ML23020A044

Insights for Risk-Informed Approaches to EPZs - IAEA_TIC2022_94

ML23020A044
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Issue date:	01/20/2023
From:	Marie Pohida, Rosenberg S, Tanya Smith, Shilp Vasavada NRC/NSIR/DPR
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IAEA-CN-308-94
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VASAVADA et al.

INSIGHTS FOR RISK-INFORMED APPROACHES TO SIZING EMERGENCY PLANNING ZONES S. VASAVADA, T. SMITH, M. POHIDA, S. ROSENBERG U.S. Nuclear Regulatory Commission, Rockville, MD, United States of America Abstract Emergency preparedness provides the final layer of defense in depth for the protection of public health and safety in the event of a radiological emergency at a nuclear power plant. Emergency plans may include an emergency planning zone (EPZ) when predetermined, prompt protective actions are warranted. The size of an EPZ can be scaled to reflect the risk profile of a particular nuclear facility as determined by a risk-informed analysis of the consequences from a spectrum of accidents with considerations given to accident frequency and other factors. The U.S. Nuclear Regulatory Commission and designers of evolutionary and innovative reactors are seeking to use a risk-informed approach to determine the appropriate EPZ size for a particular design and site. The paper discusses the merits of a risk-informed methodology for determining the EPZ size, an approach that can be supported by contemporary probabilistic risk assessments. The advantages and challenges of using these assessments to support EPZ sizing are also described. The paper also identifies strategies to address prominent challenges, such as the treatment of uncertainty, justifying screening thresholds for accident sequences, consideration of cliff-edge effects, and performance monitoring. These strategies are shown to be consistent with the principles of risk-informed regulation.

1. INTRODUCTION Emergency preparedness (EP) ensures that adequate protective measures can and will be taken in the event of a radiological emergency at a nuclear power plant. Planning requirements include the capability to recognize and classify emergency events, establish and maintain effective communications, assess radiological conditions in and around the facility, implement protective measures on site, and recommend protective actions to offsite authorities as conditions warrant. EP provides a final, independent layer of defense in depth for the protection of public health and safety.

The U.S. Nuclear Regulatory Commission (NRC) employs a graded approach to EP in which its requirements are set commensurate with the risk of the facility, among other considerations. Emergency plans sometimes include a defined emergency planning zone (EPZ). The EPZ is a tool used to support the planning of prompt protective actions that would need to be taken during an emergency. Specifically, the EPZ helps to simplify protective action decisionmaking when such decisions may be time constrained. Planning within the EPZ also provides a basis for expansion of the response beyond the EPZ, should that prove necessary. The EPZ size does not change the requirements for emergency planning; it only sets bounds on the planning. As such, EPZs are scalable in size and commensurate with the planning needs for the facility.

The size of the EPZ is risk-informed. For large light-water reactors (LLWRs), risk insights from design-basis accidents and probabilistic risk assessments (PRAs) of severe accidents led to a generic 10-mile plume exposure pathway EPZ [1]. Existing NRC regulations also provide for a case-by-case determination of the size of the plume exposure pathway EPZ for gas-cooled reactors and for reactors with an authorized power level less than 250 MW thermal. Other NRC licensed facilities such as fuel fabrication facilities, independent spent fuel storage installations, and research and test reactors have requirements for emergency planning, but may have scalable EPZs, site-boundary EPZs, or no EPZ, depending on the risks of the facility. The considerations that informed the EPZ size for LLWRs can be applied to other facilities, including evolutionary and innovative reactor designs. The risk-informed EPZ sizing methodology includes an analysis of dose vs. distance, comparing projected dose from design-basis accidents and beyond-design-basis accidentsincluding accidents with containment bypassto various dose criteria. The conditions requiring an EPZ are beyond the scope of the paper. The paper only discusses the plume exposure pathway EPZ when considered as part of the planning. The paper highlights insights for applying PRA information for use in determining an appropriate EPZ size using a risk-informed methodology. It needs to be emphasized that the context and scope of this paper is limited to risk-informed methodologies for EPZ sizing, recognizing EP as an independent line of defense in depth. The discussions should not be construed to be extended to or change the reactor design.

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2. USE OF PRA INFORMATION TO INFORM EPZ SIZE In 1978, a combined NRC and U.S. Environmental Protection Agency task force established a planning basis for EP as described in NUREG-0396, Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants [1]. The planning basis is an evaluation of the consequences from a spectrum of accidents used to scope the planning efforts for the distance, time, and radioactive materials released. The purpose of specifying a planning distance is to define the area within which detailed preplanning is needed to implement predetermined, prompt protective actions. The task force determined that the size of the plume exposure pathway EPZ could be based on the following considerations:

Projected doses from traditional design-basis accidents would not exceed Federal protective action guide levels outside the EPZ.

Projected doses from most core melt sequences would not exceed Federal protective action guide levels outside the EPZ.

For the worst core melt sequences, immediate life-threatening doses would generally not occur outside the EPZ.

Detailed planning within the EPZ would provide a substantial base for the expansion of response efforts if this proved necessary.

By applying these considerations using the available risk data, the task force recommended an EPZ of approximately 10 miles for the LLWRs at the time. Fig. 1 illustrates the dose-distance curves for core melt sequences used to support the task force recommendation [1]. These dose-distance curves were based on insights from PRA information in the reactor safety study, also known as WASH-1400 [2]. Considering the curves at 10 miles, Fig. 1 shows that the EPZ is not a boundary beyond which dose cannot exceed a certain value. Rather, the figure illustrates that the EPZ is judgment based, considering the consequences of a spectrum of accidents and tempered by probability considerations (i.e., it is risk-informed).

FIG. 1. EPZ size considerations for LLWRs using PRA insights.

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Since the 1970s, the NRC and its licensees have advanced significantly in their knowledge and implementation of and experience with PRAs. The NRC has a long history of using PRA information and insights to support risk-informed decisionmaking. PRA insights supported the rulemaking for Title 10 of the Code of Federal Regulations (10 CFR) 50.61, Fracture toughness requirements for protection against pressurized thermal shock events [3], and 10 CFR 50.62, Requirements for reduction of risk from anticipated transients without scram (ATWS) events for light-water-cooled nuclear power plants [4]. The 1985 Policy Statement on Severe Reactor Accidents Regarding Future Designs and Existing Plants encouraged advanced reactor designers to complete a PRA and consider the severe accident vulnerabilities identified using the PRA [5]. The 1995 PRA Policy Statement formalized the Commissions commitment to risk-informed regulation through the expanded use of PRA [6]. The publication of Regulatory Guide 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis (first issued in 1998 with the most recent version issued in 2018), introduced the concept of an integrated decisionmaking process that has as inputs risk information, considerations of defense in depth, sufficient safety margins, and performance monitoring [7]. PRA considers nuclear safety in a comprehensive way by examining a broad spectrum of initiating events. A PRA provides an integrated answer to the risk triplet: what can go wrong? how likely is it? what are the consequences? This naturally lends PRA to application in the planning basis for EP, and PRA insights continue to aid in risk-informing EP and the EPZ size.

2.1. Emergency preparedness for evolutionary and innovative designs The NRC staff has provided the Commission, for its approval, a risk-informed, consequence-oriented, performance-based, and technology-inclusive draft final rule for EP for small modular reactors and other new technologies [8]. The performance-based framework includes provisions for a scalable EPZ. Under the proposed performance-based framework in the draft final rule, the plume exposure pathway EPZ is defined through use of two criteria. The first criterion is that the plume exposure pathway EPZ is the area within which public dose is projected to exceed 10 millisieverts total effective dose equivalent over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> from the release of radioactive materials from the facility, considering accident likelihood and source term, timing of the accident sequence, and meteorology. The second criterion is that the plume exposure pathway EPZ is the area where predetermined, prompt protective measures are warranted.

As part of the performance-based framework, the staff has proposed a methodology to support EPZ size determinations by applicants and licensees. The methodology is consistent with the dose-distance considerations from NUREG-0396 and makes use of available PRA information. The methodology aggregates doses from different accident source terms, giving consideration to the accident likelihood. For example, for beyond-design-basis events, dose-distance results may be aggregated using frequency information from PRAs to evaluate the likelihood of exceeding various dose criteria as a function of distance. The proposed methodology also addresses the use of uncertainty analyses and considerations for accident likelihood and timing as they relate to EPZ size determination. The methodologies used for event selection, identification of source terms, modeling of releases, and aggregation of potential offsite doses were written to be appropriate for use for evolutionary and innovative designs, including small modular reactors, non-light-water reactors, and nonpower production or utilization facilities.

2.2. Challenges to use of probabilistic risk assessment information The use of PRA in risk-informed EPZ sizing decisions introduces unique challenges. Prominent among these challenges are: (1) technical acceptability of PRA information, (2) defensible screening thresholds for accident sequences, especially sequences initiated by seismic events, (3) the treatment of uncertainty in PRAs, including parametric and modeling uncertainty, (4) potential for a cliff-edge effect, and (5) performance monitoring. The context and scope of the discussions of challenges and potential frameworks for solutions hereunder is limited to risk-informed methodologies for EPZ sizing, recognizing EP as an independent line of defense in depth. The discussions should not be construed to be extended to or change the reactor design.

As with any model, PRAs include assumptions and sources of uncertainty. Parametric and modeling uncertainty in PRAs can impact the risk insights from the PRAs and subsequent EPZ sizing decisions made using those insights. The use of screening threshold(s) for PRA accident sequences for risk-informed EPZ sizing, if used in isolation from other considerations for EPZ sizing such as the need for a complete spectrum of accidents, can 3

IAEA-CN-308-94 result in a cliff-edge effect being overlooked. A cliff-edge effect, which can be explained in plain language as a small change in the input resulting in a substantial change in the output 1, could manifest itself as a change in the EPZ size with a small change in the selected screening threshold(s).

Contemporary PRAs used to support risk-informed applications approved by the NRC staff are updated regularly to reflect the as-built and as-operated plant. These updates, which are achieved through a PRA configuration control process, ensure that the risk insights from the PRAs that support risk-informed applications are reflective of the plant. Consequently, the PRAs are living models. This introduces the challenge of performance monitoring to ensure that EPZ sizing decisions made using PRAs continue to remain valid after accounting for the changes to the plant and its operations over the lifetime of the plant. The next section describes strategies to address each of the prominent challenges.

3. STRATEGIES TO ADDRESS CHALLENGES TO USING PRA TO SUPPORT EPZ SIZING Strategies exist to address each of the prominent challenges of using PRA to support EPZ sizing discussed above. These strategies are consistent with the principles of risk-informed decisionmaking and the planning basis for EP.

3.1. Probabilistic risk assessment technical acceptability The NRC staff has focused on improving and stabilizing the technical acceptability of PRAs to support risk-informed regulatory activities since the late 1990s with the NRC rulemaking in 10 CFR 50.69, Risk-informed categorization and treatment of structures, systems and components for nuclear power reactors [10]. In 2003, the Commission approved the implementation of a phased approach for achieving an appropriate technical acceptability (termed quality) for PRAs for use in risk-informed regulatory decisionmaking [11]. Consequently, the NRC staff developed an action plan to implement the phased approach to PRA quality [12]. This action plan relied on: (1) a consensus PRA standard, (2) an industry peer review process, and (3) associated regulatory guidance. The NRC staff undertook a concerted effort to establish guidance and stabilize expectations for PRA technical acceptability to support the broad use of risk-informed applications. Regulatory Guide 1.200, Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities (first issued in 2007 with the most recent draft revision issued in 2020), provides contemporary guidance on sufficient PRA technical acceptability for light-water reactors to support risk-informed applications [13]. This guidance has been successfully implemented for a variety of broad-scope risk-informed applications. Similarly, Regulatory Guide 1.247, Acceptability of Probabilistic Risk Assessment Results for Non-Light-Water Reactor Risk-Informed Activities, published for trial use in 2022, provides guidance on sufficient PRA technical acceptability to support risk-informed activities for non-light-water reactors [14]. Therefore, mature guidance and experience exist to develop technically acceptable PRAs for use in risk-informed EPZ sizing. In addition, an applicant or licensee justifies the acceptability of the PRAs for any risk-informed application.

3.2. Event selection and source term development The spectrum of accidents used to determine EPZ size should include both design-basis accidents and beyond-design-basis events; this includes internal and external initiating events, multimodule and multiunit accidents and interactions, and all sources of radioactive material whose release may result in the need to take prompt protective actions. Such information is available from PRAs and source-term models. Consistent with the PRA Policy Statement, the PRA and source-term models should be as realistic as possible so that the values and limitations of any mechanism or barrier to a radiological release for evolutionary and innovative reactor designs are not obscured.

The accident radiological source terms should be estimated for the specific facility using technically defensible analysis methods and codes. The source-term calculations should reflect the performance of the facility under normal and off-normal conditions and model the transport of fission products across all barriers and 1 A formal discussion on cliff-edge effects can be found in Section 2.3 of Appendix E of Integrated Risk-Informed Decisionmaking Process for Emergent Issues, [9].

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pathways to the environment. Evaluations of design-basis accidents can assume safety-related structures, systems, and components are available to mitigate the accident. For beyond-design-basis events, the operation of structures, systems, components, and operator actions should be modeled according to their capability under the plant conditions for the event under consideration.

3.3. Screening thresholds and seismic event screening While a full spectrum of accidents and consequences are considered to scope the planning effort, the EPZ size is tempered by probability and other considerations. As such, it may be possible to exclude accident sequences from EPZ size determinations due to an extremely low likelihood of occurrence or due to the extended time available for initiating protective actions associated with the accident sequence. For beyond-design-basis accident scenarios, the frequency of the event can be evaluated to allow quantitative consideration of the impact on the EPZ size. If accident or release frequency values from a PRA are used to determine whether to exclude the accident from the EPZ size determination analysis, then the PRA should be reviewed to establish its technical acceptability for this use in a risk-informed application. In addition, uncertainty in the scenario frequency estimate should be quantified as discussed in sections 3.1 and 3.4.

The EPZ size determination is supported by the development of aggregate dose-distance curves resulting from the consequences of a spectrum of accidents weighted by accident frequency. Typically, accident frequency is inversely proportional to the consequence; that is, higher consequence accidents tend to occur with lower frequency. The result is a relatively smooth set of weighted dose-distance curves, as was the case for the accident sequences that informed the 10-mile EPZ as illustrated in Fig. 1. However, an interesting situation that can arise for a robust design is that a single accident sequence or group of similar sequences may exist that dominates the dose-distance curve. This situation is illustrated in Fig. 2. The example in Fig. 2 is based on a hypothetical reactor design with an assumed very low cumulative probability of a severe accident (not quantified for this example) resulting in a radiological release. In this example, assumed values for the relative frequency of a spectrum of accidents (individual accident sequences or release categories) and the resulting consequences are used to develop aggregate dose-distance curves to illustrate the likelihood of exceeding 1 rem and 200 rem at various distances.

As shown in Fig. 2., for the assumed spectrum of beyond-design-basis events, the 1 rem and 200 rem dose-distance curves demonstrate that a site-boundary EPZ could be justified. However, given the robustness of the design, a single accident sequence may exist that could dominate the dose-distance determination. This is illustrated by the 200 rem - single accident curve in Fig. 2. Without due consideration, Fig. 2 would seem to imply that the hypothetical plant would need a larger EPZ.

EPZ boundary FIG. 2. Impact of a dominant accident on the dose-distance curves for EPZ size determination.

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IAEA-CN-308-94 This situation can present a particular challenge in the determination of an appropriate EPZ size, if the EPZ size were deterministic rather than risk-informed. The planning basis for EP provides an important starting point to address this challenge. The planning basis task force did not believe that it was appropriate to develop specific plans for the most severe and most improbable events. However, the task force did believe that the characteristics of severe accidents should be considered in judging whether emergency plans based primarily on lesser accidents can be expanded to cope with larger events. Recognizing that EP should not be based solely on the most severe and most improbable accident, determining the EPZ size requires considering some of the key characteristics of very large releases to ensure that capabilities exist to cope with such events. As such, it may be appropriate to set a screening threshold so that enough of the severe accident characteristics are considered to inform the planning without being overly conservative. It is important to reemphasize that the EPZ only sets the bounds of the planning activitiesthe EPZ does not set a limit on EP capabilities. Even if particular accident sequences are not used to inform the size of the EPZ, the accident characteristics determined from a PRAparticularly the timing and materials releasedwould still inform other EP functional capabilities and would ensure that some capability for response exists for even extremely improbable accidents. Importantly, this includes the capability for seeking additional assistance and support.

The situation just described may be of particular interest to designers of evolutionary and innovative reactors. The use of simplified, inherent, passive, or other innovative safety features in designs may reduce the likelihood of an internal event resulting in a significant radiological release; however, this may lead to a situation in which external eventsparticularly severe seismic eventsdominate the risk profile of the facility. This conclusion suggests a defensible seismic screening threshold is needed to ensure that enough of the seismic risk is captured to inform the EPZ size without skewing the results. If the entirety of the seismic risk (all sequences from a seismic PRA) is considered, using a screening threshold based on the sequence frequency might result in the inclusion of extremely large seismic accelerations in the EPZ sizing, which would skew the EPZ size determination but may not contribute additional insights important to EP. A viable approach to identify an appropriate seismic screening threshold would be to: (1) use the annual exceedance frequency (AEF; also known as occurrence frequency) as the screening parameter, and (2) identify the appropriate AEF screening threshold based on the seismic risk and the consequences to ensure an adequate contribution of seismic events is captured in the spectrum of events. Seismic sequences below the AEF screening threshold would be excluded from EPZ sizing considerations (characterized as the risk gap) at that threshold. The seismic risk that would be excluded from EPZ sizing considerations at a particular AEF screening threshold could then be evaluated using the dose consequences for the excluded sequences. This approach would provide a quantitative understanding of how much of the entire seismic risk was considered and how much was excluded. However, such an evaluation may not always be practicable. Further, WASH-1400, and by extension NUREG-0396, did not include quantitative seismic risk estimates in the dose-distance calculations, meaning that a direct comparison with the consequences for seismic risk considered in the currently prescribed 10-mile EPZ is unavailable. WASH-1400 recognized external events risksincluding seismic riskand dispositioned these risks as comparable to the risk from internal events for LLWRs. In contrast, seismic risk could be a dominant contributor to small modular reactors and advanced non-light-water reactors due to designs that reduce the risk from internal events and other non-seismic hazards.

Due to these practical constraints, the seismic core damage frequency or release frequency can be used for the risk gap approach only to identify the appropriate screening threshold for use in the risk-informed EPZ sizing methodology. Dose consequences of seismic events, like non-seismic events, would need to be performed for determining the EPZ size using a risk-informed EPZ sizing methodology. It is important to reiterate that the screening threshold identified using this approach would not be used to develop the seismic PRA; it would be used solely to determine the sequences from the seismic PRA for consideration in EPZ sizing.

3.4. Treatment of uncertainty and cliff-edge effects The PRA should treat uncertainties by quantifying their impacts using quantitative uncertainty analyses supported by sensitivity analyses. NRC guidance supporting the development of technically acceptable contemporary PRAs is mature and has a long history of implementation. This guidance is used to develop PRAs that have sufficient technical acceptability to support broad-scope risk-informed applications. The guidance, documented in Regulatory Guide 1.200 and NUREG-1855, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decisionmaking Revision 1, issued March 2017, addresses parametric

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and modelling uncertainties [15]. Specifically, for modelling uncertainties, the guidance provides the process to identify key assumptions and sources of uncertainty. These are assumptions and sources of uncertainty that, if changed, can impact a risk-informed decision. The guidance also provides approaches for dispositioning the identified key assumptions and sources of uncertainty in the context of the risk-informed decision. In PRAs used for EPZ sizing, key assumptions and sources of uncertainty need to be identified, assessed, and dispositioned by demonstrating that they do not change the decision (i.e., the EPZ size). Because each of the analyses supporting the evaluation can contain uncertainties, any significant uncertainties that could affect this comparison should be identified and characterized.

To ensure that radiological releases with large potential consequences that may affect the size of the EPZ are not inappropriately scoped out of the consequence assessment based on low likelihood, the applicant should consider the uncertainty of the accident likelihood. If based on PRA, the use of a low-frequency screening threshold should consider uncertainty. The PRA results should retain event sequences with frequencies below the screening threshold, and analysts should use them to confirm that there are no cliff-edge effects. In other words, if the mean frequency of a scenario is below a screening threshold, but the upper end of the frequency uncertainty range lies above that threshold, then the scenario should be considered for inclusion in the analysis.

A strategy to address cliff-edge effects in screening thresholds is to evaluate sequences below the screening threshold for each hazard. This approach can identify any sequences that provide different insights from the sequences screened in using the screening threshold. Any such sequences can be included in the subsequent steps at the release frequency at which they were identified. Since cliff-edge effects result from small changes in the input, evaluation of sequences to an acceptable percentage value below the screening threshold can provide sufficient margin to address cliff-edge effects. For example, if the screening threshold is 8E-7 per year with an acceptable margin of 50 percent, evaluation of sequences to 4E-7 per year can address cliff-edge effects.

3.5. Performance monitoring Performance monitoring is one of the five principles of risk-informed decisionmaking [7]. An effective performance management strategy ensures that EPZ sizing decisions are confirmed when the PRAs used to support these decisions are updated to reflect the as-built and as-operated plant. Such a strategy can be achieved using different means, such as inclusion in a risk-informed EPZ sizing methodology.

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SUMMARY

Evolutionary and innovative reactors are expected to have design features that enhance their safety and security functions. Even so, the safety features of these advanced reactor designs will be complemented by the operational program for EP as a final layer of defense in depth. EP is inherently risk-informed. The scope of the planning efforts and the size of the EPZ needed to support the planning are informed by the consequences from a spectrum of accidents, tempered by probability considerations. As such, PRA can provide valuable insights for EP, particularly in determining an appropriate EPZ size that is commensurate with the risk of the facility.

However, there are unique challenges in applying PRA to EPZ size determinations. Strategies exist to address each of the prominent challenges. These strategies ensure PRAs are technically acceptable, provide a spectrum of accidents for consideration, and can support defensible EPZ size determinations. In turn, meeting the requirements for EP provides reasonable assurance that adequate protective measures can and will be taken in the event of a radiological emergency.

REFERENCES

[1] U.S. NUCLEAR REGULATORY COMMISSION, Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants, NUREG-0396/EPA 520/1-78-016, NRC, Washington, DC (1978).

[2] U.S. NUCLEAR REGULATORY COMMISSION, Reactor Safety Study: An Assessment of Accident Risks in U.S.

Commercial Nuclear Power Plants, NUREG-75/014, NRC, Washington, DC (1975).

[3] U.S. NUCLEAR REGULATORY COMMISSION, Fracture toughness requirements for protection against pressurized thermal shock events, Title 10 of Code of Federal Regulations, Section 50.61.

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[4] U.S. NUCLEAR REGULATORY COMMISSION, Requirements for reduction of risk from anticipated transients without scram (ATWS) events for light-water-cooled nuclear power plants, Title 10 of Code of Federal Regulations, Section 50.62.

[5] U.S. NUCLEAR REGULATORY COMMISSION, Severe Reactor Accidents Regarding Future Designs and Existing Plants, Final Policy Statement, 50 FR 32138 (1985).

[6] U.S. NUCLEAR REGULATORY COMMISSION, Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities, Final Policy Statement, 60 FR 42622 (1995).

[7] U.S. NUCLEAR REGULATORY COMMISSION, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, Regulatory Guide 1.174, Rev. 3, NRC, Washington, DC (2018).

[8] U.S. NUCLEAR REGULATORY COMMISSION, Final Rule: Emergency Preparedness for Small Modular Reactors and Other New Technologies, SECY-22-0001, NRC, Washington, DC (2022).

[9] U.S. NUCLEAR REGULATORY COMMISSION, Integrated Risk-Informed Decisionmaking Process for Emergent Issues, LIC-504, Revision 5 (2020).

[10] U.S. NUCLEAR REGULATORY COMMISSION, Risk-informed categorization and treatment of structures, systems and components for nuclear power reactors., Title 10 of Code of Federal Regulations, Section 50.69.

[11] U.S. NUCLEAR REGULATORY COMMSSION, Staff RequirementsCOMNJD-03-0002Stabilizing the PRA Quality Expectations and Requirements, SRM-COMNJD-03-0002, NRC, Washington, DC (2003).

[12] U.S. NUCLEAR REGULATORY COMMISSION, Plan for the Implementation of the Commissions Phased Approach to Probabilistic Risk Assessment Quality, SECY-04-0118, NRC, Washington, DC (2004).

[13] U.S. NUCLEAR REGULATORY COMMISSION, Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities, Regulatory Guide 1.200, Rev. 3, NRC, Washington, DC (2020).

[14] U.S. NUCLEAR REGULATORY COMMISSION, Acceptability of Probabilistic Risk Assessment Results for Non-Light-Water Reactor Risk-Informed Activities, Regulatory Guide 1.247 (for Trial Use), NRC, Washington, DC (2022).

[15] U.S. NUCLEAR REGULATORY COMMISSION, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decisionmaking, Final Report, NUREG-1855, Rev. 1, NRC, Washington, DC (2017).