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
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
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 This activity is to review Regulatory Guide (RG) 1.174 and other related guidance documents (e.g., RGs 1.175, 1.177, and 1.178) to support updating them using revised guidance and insights from the development of PRA consensus standards, insights gained from the Risk Management Regulatory Framework activities, and considering information related to NRC's efforts to better integrate the concepts of defense-in-depth and risk-informed regulation to assure that the NRC's defense-in-depth philosophy is interpreted and implemented consistently within a risk-informed regulatory framework. More Information
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 2 – Modify Surveillance Requirement 3.0.3 to allow a delay period of 24 hours or up to the Surveillance Frequency interval, whichever is longer.
Initiative 3 - Increase Flexibility in Mode Restraints Limiting Condition for Operation 3.0.4.
Initiative 4a - Individual Risk Informed Completion Times.
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 7a - Impact of Non Technical Specification Design Features on Operability Requirements – Barriers.
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.
Interrelationships: This research is a standalone program at present. However, as this research matures it will supplement other risk informed research by addressing digital system failures in NPP PRAs.
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
Risk Informed Steering Committee (RISC) The NRC's Risk-Informed Steering Committee (RISC) is an NRC senior management committee that will provide strategic direction to the NRC staff to advance the use of risk-informed decision-making in licensing, oversight, rulemaking, and other regulatory areas, consistent with the Commission's PRA Policy Statement. The NRC's RISC is chaired by the Director of NRR, with membership of Deputy Office Directors from NRO, RES, NRR, NSIR, and NMSS, as well as the Region I Administrator. The NRC RISC has held several public meetings with the industry's own RISC. The industry's RISC is a counterpart to the NRC RISC with its membership comprised of licensee chief nuclear officers and other senior level executives, as well as representation from the Nuclear Energy Institute (NEI). The NRC and industry each agreed to form two working groups which will focus on guidance in two selected areas related to PRA technical adequacy and dealing with uncertainties in risk-informed decision-making. These working groups are holding public meetings to continue the efforts and have presented problem statements and action plans to the RISC. More Information
Risk Prioritization Initiative (RPI) In February 2013, the Commission approved SRM COMGEA-12-0001/COMWDM-12-0002 – "Proposed Initiative to Improve Nuclear Safety and Regulatory Efficiency," dated February 6, 2013 (ADAMS Accession No. ML13037A541) to further explore the idea of enhancing nuclear safety and regulatory efficiency by applying probabilistic risk assessment (PRA). This initiative could encourage the development of high-quality, plant-specific PRA models by allowing licensees to use PRA models to propose a schedule for implementing regulatory actions as an integrated set and in a way that reflects their risk significance on a plant-specific basis. The staff is developing options to present to the Commission in March 2015. More Information
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-14-0107, "Status of the Accident Sequence Precursor Program and the Standardized Plant Analysis Risk Models," dated October 6, 2014.
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 This activity involves the maintain and update the SAPHIRE software and documentation. The staff provided the Commission with an update of these activities included in SECY-14-0107, "Status of the Accident Sequence Precursor Program and the Standardized Plant Analysis Risk Models," dated October 6, 2014.
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.

Assess Debris Accumulation on PWR Sump Performance, GSI-191

Description: The generic issues program and 10CFR50.46c rulemaking are considering debris accumulation on the Emergency Core Cooling System (ECCS) sump screen that might restrict water flow to the pumps, following a LOCA. 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. Recommendation 1 was closed by the Commission without approving staff proposed improvement activities in SRM-SECY-13-0132. A status update was provided in SECY-14-0046.
Risk Basis: Recommendation 1 has been closed by the Commission (see above). Licensee's are using a probabilistic seismic hazard approach in their responses to NRC's request for updated seismic hazard information as part of NTTF Recommendation 2.1. Seismic risk insights from the Individual Plant Examination of External Events for Severe Accident Vulnerabilities are being used to determine which plants will need to perform a seismic risk assessment. For plants that need to perform the seismic risk assessment, NRC will use that 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). The Commission-directed Level 3 PRA Project is not supporting a specific risk-informed regulatory application. However, as described in SECY 12-0123, "Update on Staff Plans to Apply the Full-Scope Site Level 3 PRA Project Results to the NRC's Regulatory Framework," the results and insights of the Level 3 Project are expected to benefit a variety of ongoing regulatory initiatives. Additionally, as described in the project Technical Analysis approach Plan, the Level 3 project is leveraging ongoing and completed agency-risk informed initiatives to the extent possible. For example, the project has benefitted from previous work completed on the Spent Fuel Pool Scoping Study and the State-of-the-Art Reactor Consequence Analysis project. The leveraging of these other activities helps to avoid duplication of effort for the project and ensures that the project is utilizing the latest available information.
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

  • Collaborate with international research institutes to evaluate the NRC's Scenario Authoring, Characterization and Debriefing Application (SACADA) system in collecting operator simulator performance data to inform human reliability analysis. This work supports "Event and Condition Assessment: Technical Support" to improve human reliability analysis quality "Improvement of Human Reliability Analysis (HRA) methods and practices."

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 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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Probabilistic Flood Hazard Assessment (PFHA) Recent workshop coordinated with federal agency partners to share information on probabilistic assessments for other hazards causing or resulting in external floods such as: flood-induced dam and levee failures; tsunami flooding; riverine flooding, local intence 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.
Workshop on Probabalistic Flood Hazard Assessment and Workshop Proceedings
Risk Informing Security Workshop The staff continues to conduct workshops on risk-informing security as a follow-on to an earlier workshop.
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 Friday, April 01, 2016