification). Subsequent to this software reliability study, enhancement in the state-of-the-art for PRA modeling of digital systems (e.g., modeling of self-diagnostics, reconfiguration, and surveillance; software common-cause failure; and data for hardware failures of digital components) is considered as future research areas.
Risk Basis: This research directly contributes to the NRC's risk-informed licensing and oversight activities by including digital I&C failure risks into NPP PRAs.

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Oversight Function for Operating Reactors

Initiative, Program or Project Project Description and Major Activities Status
Sep 2013
Workshop on Probabilistic Flood Hazard A recent workshop coordinated with federal agency partners to share information on probabilistic flood-hazard assessments for extreme events such as: flood-induced dam and levee failures; tsunami flooding; riverine flooding, local intense precipitation flooding and storm surges. NUREG/CP-0302 documented the proceedings including recommendations that will be considered during the development of a NRC research plan on flooding.
Risk Basis: The development of flooding models for all types of external events along with the methods to analyze their effects on a facility will support and improve risk analysis.
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Consequential Steam Generator Tube Rupture (C-SGTR) Develop an enhanced risk-assessment procedure for C-SGTR by closing technical gaps.
Risk Basis: Steam generator tube ruptures are potentially significant containment bypass events. If the tube rupture is due to a severe accident, the tube rupture can lead to a direct fission product release path to the environment. This project is being done to improve agency risk tools for assessing the potential for severe accident induced SG tube rupture events.
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Development of Standardized Plant Analysis Risk (SPAR) Models

Maintain and develop Standardized Plant Analysis Risk (SPAR) Model Development Program. SPAR models are plant-specific PRA models that treat accident sequence progression, plant systems and components, and plant operator actions. The standardized models represent the as-built and as-operated plant. As such, they permit the staff to perform risk-informed regulatory activities by independently assessing the risk of events or degraded conditions at operating nuclear power plants. The staff provided the Commission with an update of these activities in SECY-13-0107, "Status of the Accident Sequence Precursor Program and the Standardized Plant Analysis Risk Models," dated October 4, 2013.
Risk Basis: The SPAR models are used to support a variety of agency risk-informed programs including the reactor significance determination process and the Accident Sequence Precursor program.

Maintenance and development of the Systems Analysis Programs for Hands-on Analysis Integrated Reliability Evaluations (SAPHIRE) Code Maintain and update the SAPHIRE software and documentation. The staff provided the Commission with an update of these activities included in SECY-13-0107, "Status of the Accident Sequence Precursor Program and the Standardized Plant Analysis Risk Models," dated October 4, 2013.
Risk Basis: SAPHIRE is a software application for performing probabilistic risk analyses (PRAs). SAPHIRE offers state-of-the-art capability for assessing the risk associated with core damage frequency (Level 1 PRA) and the risk from containment performance and radioactive releases (Level 2 PRA).
Event and Condition Assessment: Technical Guidance Maintain an integrated handbook for the analysis of internal, external, and low-power/shutdown operational events in support of revised methods and user needs.
Risk Basis:  Maintaining analysis tools and formal guidance associated with risk analysis, supports risk informed decision making by the staff.
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Event and Condition Assessment: Technical Support

Maintain analysis methods to support user needs, and provide on-call technical assistance to senior reactor analysts and NRR.

  • Provide event-specific methods and SPAR model modifications (MD 8.3, ROP, ASP).
  • Provide Significance Determination Process (SDP) analysis reviews, as requested.
  • Provide support (methods and models) to the Risk Assessment Standardization Project (RASP) help desk.

Risk Basis: Maintaining analysis tools and formal guidance associated with risk analysis, supports risk informed decision making by the staff.

Risk-related Generic Issues (GI) from the Generic Issues Program (GIP)

GI-191: Assessment of Debris Accumulation on PWR Sump Performance. GI-199, Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern US on Existing Plants and GI-204, Flooding of Nuclear Power Plant Sites Following Upstream Dam Failure, efforts have been rolled-up into the Fukushima Lessons Learned activities (Recommendation 2.1, flooding and seismic reassessment).
Risk Basis: Licensees calculate the portions of core damage frequency (CDF) and large early release frequency (LERF) attributable to debris and compare them to the risk acceptance guidelines in Regulatory Guide 1.174.

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Implementing Lessons Learned from Fukushima Following the accident at the Fukushima Dai-ichi nuclear plant in Japan, the NRC initiated actions to evaluate lessons learned and to implement appropriate changes in nuclear power plant designs and procedures. Initial recommendations were included in the Near Term Task Force (NTTF) report entitled "Recommendations for Enhancing Reactor Safety in the 21st Century." Several of the items (e.g., Recommendation 1 regarding improving the regulatory framework and Recommendation 2.1 on re-evaluating seismic and flooding hazards) include incorporation of risk-informed, performance-based approaches into NRC activities. The status and program plans for items identified for longer term evaluations were reported to the Commission in SECY 12-0095. A status update was provided in SECY-13-0020.
Risk Basis: Recommendation 1 is to develop a regulatory framework that appropriately balances defense-in-depth and risk considerations. The staff expects to provide options and recommendations to the Commission in December 2013. Recommendation 2.1 will use risk information as part of the determination of whether additional regulatory action is warranted.
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Full-scope site Level 3 Probabilistic Risk Assessment (PRA) Develop a Level 3 PRA, generally based on current state-of-practice methods, tools, and data, that (1) reflects technical advances in PRA and related fields, as well as plant modifications, that have occurred since the NUREG-1150 studies were published in 1990, and (2) addresses scope considerations that were not considered in NUREG-1150 (e.g., low power and shutdown, multiunit risk, and spent fuel storage).
Risk Basis: This Level 3 PRA project involves performance of a full-scope site Level 3 PRA that addresses all internal and external hazards, all plant operating modes, and all reactor units, spent fuel pools, and dry cask storage.
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Improvement of Human Reliability Analysis (HRA) methods and practices

HRA efforts address the suitability of an individual method to a regulatory application; consistency among HRA practitioners in use of methods; the absence of guidance on the rigor needed for quantifying human reliability; and the scarcity of empirical data to evaluate human performance.

  • International HRA Empirical Study for benchmarking HRA methods.
  • US Study for benchmarking HRA methods.
  • NRC/EPRI collaborative effort to address HRA model differences (SRM-M061020).
  • Develop HRA methodology for risk analyses of external events, shutdown events, and Level-3 PRA.

Risk Basis: Regulatory Guide (RG) 1.200 provides an acceptable approach for determining the technical adequacy of PRA results for risk-informed activities. HRA is as a key element in the PRA; different HRA methods often have different assumptions and approximations and, therefore, may yield different results. Thus, improving HRA methods enhances the consistency and quality of HRA and PRA.

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Operator Simulator Exercise Data Collection

Data collection for HRA

  • Implement MOU with the South Texas Project Nuclear Operation Company (STPNOC) to use STPNOC's crew and simulator for data collection.

Risk Basis: Acquiring data to improve the credibility and consistency of human error probability estimates are an integral part of determining the human contribution to overall plant risk.

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Risk Informing Emergency Preparedness (EP) Oversight: Proof of Concept (NUREG/CR-7160) Publish NUREG/CR showing feasability of quantification tool for EP significance determination.
Risk Basis: The Risk informed EP project has made it possible to quantify the protection provided by EP programs as well as determining the risk significance of some program elements. The quantification tool in conjunction with a performance based regulatory structure could enhance oversight of nuclear power plant EP programs.
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Risk Informing Emergency Preparedness Oversight: Radiological Emergency Response Plan Oversight Study feasibility of using performance based evaluation techniques for emergency response programs.
Risk Basis: This study is the final element of a risk informed and performance based EP regulatory regimen. It developed a performance based oversight regimen for evaluation of offsite radiological response programs that support nuclear power plants.
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Risk Informing Emergency Preparedness Oversight: Risk Inform Emergency Action Levels (EALs) Apply SPAR models to calculate the conditional core damage probabilities (CCDPs) of different event scenarios for each EAL at different emergency classification levels (ECLs). Determine if there is any new risk-informed insight that can be applied to update the EALs in each plant. Results were published in NUREG/CR-7154, "Risk Informing Emergency Preparedness Oversight: Evaluation of Emergency Action Levels — A Pilot Study of Peach Bottom, Surry and Sequoyah" in January 2013. Additionally Level 2 study for selected EAL scenarios were documented in a white paper titled ""Feasibility Study of Risk Informing Emergency Preparedness Using Level 2 PRA: A Pilot Study Using Peach Bottom Level 2 SPAR Models"
Risk Basis: Conditional core damage probability (CCDP) was used as a key risk metric to evaluate EAL scenarios.
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Risk Informing Security Workshop The staff plans to conduct a workshop on risk-informing security as a follow-on to the September 2010 workshop conducted at Sandia National Laboratories.
Risk Basis: Several areas will be analyzed to identify possibilities to better risk inform security, to include comparing risk implementation across disciplines, evaluating the likelihood of initiating events, and using risk to grade security based on the attractiveness of special nuclear material to the adversary.
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Page Last Reviewed/Updated Tuesday, November 12, 2013