ML25248A315

Response to Public Comments on Level 3 PRA Project, Volume 4

ML25248A315
Person / Time
Issue date:	09/24/2025
From:	Office of Nuclear Regulatory Research
To:
References
Download: ML25248A315 (33)

Text

July 24, 2024 Page 1 Response to Public Comments On August 21, 2023, the U.S. Nuclear Regulatory Commission (NRC) issued for public comment a draft report on the Level 3 Probabilistic Risk Assessment (PRA) project; specifically, Volume 4: Overview of Reactor, At-Power, Level 1, 2, and 3 PRAs for Internal Fires, Seismic Events, and High Winds (Federal Register [88 FR 56885], Docket ID NRC-2023-0140). Comments were received from the following individuals and organizations:

Robert Nicholas Urrabazo Michael Ravnitzky Nuclear Energy Institute (NEI)

NuScale Power, LLC Barry Quigley A synopsis of the comments received and the Level 3 PRA (L3PRA) project team responses to these comments are provided in the table below.

Commenter Synopsis of Comment

Response

R. Urrabazo Concern over anonymous hacking group stealing government contracts.

Not applicable to L3PRA projectno response.

M. Ravnitzky Omission of geomagnetic storms The staff agrees with the comment in that the effects of geomagnetic storms were identified as a potential candidate for future research in Section 12.5 of the report. No changes were made to the report as a result of this comment.

M. Ravnitzky Omission of EMP effects The staff disagrees with the comment because EMP effects are considered to fall under the category of malevolent acts, which was not included within the scope of the Level 3 PRA Project, as described in footnote 6 on page 11-2. No changes were made to the report as a result of this comment.

M. Ravnitzky Risk of natural gas pipelines The staff disagrees with the comment because pipeline accidents are assessed in Section 15.2 of the report. No changes were made to the report as a result of this comment.

July 24, 2024 Page 2 Commenter Synopsis of Comment

Response

M. Ravnitzky Lack of clear definition or classification of external hazards The staff disagrees with the comment in that the hazards considered are described in Table 12-1 and defined as a matter of the effects that were considered. As indicated in Section 12.2 of the report, the process of identifying hazards was based on reviews of relevant documents that have previously been used for the same purpose and are based on the current state-of-practice, such as ASME/ANS RA-Sa-2009 and NUREG/CR-2300. However, the staff agrees with the portion of the comment that a more systematic and consistent framework for identifying hazards may be useful. The staff has developed draft guidance to this effect in support of the Part 53 rulemaking activity. No changes were made to this report as a result of this comment.

M. Ravnitzky Interaction or correlation among external hazards As stated in the initial plan for the L3PRA project (ADAMS Accession No. ML121320310), consequential (linked) multiple initiating events (e.g., seismically induced fires and floods) will not be addressed in the current study, but are good candidates for further research to advance the PRA state-of-the-art.

M. Ravnitzky Lack of comprehensive uncertainty and sensitivity analyses All the supporting reports for Volume 4 (i.e., Volumes 4a-4e) include quantification of parameter uncertainty and extensive discussion and identification of model uncertainty and related sensitivity analyses. For example, for Volume 4d (Level 2 PRA for internal fires, seismic events, and high winds), Section 2.5.7 discusses parameter uncertainty for the release category frequencies, Section 2.6.3 discusses uncertainties in source term development, and Appendix B provides detailed uncertainty calculations and selected sensitivity analyses. Note, since many of the uncertainties for the Level 2 PRA for internal fires, seismic events, and high winds are carried over from the Level 2 PRA for internal events and floods, Volume 4d refers to that earlier report (Volume 3c, ADAMS Accession No. ML22067A214), for which Appendix C provides extensive documentation of the treatment of uncertainty.

July 24, 2024 Page 3 Commenter Synopsis of Comment

Response

M. Ravnitzky Applicability or transferability of the L3PRA results As stated in SECY-12-0123, [w]hile the Level 3 PRA study is for a single multi-unit site, and many of the insights obtained may not be applicable to other plant designs, other multi-unit sites, and all technical issues, the NRC staff anticipates that some insights obtained will be applicable to similar plants, and may be used to inform or enhance regulatory decision making. How the results and insights from the L3PRA project can be used for other plants or sites will be examined in the L3PRA project summary report (Volume 1).

M. Ravnitzky Communication and dissemination of L3PRA results While it is not anticipated that the individual L3PRA project technical reports will include a discussion of strategies or best practices for communicating or disseminating the results effectively and appropriately, or risk communication, in general, some discussion of this topic may be included in the L3PRA project summary report (Volume 1).

NEI Many of the insights from the Level 3 PRA Project were achieved without exercising the Level 3 PRA portion of the study; that is, the insights were produced during the Level 1 or Level 2 portions of the PRA. This illustrates that there does not yet appear to be any insight to be gained from spending multiple years developing a Level 3 PRA for an operating reactor, and neither industry nor NRC resources should be invested in full-scope Level 3 PRAs for the operating fleet.

The staff understands this comment to include two related but distinct comments, paraphrased as follows:

many of the Level 3 PRA project insights were gained from the Level 1 or Level 2 portions of the PRA but limited or no insights were gained from exercising the Level 3 PRA portion of the study neither industry nor NRC resources should be invested in full-scope Level 3 PRAs for the operating fleet Staff does not agree with the first statement. The fact that insights were gained in the Level 1 or Level 2 portions of the PRA does not imply that limited or no insights were gained from the Level 3 portion of the PRA. An example of insights gained from performing the Level 3 portion of the PRA is illustrated in the comments

July 24, 2024 Page 4 Commenter Synopsis of Comment

Response

received from NEI (Anderson, 2022) on the Volume 3 (internal events and floods) reports, stating:

"This study has supported some important insights that can enhance regulatory decision making, and this objective has been met. Specific notable insights include: (...)

o Significant margin exists to the QHOs for the analyzed hazards at this plant.

o Current methods show that LERF is a very conservative surrogate for the acute fatality QHO."

Because both QHO metrics rely on quantification of health effects, both insights rely on the performance of the Level 3 portion of the PRA and would not be supported by reliance only the Level 1 or Level 2 portions of the PRA.

Staff also notes that the statement, "neither industry nor NRC resources should be invested in full-scope Level 3 PRAs for the operating fleet," goes beyond a comment on the Level 3 PRA project to comment more broadly on the use of PRA generally.

Depending on the definition of the term "full-scope Level 3 PRA,"

this statement can be interpreted in two ways. In the first interpretation, which is that used in the Level 3 PRA project, the term "full-scope" means consideration of all sources, all hazards, and all plant operating states. Staff notes that performance of a "full-scope Level 3 PRA" in this sense means an analysis of the risk of offsite consequences from releases occurring from all sources, all hazards, and all plant operating states. Staff experience with the Level 3 PRA project has demonstrated the challenges of performing a full-scope PRA, whether it is limited to a Level 1 PRA or extends to a Level 3 estimation of offsite consequence risk.

However, staff notes that the challenges of performing a full-scope PRA in this sense arise largely in the Level 1 and Level 2 portion of

July 24, 2024 Page 5 Commenter Synopsis of Comment

Response

the analysis. The Level 3 portion of the analysis, which starts with an identified set of release categories and associated frequencies and radiological source terms, would largely be similar across all sources, all hazards, and all plant operating states.

In an alternate interpretation, the term "full-scope Level 3 PRA" could also be interpreted as referring to a combined Level 1/2/3 analysis of any subset of sources, hazards, and plant operating states. However, staff notes that combined Level 1/2/3 PRAs, typically limited to consideration of internal events affecting an at-power reactor, have long been performed by both the industry and the NRC for the operating fleet and external events have been assessed in new reactor applications. Combined Level 1/2/3 PRAs have typically been performed by applicants and licensees as part of the NEPA analyses documented in an environmental report and reviewed by the NRC staff for numerous COL/DC/ESP and license renewal applications in accordance with NRC guidance (e.g.,

NUREG-1555, RG 4.2, and supplements to each for license renewals) for severe accidents and severe accident mitigation alternatives. Combined Level 1/2/3 PRAs are also used by the NRC for regulatory analyses (focusing on cost-benefit analysis) and backfit analyses (focused on the determination of whether a substantial safety benefit, as reflected by an evaluation of the safety margin exceeds the quantitative health objectives, warrants regulatory action). Indirectly, the use of Level 1/2/3 PRAs also support the use of surrogates for the quantitative health objectives, such as CDF and LERF, that are used in risk-informed applications.

NEI Despite over 11 years of work, substantial uncertainties remain in the Level 3 PRA portion of the study.

Given these substantial uncertainties, the existence of this study should not be construed as evidence that Level 3 Staff notes that the documents provided for public comment do not draw any conclusions about whether Level 3 PRA methods are sufficiently advanced to support regulatory application. Therefore, staff takes this comment as going beyond the Level 3 PRA project, specifically to comment more broadly on the practice of Level 3 PRA generally.

July 24, 2024 Page 6 Commenter Synopsis of Comment

Response

PRA methods are sufficiently advanced to support regulatory application of a fully quantified Level 3 PRA. While advanced reactor designers will be using some of the techniques in Level 3 PRAs to support licensing, fully quantified Level 3 PRAs are not needed, or necessarily appropriate, for advanced reactors licensing.

Staff notes that the focus of the comment is on uncertainties in the Level 3 PRA portion of the study. However, staff is unaware of a documented technical basis for concluding that the uncertainties in the Level 3 portion of a PRA are substantially greater than uncertainties in the Level 1 or Level 2 portion of the analysis. Staff notes that the contribution of uncertainties from Level 2 and Level 3 analyses was assessed as part of the SOARCA project (see Bixler, et al., 2014), which noted that "If considering separately the epistemic uncertainty from the Level-2 parameters, the epistemic uncertainty from the Level-3 parameters, and uncertainty due to weather, the largest contribution to uncertainty is from weather variability, which is a type of aleatory or stochastic uncertainty. The smallest contributor is the epistemic uncertainty from the Level-3 input parameters, i.e., those not related to source term uncertainty.

The intermediate contributor to the overall uncertainty is from Level-2 uncertainties that influence the source terms." If the focus of the analysis is on the mean value of the consequences over all sampled weather conditions, as is common practice in Level 3 PRA, the remaining uncertainty arises largely from the Level 2 portion of the analysis. Staff recognizes that an uncertainty analysis of a single scenario at a single site does not provide evidence to conclude that the uncertainties in the Level 3 portion will always be less than those of the Level 2, or even Level 1 portion of a PRA; however, that study does provide evidence that the uncertainties arising from epistemic uncertainty in the Level 3 portion of a PRA are not always inherently greater than those from other portions of a PRA.

Staff also notes that out of the technical elements comprising the Level 3 portion of a combined Level 1/2/3 PRA (meteorology, atmospheric dispersion, dosimetry, health effects, and protective actions/site data and economic factors), most (meteorology, atmospheric dispersion, dosimetry, and health effects) are based on well-established technical disciplines with recognized sources of guidance and extensive regulatory application in the Federal

July 24, 2024 Page 7 Commenter Synopsis of Comment

Response

Government. Staff also notes that the modeling practices of the remaining technical elements (protective actions/site data and economic factors) have a long pedigree in severe accident consequence analysis and have been used in accepted regulatory applications of Level 3 PRA.

Finally, staff also notes that the NRC has recently endorsed, in the trial use Regulatory Guide 1.247, a voluntary consensus standard for NLWR PRA (ASME/ANS RA-S-1.4-2021, Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants). No unique limitations on the Radiological Consequences technical element of this standard were noted relative to the other technical elements.

To summarize, while this study does not draw any conclusions about whether Level 3 PRA methods are sufficiently advanced to support regulatory application, staff believes that the technical basis for concluding that the uncertainties in the Level 3 portion of a PRA broadly preclude regulatory application of Level 3 PRA methods has not yet been clearly documented.

NEI The insights developed in this study cannot necessarily be applied on a generic basis. For example, the study cites the quantitative impact of crediting FLEX equipment; however, this quantitative impact will vary substantially depending on site-specific strategies.

As stated in SECY-12-0123, [w]hile the Level 3 PRA study is for a single multi-unit site, and many of the insights obtained may not be applicable to other plant designs, other multi-unit sites, and all technical issues, the NRC staff anticipates that some insights obtained will be applicable to similar plants, and may be used to inform or enhance regulatory decision making. How the results and insights from the L3PRA project can be used for other plants or sites will be examined in the L3PRA project summary report (Volume 1).

NEI The industry understands that the NRC had a peer review of the Level 3 PRA study conducted, with multiple Due to project-specific constraints on the Level 3 PRA study (e.g.,

practical limits on access to the plant and plant information and the need in some instances to rely on analyses and PRA models

July 24, 2024 Page 8 Commenter Synopsis of Comment

Response

findings issued. The NRC should close these findings prior to drawing any conclusions from the study to ensure technical fidelity.

previously performed by the volunteer licensee), it was not possible to adhere to every requirement in the ASME/ANS PRA standards.

However, it is not believed that these shortcomings preclude meeting the objectives of this research study.

NEI Vol. 4 (General): The uncertainty analysis is inconsistent with the guidance in NUREG-1855, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decision Making, to carefully evaluate epistemic uncertainty. The report should include adequate consideration of epistemic uncertainty.

All the supporting reports for Volume 4 (i.e., Volumes 4a-4e) include quantification of parameter uncertainty and extensive discussion and identification of model uncertainty and related sensitivity analyses. For example, for Volume 4d (Level 2 PRA for internal fires, seismic events, and high winds), Section 2.5.7 discusses parameter uncertainty for the release category frequencies, Section 2.6.3 discusses uncertainties in source term development, and Appendix B provides detailed uncertainty calculations and selected sensitivity analyses. Note, since many of the uncertainties for the Level 2 PRA for internal fires, seismic events, and high winds are carried over from the Level 2 PRA for internal events and floods, Volume 4d refers to that earlier report (Volume 3c, ADAMS Accession No. ML22067A214), for which Appendix C provides extensive documentation of the treatment of uncertainty.

NEI Volume 4, General: Computed uncertainty ranges in latent cancer risk calculations in this report are generally less than the uncertainties in core damage frequency in this report. The report should describe the source of this discrepancy in detail.

The authors do not have a full understanding of why the uncertainty distributions for the calculated quantities in the study (e.g., core damage frequency or various risk metrics) are so tight. In Section 3.1.1, the authors hypothesize that the relatively large number of basic events and cutsets used in the parametric uncertainty analysis dilutes (masks) the effect of those basic events with higher uncertainties. This hypothesis is supported by a test documented in the overview report for internal events and floods (Volume 3).

NEI Volume 4, General: The report should compare the uncertainty results from the Level 3 PRA study to those from NUREG-1150, Severe Accident The L3PRA project parametric uncertainty results were compared to those for the pressurized-water reactors in NUREG-1150 for internal events (the only hazard category addressed for all plants in NUREG-1150). In the L3PRA project, the range (95th/5th) is

July 24, 2024 Page 9 Commenter Synopsis of Comment

Response

Risks: An Assessment for Five U.S.

Nuclear Power Plants. The uncertainty bands from the current study are far smaller than those from NUREG-1150, and a discussion on the reasoning for those smaller uncertainty bands should be included in the report.

approximately 9, which is fairly consistent with the results for the PWRs in NUREG 1150, where the ranges for the three PWRs are Surry: ~20, Sequoyah: ~14, Zion: ~8). This information has been added to Section 3.1.1 of the report.

NEI Volume 4, General: The report should include additional sensitivity studies exploring how the results are affected by alternative assumptions for those Level 3 modeling parameters where the initiating hazard can affect the outcome, such as those related to protective actions. For example, in Section 3.4.2.3, the time needed for evacuating cohorts is assumed to be twice as long as in the nominal evacuation model but no sensitivity study results were provided.

Two sensitivity analyses identified in Table 3-33 of Volume 4e address how the results are affected by alternative assumptions for those Level 3 modeling parameters where the initiating hazard can affect the outcome. PA_6 examines uncertainty introduced by delayed GE declaration due to all-hazards impacts. PA_7 examines uncertainty associated with the potential effect of structural damage associated with seismic or high wind events. Of these two, PA_7 was exercised in the report, as described in Chapter 4 of Volume 4e.

NEI Volume 4, pg 1-1: The fourth objective of the Level 3 PRA project includes the evaluation of the realistic cost of developing new Level 3 PRAs. No further discussion of this item was found in Vol. 4 (Overview of Reactor, At-Power, Level 1, 2, and 3 PRAs for Internal Fires, Seismic Events, and High Winds). Since the total cost will be composed of the various constituents, this report should include While the NRC has tracked the costs of the L3PRA project, there are many reasons why these costs would not be representative of the costs for an independent entity to perform a Level 3 PRA.

These include:

the large scope of the L3PRA project (i.e., all hazards, all plant operating states, and all large radiological sources on stie) the make-up of the project team (i.e., the NRC attempted to incorporate a significant number of less experienced staff for training purposes)

July 24, 2024 Page 10 Commenter Synopsis of Comment

Response

some discussion of cost / effort in each volume to inform the process along the way, and provide for public dissemination / comment.

the lack of a dedicated team, resulting in significant inefficiencies as critical staff were directed to other higher priority work the unique starting point of the study (in several instances, the staff needed to rely on and get familiar with reference plant models) the lack of full access to the reference plant and associated information For the above reasons, and since the L3PRA project costs are only tracked project-wide (i.e., not categorized in any way), the discussion of the costs for Level 3 PRAs will be very limited and will only be included in the final summary NUREG report (Volume 1).

NEI Volume 4, pg 2-1: It would be helpful to indicate that these key messages are specific to the plant being analyzed and may be different for other plants.

This clarification has been added to Section 2.

NEI Volume 4, pg 2-2: Under the Level 2 key messages, acknowledgement should be included for the major investment made by the industry to implement Severe Accident Management Guidelines (SAMGs) which do provide for mitigation beyond vessel breach.

The examples of uncredited post-vessel-breach actions provided in the text are SAMG actions. This has been clarified in the report.

NEI Volume 4, pg 2-2: Early fatality (EF) risks to individuals are described as far below the QHO whereas in Tables 2-1 and 2-2 they are listed as The text has been modified to read, Early fatality risks to individuals are far (almost 6 orders of magnitude) below the QHO associated with the safety goals."

July 24, 2024 Page 11 Commenter Synopsis of Comment

Response

essentially zero, with footnotes 7 & 8 providing the very low values (<E-12).

The information and notes in Tables 2-1 and 2-2 are very helpful. It is recommended that the text discussion for EFs include more quantitative indication of the margin to the safety goal (e.g., x orders of magnitude,

<0.000x%) similar to the last sub-bullet for LCFs.

NEI Volume 4, pg 2-3: The Level 3 results for the reactors in this study appear to be promising, which generated large margins to the safety goals to both early and latent fatality values.

Although the users of this report will note the limitations, one aspect of the non-Level 3 focused PRAs should be discussed in that some low frequency but high consequence scenarios may not have been modeled or quantified, which could result in direct release of radionuclides to the environment.

Nevertheless, the total contributions should still be expected to be low, but may not be shown as negligible in Table 2-1 of the Vol 4 report.

Many low frequency but high consequence events have been modeled in the study (e.g., unisolated steam generator tube ruptures [SGTRs] and interfacing systems LOCAs).

Some other low frequency events were screened out (as documented in Volume 4c) using screening criteria based primarily on guidance from Revision 1 of NUREG-1855 and the related supporting requirements in Part 6 of ASME/ANS RA-Sa-2009.

Additionally, the screening criteria selection process was informed by Part 6 of ASME/ANS RA-Sb-2013 and insights from the development of the NRC Interim Staff Guidance document DC/COL-ISG-028, Assessing the Technical Adequacy of the Advanced Light-Water Reactor Probabilistic Risk Assessment for the Design Certification Application and Combined License Application.

Lastly, it is acknowledged that evaluation of some low frequency, high consequence events is out of the scope of the Level 3 PRA project (e.g., multi-tube SGTRs).

NEI Volume 4, pg 3-1: It appears that no changes were made when risk was overestimated due to rare event Section 2.5.6 of Volume 4d provides a detailed discussion of the overestimation (inflation) associated with the high failure probabilities assigned to many seismic basic events, particularly for

July 24, 2024 Page 12 Commenter Synopsis of Comment

Response

approximations for seismic hazard based on the risk contribution of failure to implement FLEX and continue TDAFW to latent cancer fatality risk.

the higher seismic bins, and what steps were taken in the analysis to minimize this inflation.

NEI Volume 4, pg 3-1 and pg 3-14: There is insufficient evidence to support the inverse trend in FLEX reduction.

There appears to be a greater reduction in cases such as internal events, floods, and high winds, where there is a greater chance of FLEX failure due to a lack of concrete cues for SBO failure or high winds causing higher failure probabilities.

The coarse FLEX model used in the L3PRA project results in FLEX (and extended TDAFW operation) failure probabilities that are large enough that the likelihood of failing to identify the occurrence of SBO conditions does not make a significant contribution, regardless of the hazard being analyzed. The failure probabilities for FLEX (and extended TDAFW operation) are, therefore, dominated by the assumed impact of the specific hazard on plant equipment and the likelihood of operators being able to successfully implement these strategies.

NEI Volume 4, pg 3-1 and pg 3-14: Even though FLEX was designed with seismic-like scenarios in mind, the use of higher seismic parameters skews the impact of seismic on latent.

See previous response.

NEI Volume 4, pg 3-2: As indicated in Section 2, it would be helpful to indicate that these results are specific to the plant being analyzed and may be different for other plants.

Clarification has been added to Section 3.

NEI Volume 4, pg 3-34: The discussion of individual early fatality risk (with references to Tables 3-18 to 3-21) is helpful; however, the risk values listed Additional clarification has been added to the discussion of individual early fatality risk.

July 24, 2024 Page 13 Commenter Synopsis of Comment

Response

in those tables are extremely low (e.g.,

E-13 to E-15 range). The report should acknowledge these very low values in the text and clarify if there are any potential cliff-edge effects associated with modeling issues such as truncation levels, which would impact the comparisons being made.

NEI Volume 4, pg 3-38: Inclusion of the alternate dose truncation model is very useful with respect to insights.

Discussion in this paragraph regarding risks from moderate (>10 rem) to high

(>100 rem) lifetime doses however is not very clear with respect to which threshold was used for the alternate dose truncation model (i.e., 10 rem or 100 rem). The dose threshold used was not found discussed in other portions of this volume. Portions of the SOARCA study (e.g., Peach Bottom volume Section 7.3) included an additional dose truncation level of 620 mrem/year based on natural background and medical sources.

Including this level as an additional sensitivity case would provide significant benefit for insights, especially in light of footnote 19 on page 3-38 where the Health Physics Society has updated their statement The dose truncation sensitivity analysis was conducted using the values provided in the last sentence of the paragraph in question.

To eliminate potential confusion, the pertinent discussion was revised.

Staff recognizes that additional sensitivity analyses could provide additional insights; however, the dose truncation sensitivity analysis that was performed, coupled with interpretation of intermediate results such as the size of the population exceeding 10 rem, provide sufficient evidence for the conclusion that most of the latent fatality risk arises from doses in or below the low-dose range.

July 24, 2024 Page 14 Commenter Synopsis of Comment

Response

on radiation risk to specifically mention natural background levels.

NEI Volume 4, pg 3-39: Section 3.3.3 identifies initial insights with a focus on significant risk contributors (comparative in nature). Given the very low early fatality risk, it would seem that more discussion is merited related to the margin to the QHO. On page 2-2 there is a parenthetical where the low EF risk is atributed to sufficient warning time for effective evacuation. Further discussion would seem warranted (e.g., discussion of results from non-evacuating cohort).

Additionally, as an area for future evaluation (e.g., Table 6-2), it would be beneficial if MACCS was able to calculate an estimate for physical injuries and fatalities associated with evacuation for perspectives on competing risks. Notionally, this would seem achievable by using either historical evacuation injury/fatality data or automotive injury/fatality data to develop a hazard curve that could be applied to the population modeled to evacuate.

With respect to the comment related to the margin to the early fatality risk QHO, additional discussion has been added to the Additional Insights subsection of Section 3.3.3.

With respect to the comment related to tradeoffs between radiological risk and non-radiological risks associated with protective actions, staff acknowledges that such an analysis could provide useful perspectives. Staff notes that some of the issues raised in this comment are related to the issues raised in a petition for rulemaking (PRM-50-123), requesting that the NRC revise its regulations so that protective actions implemented during a General Emergency at a nuclear power plant will most likely do more good than harm when the possible physical health effects of radiation exposure and protective actions are taken into consideration). That petition is currently being considered by the NRC. However, because staff has not performed an assessment of non-radiological risks associated with protective actions as a part of the Level 3 PRA project, staff does not plan to explore this insight or identify this specific potential MACCS improvement in the present report.

However, staff anticipates that many such insights can be identified and explored in the future based on the information provided in the Level 3 PRA project reports. The low early fatality risk documented in the report could support a justification for undertaking such an effort in the future.

NEI Volume 4, pg 3-63: The Level 3 results do not report the uncertainty distributions (i.e., 5% and 95%

values). The report should document The uncertainty distributions (i.e., 5th and 95th percentile values) for each hazard category for early fatality risk and latent cancer fatality

July 24, 2024 Page 15 Commenter Synopsis of Comment

Response

the Level 3 results with uncertainty distributions.

risk are provided in Table 3-12 (p. 3-64) and Table 3-29 (p. 3-67),

respectively.

NEI Volume 4, pg 4-4: Existing representative source terms from internal events, which are generally less applicable to fire, seismic, and wind, especially with the credit of flex, and have a significant impact on the source term for a release category in terms of timing and magnitude, require at least a sensitivity study.

Sensitivity studies on representative source terms were conducted for the internal events model, and are described in Volume 3c, Appendix C, with high level insights discussed in Volume 3c, Section 2.5.3. The range of uncertainty displayed there is applicable to fire, seismic, and wind events, given that these hazards generally affect the probability of system failures rather than creating new containment failure modes. The main possible exception is containment leakage size following a severe seismic event. This parameter is discussed in Table C-47 of Volume 3c, Appendix C. The sensitivity case showed that releases increase roughly linear with the leakage area.

Credit for FLEX is not expected to have a major impact on the applicability of the source terms, since all plant damage states that contribute significantly to FLEX were also significant contributors to internal events and were considered in choosing the internal events source terms.

NEI Volume 4c, pg 12-6: In Table 12-2, Criterion 4, remove the parenthetical (i.e., 1 percent) as it be confusing.

One could interpret it as 1 percent less than which is not the intent.

The staff agrees with the comment and has revised the last sentence of Criterion 4 as follows.

Significantly lower infers that the evaluated hazard has a mean frequency of occurrence that is at least two orders of magnitude (i.e., 1 percent) less than the mean frequency of occurrence of the compared hazard.

NEI Volume 4c, pg 12-7: When discussing Criterion 1 of EXT-B1 of the 2009 Standard, it is mentioned that No screening criterion was used in the L3PRA project relating to Criterion 1 of The staff agrees with the comment and has clarified the language in the Comparison column to indicate that the application of EXT-B1, Criterion 1, is not considered appropriate for the purposes of the Level 3 PRA project.

July 24, 2024 Page 16 Commenter Synopsis of Comment

Response

EXT-B1, given that the NRC does not find it appropriate8 to screen hazards based on the analysis of a design-basis event, except as qualified in the comments above for the second criterion related to Criterion B of EXT-C1. It seems appropriate to exclude this criterion for the purpose of the Level 3 PRA project. However, consider clarifying the bolded text and the footnote as it applies to currently operating plants; the referenced ISG appears to be limited to ALWR applications. Per RG 1.200, Revision 2 and Revision 3, there was no objection to criterion 1 of EXT-B1 of the 2009 Standard.

No screening criterion was used in the L3PRA project relating to Criterion 1 of EXT-B1, given that the NRC does not find it appropriate8 to screen hazards based on the analysis of a design-basis event for the purposes of the L3PRA project, except as qualified in the comments above for the second criterion related to Criterion B of EXT-C1.

NEI Volume 4c, pg 12-11: It is not clear if the postulated wildfire considered in the internal fire PRA as a bounding fire adequately represents the potential magnitude of a wildfire applicable to the site conditions, e.g.,

extent to which forested area is cleared from the site. The report should include discussion of, or reference to, the magnitude of a potential wildfire and whether or not the bounding fire considered in the internal fire PRA is adequate to represent such a fire.

The staff agrees with the comment. The staff identified in Section 12.5, Candidates for Potential Future Research, the need to evaluate forest fire events of greater magnitude and associated frequencies. No changes were made to this report as a result of this comment.

July 24, 2024 Page 17 Commenter Synopsis of Comment

Response

NEI Volume 4c, pg 12-7 through pg 12-22:

In several portions of the external hazard screening analysis, criterion 3 is used to support screening out an external hazard on the basis that the internal events LOOP includes the hazard. It is not clear that the generic LOOP frequency from the referenced documents (e.g., NUREG/CR-5750) is applicable for the conditions specific to this site. The report should include an assessment of whether the generic data adequately represents the site in terms of LOOP frequency due to specific external hazards.

Upon further consideration, the staff agrees that is not certain whether the generic data adequately represents the site in terms of LOOP frequency due to some external hazards (in particular, biological events, wildfire, ice cover, and lightning. An argument can be made that lightning and wildfires may be covered by the current generic LOOP frequencies, but ancillary effects of these hazards may not be properly accounted for. As such, this item is now identified as a candidate for future study under Section 12.5.

NEI Volume 4c, pg 12-15: The following statement does not have adequate technical basis, since the applicable area for such a statement is over the entire surface of the Earth and should therefore be further justified in the report: The frequency of a given meteor or satellite sufficient to damage SSCs is generally expected to be less than one occurrence per year and in most cases is much less than once per year.

The staff disagrees with the comment in that the probability results from the analysis tool used to assess indirect impacts of a meteor strike also support the cited statement about direct impacts. The report was revised as follows:

Based on the probability results from the analysis tool used to assess indirect impacts, Tthe frequency of a given meteor or satellite sufficient to damage SSCs is generally expected to be less than one occurrence per year and in most cases is much less than once per year.

NEI Volume 4c, pg 14-12 [Ed. note:

pg 14-2?]: It is not clear what value of N P f(x, y) was used when calculating the annual aircraft impact frequency into structures. It is noted earlier in this The staff agrees with the comment and the aircraft impact hazard analysis has been revised to indicate that the average CONUS values of N P f(x,y) for the respective aircraft categories were used sourced from Tables B-14 or B-15 of DOE-STD-3014-06. The staff agrees that refining the probabilities used for the average CONUS

July 24, 2024 Page 18 Commenter Synopsis of Comment

Response

section that these values for general aviation in-flight are very conservative.

Consider adding refinement of the NPf(x,y) probability or similar to Section 14.5 as a candidate for potential future research.

values of N P f(x,y) could be a candidate for potential future research to assess the effects of the most current state of knowledge in this area. The following statement was added to Section 14.5 to this effect:

Evaluation of current aircraft activity data related to this hazard, including consideration of aspects that may not be represented in the underlying data sets used for this analysis (e.g., widespread adoption of GPS navigation).

NEI Volume 4c, pg 14-11: The following sentence appears to be missing a word in the bolded part: As related to airport operations, aside from the nearby helipad, the nearby that remains in use is the only other airport that could potentially result in an aircraft impact due to airport operations. The report should be edited to provide clarity in this sentence.

The staff agrees with the comment and has revised the cited statement as follows:

As related to airport operations, aside from the nearby helipad, the a nearby airport that remains in use is the only other airport that could potentially result in an aircraft impact due to airport operations.

NEI Volume 4c, pg 15-3: This section appears to be missing sufficient detail to support the screening conclusion. It is not clear what method was used to determine that these pipelines are sufficiently distant from the reference plant site, how close are the pipelines at the nearest approach, or how the approach used to determine sufficient distance treats release of stored The staff disagrees with the comment in that the guidance cited in the introduction to Section 15 on page 15-1 was used as the basis for the concluding statement. No changes were made to the report as a result of this comment.

July 24, 2024 Page 19 Commenter Synopsis of Comment

Response

material, atmospheric transport, and subsequent ignition.

NEI Volume 4d, pg 2-77 [2-50] and 2-80

[2-52]: The Table 2-16 and Table 2-17 table headers appear to be in error.

The table headers have been corrected.

NEI Volume 4d, pg 2-82: Among the discussion of key conclusions is that thermally induced SGTR are important to the LERF result. Volume 3c describes the sensitivity analysis associated with the timing of TI-SGTR (MU-11.1) and decontamination within the SG (MU-11.2). The report should describe how these key sensitivities have been incorporated into the overall Level 3 PRA results.

Volume 3c, Section 2.5.2 addresses source term development for internal events, and the choices there regarding TI-SGTR are directly applicable to the Level 2 fire, seismic, and wind analysis, given that creep rupture of both the hot leg and the steam generator tube are largely phenomenological and independent of hazard effects. This combined treatment of source term uncertainties across hazards is discussed briefly in Section 2.6.3 of Volume 4d. The two sensitivity cases on hot leg rupture timing and steam generator decontamination were considered in the original evaluation of possible source terms for TI-SGTR (Volume 3c, Section 2.5.2.6), and the decision to use the largest source term is consistent with the principle of trying to conservatively bound the range of uncertainty. The Level 3 PRA results show that despite this choice of a high source term for the 1-REL-ISGTR release category, and its significant contribution to LERF across all hazards, the potential for early fatalities due to the posited source term remains minimal. Therefore, reducing conservatism in this source term would have only minor risk significance.

NEI Volume 4e, pg 3-12: The last paragraph of Section 3.1.2.2 identifies additional source term characteristics that may be important but have not yet been not specifically analyzed including particle size distribution and dry deposition velocity. If these are not The potential significance of uncertainties of the treatment of deposition is discussed in Section 3.3.4 of Volume 4e, which references Helton (1995a, 1995b) and SNL (2016, 2019) to describe the significance of uncertainties in deposition modeling. A pointer to this discussion has been added into Section 3.1.2.2: "The potential significance of uncertainties arising from the treatment of

July 24, 2024 Page 20 Commenter Synopsis of Comment

Response

to be assessed as part of the NRC Level 3 project, the report should reference the basis for their potential importance (e.g., reference SOARCA documents if applicable).

plume rise and deposition is discussed in further detail in Section 3.3.4."

NEI Volume 4e, pg 3-14: Table 3-6 footnote 2 notes that the containment overpressure release point is uncertain, ranging from 0m to 18m, but was modeled as a release at 0m.

It would be helpful to briefly add the reasoning for modeling the ground level release rather than a higher elevation release (e.g., 18m). A ground level release might be bounding for early releases for early fatality considerations, but containment overpressure scenarios would be expected to be most aligned with latent fatality considerations and a higher elevation release might distribute the release further distances or more widely geographically.

Revised footnote 2 to add the statement Lower release elevations, such as those assumed in this analysis, can result in increased consequences at very close range but somewhat lower consequences at longer distances. Conversely, higher release elevations could result in lower consequences at very close range but somewhat higher consequences at longer distances.

NEI Volume 4e, pg 3-21: Table 3-9 further discusses the potential sensitivity cases that could be evaluated and references the report section. No sensitivities were identified referencing Section 3.1.2.2 in Table 3-9, although Section 3.1.2.2 identifies modeling aspects that could be evaluated with sensitivity cases. The report should These uncertainties are manifested in the atmospheric dispersion modeling section where plume rise and plume deposition are discussed and are addressed in Section 3.3.4. Added a pointer to this discussion into Section 3.1.2.2: "The potential significance of uncertainties arising from the treatment of plume rise and deposition is discussed in further detail in Section 3.3.4."

July 24, 2024 Page 21 Commenter Synopsis of Comment

Response

add discussion to Table 3-9 based on Section 3.1.2.2 (e.g., deposition velocity sensitivity case).

NEI Volume 4e, pg 3-39: Table 3-16 entry for Section 3.3.2.3 (Plume Rise) mentions the NRC/CEC pilot study without further reference. The report should clarify the reference for this study.

Revised reference to NRC/CEC pilot study to read Harper (1995). This reference documents the Probabilistic Accident Consequence Uncertainty Analysis Dispersion and Deposition Uncertainty Assessment.

NEI Volume 4e, pg 3-45: In the list of assumptions for protective action /

economic modeling, it would be beneficial to discuss the varied aspects of seismic intensity and the components of high winds (e.g.,

straight line, tornadic, hurricane). The discussion could elaborate on the degree to which varied seismic intensity / high wind components have the potential to impact the protective actions at a regional level. Particularly with respect to high winds (whose protective action degradations were treated the same as seismic), it might seem that tornadic winds would be expected to have a reasonably small geographic footprint near the plant site (to generate a plant trip) minimizing the degradation of protective actions throughout the EPZ and beyond. With respect to hurricanes, the advance warning available would impact The potential benefit of additional discussion of geographic aspects of seismic intensity and high winds is acknowledged, but current project schedule and resources limit the ability to examine these aspects in greater detail. Given that the impact of degraded evacuation on consequences for most release categories is very limited at this site, staff considers that the resources needed to develop additional discussion are not warranted.

July 24, 2024 Page 22 Commenter Synopsis of Comment

Response

protective actions (e.g., schools canceled in advance, substantial pre-evacuation due to the storm).

NEI Volume 4e, pg 3-46: It is assumed that events initiated by seismic events or high wind events fail all bridges in the EPZ. This appears to be a very conservative assumption, particularly for high wind events. It would be beneficial to briefly discuss the number of bridges failed (e.g, a few vs many), their general location, and a depiction of what classifies as bridge (e.g., highway overpasses, small bridge over creek, bridges over some length, etc.)

Added a footnote to Section 3.4.1 stating Examination of the road network indicates many of the roads within the 10-mile EPZ are not anticipated to have an adverse impact due to the seismic event which would make the route impassable. There were five bridges and several low-lying areas in the EPZ that are assumed to be impassable due to the seismic event, and several low-lying areas, but alternate evacuation routes are generally available.

NEI Volume 4e, pg 3-46: It is assumed that EPZ evacuees would not encounter congestion for seismic or high wind events except at the exit routes for the region, yet on page 3-73 it was mentioned that a traffic model was developed. It would be beneficial to clarify in this assumption the degree to which the traffic model validates this lack of congestion.

A summary of the capacity analysis is provided in Section 3.4.2.3.2.

The assumption section is revised to state "it is assumed (based on review of the traffic analysis documented in the site evacuation time estimate) that the evacuation network is not limited by the road capacity."

NEI Volume 4e, pg 3-46: In the last assumption it is noted that normal activity shielding factors are applied for seismic events. However, in Section 3.4.2.2.2, there is discussion Revised "Following a seismic event, it is assumed that individuals do not remain outdoors after the event. That is, normal activity shielding factors, which assume a large fraction of the population is indoors, are assumed to be applicable" to "Following a seismic event, it is assumed that the majority (95 percent) of individuals do

July 24, 2024 Page 23 Commenter Synopsis of Comment

Response

of modifying the normal shielding factors to account for an assumed 5%

that cannot shelter due to seismic induced damage. This should be mentioned in the listed assumptions.

not remain outdoors after the event. That is, normal activity shielding factors, which assume a large fraction of the population is indoors, are assumed to be applicable. These shielding factors are adjusted to account for the 5 percent of the population that is assumed to be unable to shelter due to seismic events."

NEI Volume 4e, pg 3-52: Per Table 3-19, the modeling appears to include 45 minutes to address the time between the GE declaration and the EPZ Notification. It would be beneficial to include a brief discussion or reference (e.g., SOARCA) to the development of this timeline.

Added assumption stating, "It is assumed, based on a review of site-specific FEMA after-action reports, that there would be a period of about 45 minutes from plant declaration of SAE or GE until the offsite emergency messages are broadcast."

NEI Volume 4e, pg 3-59: Table 3-26 Row 16 for 3GPT has a considerably faster time for ESPEED2 as compared to ESPEED1, in contrast to most of the other entries in the table. Rows 5 (IND) and 10 (2GPT) have ESPEED2 values slightly faster than ESPEED1.

A footnote summarizing the reason for ESPEED2 > ESPEED1 would be beneficial.

The evacuation model parameters were developed by balancing consideration of multiple factors such as travel delays, approximate travel distances, roadway congestion, and evacuation zone clearance time. The resulting parameter set was then checked for reasonableness. Staff notes that the significantly higher travel speed for ESPEED2 for the 15-20 mile evacuation tail cohort is consistent with a cohort which encounters congestion arising from previously departing cohorts immediately upon commencing travel, resulting in a lower ESPEED1 value, but which is the last cohort to clear the evacuation zone, resulting in less overall roadway congestion.

NEI Volume 4e, pg 3-62: Table 3-29 last row 3/3S* appears to need a footnote to explain the *. The last row Added footnote 1 to Table 3-29: "1. Used for cases with more than 1,000 projected normal evacuation relocatees beyond the evacuation zone."

July 24, 2024 Page 24 Commenter Synopsis of Comment

Response

of Table 3-28 is for 3/3S1 and includes a footnote.

NEI Volume 4e, pg 3-64: Sections 3.4.2.4.4 (Estimation of Loss of Use Costs) and 3.4.2.5 (Decontamination) note there were no changes made for the Seismic/High Winds analysis as compared to Internal Events/Internal Flooding. It is recommended that some discussion be added with respect to the fact that physical damage due to the initiating events would be expected to reduce costs associated with loss of use and decontamination from radiological contamination alone. Structures that are physically damaged by the initiating event (e.g., 5% assumed unavailable for shelter in Section 3.4.2.2.2) would not be expected to be decontaminated in the same manner, and there would be loss of use unrelated to the contamination. This could also be included in Table 3-32 (uncertainties).

Added discussion of this source of uncertainty to Section 3.4.4 and Table 3-32: The impact of the initiating event (e.g., seismic or high winds) on loss-of-use and decontamination costs is a source of uncertainty. Physical damage due to the initiating events could affect costs associated with loss of use and decontamination from radiological contamination alone, as structures that are physically damaged by the initiating event may not be decontaminated in the same manner as those not impacted by the initiating event and could be subject to loss of use from the impact of the initiating event alone.

NEI Volume 4e, pg 3-65: Table 3-31 values for DOSEMILK, DOSEOTHER, DOSELONG reference Section 3.4.2.2 for their bases, but no discussion was found in Section 3.4.2.2.

Entry in "Source" column has been corrected to "Sect. 3.4.2.2 of NRC (2022b)."

July 24, 2024 Page 25 Commenter Synopsis of Comment

Response

NEI Volume 4e, pg 3-67: Table 3-31 identifies the use of decontamination efficiency (DSRFCT) values of 3, 5 or

15. NUREG/CR-7270 (2022) recommends values of 2, 4, or 8.

NUREG/CR-7270 was listed as a reference in the draft Vol 4e but call outs to this reference were not located in Vol 4e. It is recommended that some discussion be included with respect to NUREG/CR-7270 recommended values versus those used in the Level 3 PRA project. This could also be included in Table 3-32 (Uncertainties).

NUREG/CR-7270 is mentioned in Table 6-1, where it is noted that ongoing work to update the technical basis for decontamination model parameters was completed and published in Bixler (2022).

Because the values in NUREG/CR-7270 were not used in the report, it is not discussed in detail in Chapter 3 in order maintain consistency across project volumes that were completed before issuance of NUREG/CR-7270. Although this is not a technical error (like other entries in Table 6-1), the public availability of this update was judged to be of sufficient significance to warrant mention in this report.

NEI Volume 4e, pg 4-66: Section 4-2 (Early Health Effects) presents early fatality results and prodromal vomiting results (early radiological injury to examine early dose impact dose threshold effects) which indicate close to zero cases. While these results are based on the specific population distribution of this plant (e.g., sparse population near the site), the Level 3 PRA model also included mandatory evacuation out to 20 miles (p. 3-44) based on projected dose levels. Thus, the Level 3 results appear to indicate that while protective actions for early phase radiological impacts may be substantial in magnitude (e.g., well beyond the 10 mile EPZ), the benefit The commenter identifies an insight which can be gleaned from the detailed results provided in Chapter 4 and suggests that this insight can inform protective action policy related to tradeoffs between radiological risk and non-radiological risks associated with protective actions. Because staff has not performed an assessment of non-radiological risks associated with protective actions, staff does not plan to explore this insight in the present report; however, staff anticipates that many such insights can be identified and explored in the future based on the information provided in the Level 3 PRA project reports. A discussion of the use of the Level 3 PRA project reports as a source of insights for future analysis will be added to the project summary report (Volume 1).

July 24, 2024 Page 26 Commenter Synopsis of Comment

Response

of preventing radiological fatalities or radiological injuries may be very limited (within a few miles with 95%

weather, per Tables 4-44 & 4-45). The protective action policy aspect (not the Level 3 modeling which implements the policy) which would lead to evacuations beyond the 10 mile EPZ would seem to introduce greater risk (e.g., evacuation related injuries &

fatalities) than benefit (e.g.,

radiological injuries and fatalities are zero beyond 10 miles). Further discussion of the competing risk (e.g.,

on page 4-72 where early fatality results are further discussed) would be beneficial to inform future discussions of protective action policy.

NEI Volume 4e, pg 5-9: The following text mixes internal fires and seismic, and is likely in need of clarification or editing.

As for internal fires, the basic events in the integrated Level 1 and Level 2 PRA logic models that have a composite FV importance greater than 0.005 for mean annual risk of population dose within 50 miles and 100 miles, for seismic events, are also essentially in common for both risk metrics. Many of the most significant contributors are related to combustion The text was clarified as follows (similar changes were made to the discussions of the other risk metrics):

The basic events in the integrated Level 1 and Level 2 PRA logic models that have a composite FV importance greater than 0.005 for mean annual risk of population dose within 50 miles and 100 miles, for seismic events, are essentially in common for both risk metrics.

Many of the most significant contributors are related to combustion within containment. These findings are consistent with those previously discussed for internal fires.

July 24, 2024 Page 27 Commenter Synopsis of Comment

Response

within containment, as described above for internal fires.

NEI Volume 4e, pg 5-9: Discussion of the composite Fussell-Vesely (FV) importance measure developed to ascertain importance of Level 1 and Level 2 basic events in the Level 3 results references the report Compton, 2020, however no such document is found in the list of references. The report should include a brief appendix demonstrating the development of the composite FV results used to derive either Table 5-50, 5-51 or 5-52.

The reference Compton, 2020 was supposed to be changed to NRC, 2022b. This correction has been made to the text.

As stated in the footnote on p. 5-9, the Level 3 PRA report for internal events and floods (NRC, 2022b) provides a detailed discussion of the derivation of the composite FV importance measure. However, the detailed composite FV importance listings specifically for the internal fire, seismic event, and high wind PRAs, which were used to generate Tables 5-50, 5-51, and 5-52, are not being provided publicly due to the level of effort that would be required.

NEI Volume 4e, pg 5-10: In Section 5.2 it would be helpful to specify the early health effect QHO value for ready comparison. It would also be helpful to indicate the value in Table 5-9 (e.g.,

via footnote) to highlight the significant margin present in the Level 3 results as compared to the QHO.

The early fatality risk QHO has been added to Section 5.2 (no change was made to Table 5-9).

NEI Volume 4e, pg 5-18: In Section 5.3 it would be helpful to specify the latent health effect QHO value for ready comparison. It would also be helpful to indicate the value in Table 5-16 (e.g.,

via footnote) to highlight the significant The latent fatality risk QHO has been added to Section 5.3 (no change was made to Table 5-16).

July 24, 2024 Page 28 Commenter Synopsis of Comment

Response

margin present in the Level 3 results as compared to the QHO.

NEI Volume 4e, pg 7-1: The report text calls out Compton, 2020 in several locations, but no such report was found in Section 7 (References).

The reference Compton, 2020 was supposed to be changed to NRC, 2022b. This correction has been made throughout the text and to the list of references.

NuScale Power

1. Volume 4, Abstract, pg iii: The caution states that due to limitations in time, resources, and plant information, some technical aspects were subjected to simplifications or not fully addressed, and the L3PRA project documentation should not be viewed as an endorsement of these approaches for regulatory purposes.

In any PRA, there are limited resources. Documentation should help focus the limited resources on issues most directly related to protecting public health and safety; spending resources on items and issues that have little material impact on the risk to the public results in adverse net-risk impacts since those resources are not available to While providing guidance for the performance of PRAs is not a stated objective of the L3PRA project, consideration will be given to this comment when preparing the project summary report (Volume 1).

July 24, 2024 Page 29 Commenter Synopsis of Comment

Response

mitigate the risks that truly matter. Consider replacing the caution with guidance on how and where to implement simplifications.

NuScale Power

2. Volume 4, pg 3-16, 2nd bullet:

The reduction in fire and seismic are discussed; the discussion includes the following text: the influence of model changes is relatively noisy.

Consider adding a footnote to explain this text.

The following footnote has been added:

In particular, the cutset truncation limit used for the fire and seismic models is lower than for internal events, leading to the inclusion of a larger number of very low frequency cutsets. Some of these cutsets may be reduced in frequency when crediting FLEX, but the corresponding effect for internal events would not be detectable in the release category frequency.

NuScale Power

3. Volume 4, Section 4.1.1, pg 4-2:

The 8th bullet starts Highly accurate human error probabilities could not be assigned Consider changing to Detailed human error probabilities This change has been made.

NuScale Power

4. Volume 4, Section 4.1.1, pg 4-1:

Some assumptions do not include any basis (e.g., ELAP may be declared as early as the 1st hour, to as late as the 4th hour, following a reactor trip, mission time is 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />).

As stated in the report, some of the points are only included to elaborate on the context and scope of the FLEX model basic events used in this sensitivity analysis and not all the modeling points are necessarily used explicitly in the FLEX model parametric sensitivity analyses performed for the L3PRA project. To avoid confusion, some of the points not directly relevant to the FLEX model parametric sensitivity analyses performed for the L3PRA project have been removed.

July 24, 2024 Page 30 Commenter Synopsis of Comment

Response

It would help readers if a basis is included for all key assumptions described.

NuScale Power

5. Volume 4, Section 4.1.2, pg 4-3:

Key uncertainties are discussed; however, it fails to identify that Level 3 PRA methods are not fully developed or mature, and therefore include more uncertainty.

Include general discussion that Level 3 PRA methods are not fully developed or mature, and therefore include more uncertainty.

See earlier response to second NEI comment.

NuScale Power

6. Volume 4, Section 4.1.2, pg 4-3:

This Level 3 analysis applies to a specific plant/specific design in which FLEX measurably reduces risk. There is no discussion that the results of this analysis may not apply to other plants.

Include limitations that the analysis applies to a specific plant such that general conclusions may not apply to other plants or designs.

This clarification has been added to Section 2.

NuScale Power

7. Volume 4, Section 4.2.3, pg 4-6:

The 2nd paragraph notes that The subject paragraph references Section 3.2.2, where it is stated that releases could possibly be terminated earlier due to onsite and

July 24, 2024 Page 31 Commenter Synopsis of Comment

Response

assumptions regarding accident termination significantly impact the surrogate risk metric results for large release frequency and conditional containment failure probability.

Consider including example credible reasons for modeling different accident scenario termination times.

offsite resources and recovery actions that are not included in the scope this model.

NuScale Power

8. Volume 4, Tables 2-1 and 2-2:

Tables 2-1 and 2-2 show early fatality risk as approximately zero, with quantified values of approximately 1E-13/year in the footnotes. There is no discussion on the potential for extreme external hazards to impact this quantified result.

Please include a discussion of extreme external hazards and their influence on the quantified results. For example, page 3-7 describes how 2 percent of the seismic core damage frequency is from sequences with a conditional core damage probability of 1.0. How are the consequences of these Additional clarification was added to the footnotes for Tables 2-1 and 2-2.

Seismic bin 8 is assumed to have a conditional core damage probability of 1.0 due to major structural failures at the plant. As discussed in Volume 4d, seismic bin 8 sequences are also assumed to result in the release category for interfacing systems LOCAs with the auxiliary building failed (1-REL-V-F). This release category is considered the most conservative end state with respect to early fatalities, based on results from the Level 3 PRA.

Given the extreme accelerations associated with bin 8, it is appropriate to conservatively assume failure of all relevant components, including some that are normally evaluated as seismically robust (e.g., check valves). Therefore, the team decided that seismic bin 8 should be directed to whichever end state (release category) has the most severe consequences.

July 24, 2024 Page 32 Commenter Synopsis of Comment

Response

sequences captured in the risk estimates in Tables 2-1 and 2-2?

B. Quigley Fire PRA conclusions may be understated if effects of confusion in control room due to fire-induced failures are not fully accounted for.

In general, the comment is correct. The L3PRA project performed HRA work that is generally consistent with the state-of-practice for fire HRA. The realism of fire HRA modeling for fire events could be said to be, in general, an area of weakness in fire HRA/PRAs.

Without realistic exercises (simulator or otherwise) for fire events, actual operator response to fire-induced failures is an area of uncertainty.

The fire HRA performed relied on the reference plants prior fire PRA effort. In addition, the L3PRA project included a plant site visit and operator interviews were conducted to support the L3PRA projects fire HRA/PRA.

As stated in Section 16.2 of Volume 4a, [t]he RP-FPRA documentation also discusses the potential for undesired operator actions in response to spurious indications. However, based on guidance in NUREG/CR-6850 (NRC, 2005) and the ASME/ANS PRA Standard (ASME, 2009), and discussions with a recently retired representative from the Operations and Operations Training departments, the reference plant assumed that no such actions would be taken that would result in the change of state of equipment.

Furthermore, the EPRI/NRC-RES Fire Human Reliability Guidelines (NUREG-1921) performed some procedure reviews to identify possible errors of commission (EOCs) due to spurious indications.

The illustrative results documented in NUREG-1921 indicate that there are seldom many spurious indications and associated proceduralized response guidance that would require modeling of EOCs in the fire HRA.

July 24, 2024 Page 33 Commenter Synopsis of Comment

Response

To further clarify this issue, additional discussion was added to the description and characterization entries for the Operator ability to perform tasks topic under the Human Reliability Analysis technical element in the modeling uncertainty table in Volume 4a (Table 19-2).