ML20213A749

Comment (3) of Marcus Nichol, on Behalf of Nuclear Energy Institute (Nei), on Draft Regulatory Guide DG-4028, Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites

ML20213A749
Person / Time
Site:	Nuclear Energy Institute
Issue date:	07/21/2020
From:	Nichol M Nuclear Energy Institute
To:	Renee Taylor Office of Administration, Office of Nuclear Reactor Regulation
References
85FR16147 00003, DG-4028, NRC-2020-0076
Download: ML20213A749 (13)

Text

MARCUS R. NICHOL Senior Director, New Reactors 1201 F Street, NW, Suite 1100 Washington, DC 20004 P: 202.739.8131 mrn@nei.org nei.org July 21, 2020 Mr. Robert M. Taylor Deputy Office Director, New Reactors Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

Subject:

NEI Comments on Draft Regulatory Guide DG-4028, Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites, 85 Fed. Reg. 16147; Docket ID NRC-2020-0076 Project Number: 689

Dear Mr. Taylor:

The Nuclear Energy Institute (NEI)1 appreciates the opportunity to comment on draft regulatory guide DG-4028, Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites. While it seeks to implement important concepts in risk-informing the consideration of volcanic hazards, we have significant concerns that the approach would be unworkable for applicants. We believe that the proposed approach would force potential applicants in areas without potential volcanic hazards (e.g., the eastern United States) to perform unnecessary analyses, and creates implementation challenges for applicants in regions with potential volcanic hazards, such as the northwestern United States.

Given these concerns we recommend that the NRC work with industry to revise and reissue the draft regulatory guide for public comment. We think significant changes are needed to align DG-4028 with the regulatory framework for addressing other external hazards. While the IAEA guidance that is cited in DG-4028 does have useful insights, its use should be limited as there are several areas where it differs from the regulatory approach to external events in the U.S. We recommend that the guidance also consider the technology-inclusive, risk-informed, performance-based approach in the NRC RG 1.233 endorsement of NEI 18-04, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development.

1 The Nuclear Energy Institute (NEI) is responsible for establishing unified policy on behalf of its members relating to matters affecting the nuclear energy industry, including the regulatory aspects of generic operational and technical issues. NEIs members include entities licensed to operate commercial nuclear power plants in the United States, nuclear plant designers, major architect and engineering firms, fuel cycle facilities, nuclear materials licensees, and other organizations involved in the nuclear energy industry.

SUNSI Review Complete Template = ADM-013 E-RIDS=ADM-03 ADD: Stanley Gardocki COMMENT (3)

PUBLICATION DATE: 3/20/2020 CITATION 85 FR 16147

Mr. Robert Taylor July 21, 2020 Page 2 A table of initial comments which further detail the challenges with the guidance is also provided in the attachment. We did not develop an exhaustive set of comments in anticipation that the guidance may be substantially revised.

We appreciate the NRCs effort in developing this draft guidance for the future siting of nuclear power plants.

We support the goal of developing risk-informed guidance for assessing volcanic hazards. We look forward to working with the staff as NRC engages stakeholders in determining the path forward for guidance for volcanic hazards assessment. If you have any questions concerning the industrys comments, please contact me or Kati Austgen (202.739.8068; kra@nei.org).

Sincerely, Marcus Nichol Attachment c:

Mr. John Monninger, NRR/DANU, NRC Ms. Anna Bradford, NRR/DNRL, NRC Ms. Michele Sampson, NSIR/DPCP/RSB, NRC Mr. Thomas Boyce, RES/DE/RGGIB, NRC Dr. Barbara Hayes, NRR/DEX/EXHB, NRC Ms. Jenise Thompson, NRR/DEX/EXHB, NRC Mr. Edward ODonnell, RES/DE/RGGIB, NRC NRC Document Control Desk

Attachment Page 1 of 11 Nuclear Energy Institute Comments on DG-4028, Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites Introduction Under traditional licensing practice, external hazards are investigated and characterized for a siting application (10 CFR 100.23(c) and, e.g.,

52.79(a)(1)). External hazards are then addressed by the facilitys design (e.g., 10 CFR 52.79(a)(4)(ii)) by including the hazard in the design bases of structures, systems, and components important to safety (10 CFR 50 Appendix A, GDC 2). In addition to evaluating adequacy of the facilitys design, NRC also considers the geological characteristics of the site in determining site suitability (10 CFR 100.20(c)(1)). In the case of a design approval, the process is similar except that external hazards are postulated as site parameters and included in the design bases (e.g., 10 CFR 52.47(a)(1)), and a license applicant subsequently evaluates the actual site characteristics against those site parameters to ensure the design is adequate for the proposed site (10 CFR 52.79(d)(1)).

Under this traditional approach, some of a proposed facilitys SSCs (generally speaking, safety-related SSCs) would be designed to accommodate, without a loss of their safety function, external hazards like volcanism where a hazard is determined to be within the facilitys design bases. This approach ensures that an accident resulting from that hazard is prevented and, if prevention fails, can be mitigated. This approach does not expressly consider the radiological consequences of such an occurrence, but implicitly ensures those consequences would be acceptable. The traditional approach is informed by the hazard probability in characterizing the magnitude of a hazard at the site.

The approach of the draft guidance is a substantial departure from ordinary practice. The basic framework of the draft guidance is that credible volcanic hazards are evaluated in simplified PRA analyses to determine if total system performance is acceptable at several stages of analysis. If it is not, then either the design of SSCs must be shown capable for those hazards, or mitigation actions provided, or both, to demonstrate the potential impacts to the facility from that hazard are acceptable. If that is not achievable, then an alternative site is necessary.

Like the traditional approach, under the proposed guidance the potential radiological consequences resulting from volcanic hazards are not considered, despite the stated purpose of being risk-informed. Unlike the traditional approach, the probability of volcanic hazards is more clearly weighed, and mitigation actions can be expressly considered. Although those considerations are potential improvements in practice, there are challenges with the draft guidance and, as illustrated below, the guidance in its current form would likely lead to confusion and uncertainty in its implementation.

An example is provided at the end of the comments to illustrate the challenges with implementation of the guidance.

Attachment Page 2 of 11 Table of Initial Comments Which Further Detail the Challenges with The Guidance Affected Section Comment/Basis Recommendation

1. General The regulatory guide is not sufficiently risk-informed because it does not consider the potential radiological consequences.

It is noted that the NRC is exploring development of an enhanced technology-inclusive (TI) and risk-informed and performance based (RIPB) conceptual seismic design approach to achieve desired seismic safety for advanced non-light-water reactors that would align with the NEI 18-04, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development, approach endorsed in RG 1.233.

The regulatory guide should provide a more clearly risk-informed process that considers potential radiological consequences and the ability to design for, or mitigate, those potential radiological consequences due to volcanic hazards. The example that follows this table illustrates the need to address options for both design and mitigation against volcanic hazards in order to provide guidance for various application scenarios.

Additionally, as the NRC approach to apply RG-1.233 to seismic safety moves forward, the NRC should consider how it could also be applied to other external events, like volcanic hazards.

2. General As cited by the Staffs October 21, 2019 presentation, applicants must assess, Geological, seismological, and engineering characteristics. The geological, seismological, and engineering characteristics of a site and its environs must be investigated in sufficient scope and detail to permit an adequate evaluation of the proposed sitepermit adequate engineering solutions to actual or potential geologic and seismic effects at the proposed site. The size of the region to be investigated and the type of data pertinent to the investigations must be determined based on the nature of the region surrounding the proposed site. each applicant shall investigate all geologic and seismic factors (for example, volcanic activity) that may affect the design and operation of the proposed nuclear power plant

[excerpted from 10 CFR 100.23(c)].

Use the USGS National Volcanic Threat Assessment, as a technical basis to state in the Regulatory Guide that applications for sites located in the continental US east of 98 degrees 30 minutes W longitude do not have to prepare application content regarding volcanic hazards. Referencing the USGS National Volcanic Threat Assessment, in defining the region of interest is in keeping with the NRC Principles of Good Regulation of regulatory clarity and efficiency. Applications for proposed sites west of 98 degrees 30 minutes W longitude and all US territories would be expected to use the USGS National Volcanic Threat Assessment, to characterize the volcanic threat, if any, and then address volcanic hazards in their application with the contents of the proposed Regulatory Guide

Attachment Page 3 of 11 Affected Section Comment/Basis Recommendation The US Geologic Survey (USGS) regularly updates their publication, National Volcanic Threat Assessment, including the most recent update in 2018

[1]. The USGS systematically evaluates the potential hazards posed by 165 individual volcanic features across the US, including territories. Of those 165 features, 48 are located in ten western states; ranging from 1 each in Colorado, Nevada, and Wyoming, to 14 in Oregon. Using the aviation planning tool Great Circle Mapper (gcmap.com) and the 2018 report Appendix longitude data of each feature, the eastern edge of a 400 mile (644 km) radius from the three easternmost features depicted in Figure 6 of the 2018 update was generated. This effort found that no sites in the continental US east of 98 degrees 30 minutes W longitude (roughly the longitude of Wichita Falls, TX) were included within the above parameters.

This result corresponds with the academic literature regarding volcanism in the central and eastern US; the consensus being that volcanic activity ceased long before the 2.6 million years ago (2.6 Ma) interval discussed by the Staff. Even recent literature regarding the youngest volcanic features in the CEUS [Mazza, et. al. DOI:

10.1130/G35407.1] found that these features date to at least 47 Ma.

The relevant Title 10 regulations apply uniformly to all sites within the jurisdiction of the NRC. As the proposed Regulatory Guide is an appropriate tool to provide guidance to potential applicants on this topic, it would be in the interest of the public, NRC, and applicants to, with sound scientific basis, specifically delineate the region of the continental United States in which this particular offering one acceptable process to prepare such application content.

Attachment Page 4 of 11 Affected Section Comment/Basis Recommendation hazard does not apply and thus, applicants are not expected to prepare, nor Staff review, any application content regarding volcanic hazards.

3. General The DG states that the NRC staff relied heavily on detailed information in IAEA-TECDOC-1795 and SSG-21.

While this may provide useful insights, there are a number of technical areas that are not aligned with the NRC approach to regulate other external events and/or are not consistent with a risk-informed approach. For example, the IAEA-TECDOC-1795 look at 10 million years to identify hazards does not align with NRCs use of 2.6 million years in DG-4028. Application of the IAEA guidance in its entirety would result in excessive conservatism and could preclude the ability to site advanced reactors in much of the western United States.

The NRC should avoid referencing IAEA-TECDOC-1795 and SSG-21, since there are portions of those guidance documents that are inconsistent with the NRC approach to regulating other external events.

If the NRC references IAEA-TECDOC-1795 or SSG-21, it should be limited to areas where they are consistent with the NRC regulation of other external events.

4. General In several areas, the draft guide uses subjective criteria, for example:

(1) Pg. 9, 2nd paragraph under Harmonization: the stepwise approach uses available information to conduct a screening evaluation (2) Pg. 11, Figure 1, which simply ends if risk insights are developed and plant performance is acceptable.

(3) Pg. 13, under Step 3: evaluated to determine whether the total system performance would be acceptable for volcanically induced failures. Similar statement on pg.

17, 2nd paragraph.

(4) Pg. 14, 1st full sentence: shows that the potential volcanic hazards did not significantly affect safety, then additional analyses would not be warranted.

While subjective criteria is not as straightforward for an applicant or reviewer, as weve learned with the defense in depth assessment for emergency planning zones Provide some guidance for what would be acceptable to meet these criteria:

(1) The use of available information could eliminate the need to develop a model (conceptual model of tectono-magmatic processes).

(2) and (3) Should allow for complete SSC failure if response of other SSCs or plant is acceptable.

(4) Significantly affect safety could be judged by comparison to meeting the safety goals.

Attachment Page 5 of 11 Affected Section Comment/Basis Recommendation (EPZ), a subjective approach may be the best option in some cases.

5. Section B, Overview of Volcanic Hazards, Other Proximal Hazards, Page 6 Hydrothermal proximal hazards is only mentioned once in this document without any guidance on acceptable modeling of this volcanic phenomenon. The DG states that the NRC staff relied heavily on detailed information in IAEA-TECDOC-1795, but this document identifies hydrothermal activity as the one phenomenon without a practicable model (see Conclusion, page 189, 2nd paragraph). The DG needs to provide specific modeling guidance on hydrothermal hazard evaluation.

Specify one or more acceptable methodologies for an applicant to evaluate hydrothermal volcanic hazards.

6. Section B, Risk Informed Regulation, Page 8 The section titled Risk Informed Regulation ends with the following statement: The significance of the volcanic hazards assessment could then be determined using the suite of information available to support risk-informed decision-making (i.e., items a-e in SECY-98-144).

The items in SECY-98-144 are denoted with numbers, so it is not clear which items are being referenced here.

Revise the reference for clarity.

Also, given the discussion of risk significance supporting risk-informed performance-based licensing basis development in RG 1.233, there is an opportunity to include recognition of the potential use of this reference as well.

7. Section B, Senior Seismic Hazards Analysis Committee Study Guidelines, Page 9 NRC staff guidance for SSHAC study level uses qualitative adjectives low-level, modest, high confidence, straightforward, intermediate, modest, moderate confidence, potentially large, potentially significant, moderate-to-low confidence, etc.

To the extent that this guidance may be modelled on the SSHAC process, clarify by specific quantification and detailed definition the adjective descriptors used in staff guidance for SSHAC study level. For example, a low-level source term is one which would not exceed the 1 rem Total Effective Dose Equivalent (TEDE) Protective Action Guide (PAG) at the EPZ plume exposure distance.

8. Section B, Senior Seismic Hazards Analysis Committee Study Guidelines, Page 9 The Level 1 SSHAC is defined as having a high confidence in the completeness and accuracy of the geologic record. It is not clear what high confidence is when the recommended volcanic assessment covers the Quaternary Period, defined as the geologic timeframe ranging from 2.6 million years ago.

Define per Comment No. 7, above, and consider changing to moderate confidence.

Attachment Page 6 of 11 Affected Section Comment/Basis Recommendation

9. Section B, Harmonization with International Standards, item (3),

Page 10 This section does not endorse IAEA requirements for monitoring volcanoes in the U.S. for nuclear reactors, stating that it does not appear applicable for nuclear reactors in the United States. It goes on to point to the USGS statutory authority and what actions would be taken if there is a perceived gap in monitoring at a proposed nuclear reactor site. These statements are contradictory and confusing. If the USGS has statutory volcanic activity monitoring authority and IAEA requirements are not applicable, how would there be a perceived gap in monitoring that would be filled in by the USGS? Perhaps the intent was that individual nuclear reactors are not required to monitor volcanoes themselves because the USGS performs this role nationally. The USGS fulfilling this role (including regular publication of their, National Volcanic Threat Assessment) meets NRC requirements for volcano monitoring.

Revise the statements in item (3) on page 10 to explain the role, authority, requirements, and acceptability of USGS volcanic monitoring as applied to U.S. commercial nuclear reactor sites being proposed. Delete If there is a perceived gap in monitoring activities at a proposed commercial nuclear reactor, the U.S. Geological Survey will fill that gap.

10. Figure 1, Page 11 Step 1 should be consistent with Step 1 in the text.

Please refer to Step 1 in the Figure as Perform Initial Characterization.

11. Section C, Page 12, Step 1, Paragraph 2 The draft guidance states: For the purpose of the initial evaluation of potential hazards from volcanic ash falls, the region of interest for the volcanic hazards assessment should extend a sufficient distance beyond 320 km (200 mi) to encompass those Quaternary volcanic systems that have the potential to affect the design or operation of the proposed reactor. This leaves a lot of ambiguity surrounding the sufficient distance to consider ash fall a potential hazard.

Suggest the guidance characterize volcanic hazard regions of effects to determine the areas where they need to be considered, e.g., using the USGS National Volcanic Threat Assessment, rather than the current draft guidance to search for hazards based on distance from the site selected.

12. Section C, Page 12, Step 1, Paragraph 3 In performing initial characterization, when there is evidence of Quaternary volcanism in the regions of interest, a conceptual model of tectono-magmatic processes is recommended to be developed to determine how past patterns of volcanism should be projected to Provide reference(s) of acceptable models and examples of use and applicability.

Attachment Page 7 of 11 Affected Section Comment/Basis Recommendation estimate future activity. It is unclear whether a 2D or 3D model is required, or if applicants need to use LaMEM, FDSTAG, or other software to prepare the model.

13. Section C, Page 12, Step 1, Paragraph 4 The draft guidance states: The hazard analysis can screen out volcanic systems that are not consistent with the tectono-magmatic model. This screening, however, is dependent on establishing sufficient confidence in the underlying technical basis showing that future volcanism is not a credible event. This suggests that future reactors will need an increased burden of proof over the existing fleet (excluding Columbia) to screen volcanic hazards.

Additionally, the term, sufficient confidence, though followed by one example, is ambiguous and subject to a wide range of interpretation by applicants and NRC reviewers.

The guidance should provide more flexibility to screen volcanic hazards without this model in places where: 1. nuclear facilities already exist, 2.

entire regions (e.g. the eastern United States) where consideration of volcanic hazards is excluded based on the USGS, National Volcanic Threat Assessment, and 3. locations where there are no significant proximal volcanic hazards. Suggesting that a tectono-magmatic model is required for those areas is counter to a risk-informed performance-based approach.

Clarify with a specific quantifiable definition what is meant by sufficient confidence.

14. Section C, Page 13, Step 2, Paragraph 1 volcanoes in the region of interest are consistent with the tectono-magmatic model lacks the same clarity addressed in Comment No. 12 on Step 1.

Once the acceptable type(s) of models have been provided in Step 1, provide a reference here back to that section.

15. Section C, Page 13, Step 2, Paragraph 1 Quantify what credible means. credible distancecredible phenomena Credible is defined as an annual frequency of occurrence of 1 x 10-5 per year in Section 6-2.3 of the ASME/ANS PRA Standard.

16. Section C, Page 13, Step 2, Paragraph 4 The draft guidance states: the screening analysis should evaluate An example of a representative screening analysis should be provided as a go-by reference.

17. Section C, Page 14, Step 3 The term beyond-design-basis event is used with no definition.

Recommend including a metric for beyond-design-basis event based on probability of occurrence; for example, RG 1.233 and NEI 18-04, define beyond-design-basis event as event sequences with mean frequencies of 5x10-7/plant-year to 1x10-4/plant-year.

18. Section C, Page 18, first sentence Typo: IAEA-TECCOC-1795 Revise to IAEA-TECDOC-1795

Attachment Page 8 of 11 Affected Section Comment/Basis Recommendation

19. Section C, Page 18, Step 7, Paragraph 2 This section requires a robust technical basis for the amount of time available for mitigative actions, but in the same paragraph, acknowledges that considerable uncertainties exist in any such predictions based on historical data. There is no specification of how a reviewer or applicant can provide a robust technical basis.

This section identifies appropriate monitoring resources to accommodate these uncertainties without defining what is appropriate. USGS monitoring is sufficient and no additional monitoring from the site is needed.

Considering the effort to be risk-informed, specify acceptable methodologies and appropriate monitoring resources for providing a robust technical basis to estimate the amount of time available for mitigative actions. NRC should work with stakeholders to determine an appropriate process for receiving timely information from the USGS on volcanic activity from sources that could impact a plant.

20. Section C, Page 18, Step 7, first bullet The first bulleted item in Step 7 indicates monitoring resources are established to provide early indication of potential eruption, yet item (3) on page 10 states, if there is a perceived gap in monitoring activities at a proposed commercial nuclear reactor, the U.S. Geological Survey will fill that gap. See Comment No. 9. Here it is implied the plant is responsible, at least in part, for its own monitoring activities; however, USGS monitoring is sufficient and no additional monitoring from the site is needed.

NRC should work with stakeholders to determine an appropriate process for receiving timely information from the USGS on volcanic activity from sources that could impact a plant.

21. Consideration of Alternative Sites, Page 20 This section states that if the outcome of the volcanic hazards assessment indicates that volcanic hazards are beyond the facilitys design basis and cannot be mitigated effectively, then alternative sites should be investigated.

This does not appear to accurately reflect the general approach for addressing external events. We would expect that an applicant would use an assessment to understand hazards and ensure the design protects against those hazards. An applicant always has the option to select a more suitable site. This section is unnecessary; if the plant is unable to achieve acceptable results, then the applicant will not pursue a site that would not be approved in the NRC safety review.

Delete the section on Consideration of Alternative Sites. If the staff wish to retain this section, work with stakeholders to clarify that this is not related to the alternatives analysis required per §51.50(b) or (c). The focus of this guidance is volcanic hazards assessment as an input to the safety review.

Attachment Page 9 of 11 Example Use of Guidance Some problems with implementing the guidance can be best illustrated by a simplistic example. Consider a hypothetical plant site near Lincoln, Nebraska. Such a site probably would not have considered volcanic hazards without DG-4028as noted, only a few sites in close proximity to active volcanoes have. However, DG-4028 requires such a site to spend significant effort just to demonstrate that volcanic hazards do not need to be considered.

Under Step 1, the Lincoln Site applicant would consider ash fall from a region of interest (ROI) that encompasses those Quaternary volcanic systems that have the potential to affect the design or operation of the facility. All of the Yellowstone supereruptions occurred in the Quaternary period, and such an eruption has the potential to cause ash fall in most of the United States. Any ash fall could be considered to potentially affect design or operation of nearly every site in the U.S. because that phrase is undefined and because most facilities would not be specifically designed for an ash fall. Because ash fall would probably affect operation of a facility not designed for it, Yellowstone is within the ROI for the Lincoln Site and the applicant would move to Step 2.

The Lincoln Site would proceed to Step 2, where it would determine a maximum credible distance that a credible hazard can travel.

Credible is undefined and unexplained. One model shows the potential for almost 4 inches of ash fall in Lincoln,1 over 700 miles from the volcano; thus, it would seem that substantial ash fall is credible.

In Step 3, the Lincoln Site applicant would assume such ash fall is a beyond design basis event2 and assume the probability of SSC failure is 1. Which SSCs are assumed to fail? Surely not all of them, because that by definition would yield unacceptable results. But determining which ones fail would require considering the capacity of SSCs with respect to the hazardthe roof loading on buildings, the filter capacity on engines, etc.which doesnt occur until Step 6. And even if it were done here, that comparison would require defining the magnitude of the credible hazard (i.e., how much ash fall is credible), which is not addressed by the guidance. Assuming the analyst knows which SSCs are assumed to fail, they must then determine whether total system performance is acceptable. This critical phrase is wholly undefined.

The context implies this is a probabilistic criterionwhether the calculated probability of failure of the total system is low enoughbut 1 https://www.usgs.gov/center-news/modeling-ash-distribution-a-yellowstone-supereruption?qt-news_science_products=3#qt-news_science_products 2 Paradoxically, beyond design basis event normally describes a hazard outside of a facilitys design bases and thus not directly considered in developing the plant design, but here seems to refer to a hazard that must be addressed in some manner (by design, mitigation, or alternative siting as the ensuing evaluation indicates).

Attachment Page 10 of 11 there is no guidance at all on what such an acceptable probability would be, or what system performance means. Is the measure of performance core damage, large release, or something else?

In Step 4, the Lincoln Site applicant would determine either a probability of eruption (PE) or a probability of a volcanic phenomenon reaching the site (PH). Determining such probabilities would present significant challenges, and the guidance offers little clarification on acceptable methods for so doing. But assuming the analyst is able to determine these probabilities, they would then determine whether the resulting PE or PH significantly affects safety. Once again, this seems to be a probability threshold without any indication of what the threshold may be. And once again, affects safety is hopelessly vague.

Step 5 is a repeat of Step 3, but with probabilities adjusted by either PE or PH or, if that yielded unacceptable total system performance, then by the product of PE and PH. This step has all the same problems as Step 3. Given all the uncertainties in the methodology, its impossible to speculate where the hypothetical Lincoln Site applicant stands at this point, but presuming the PE and PH have not offered an exit from the methodology, they would proceed to Step 6.

In Step 6 the analyst is finally to evaluate SSC design bases to actually withstand the hazard. This Step is deemed optional, and can consider both the design basis and safety margins in the design basis. Here, as alluded to with respect to Step 3, it becomes imperative to characterize the magnitude of the hazard under consideration. How much ash fall exactly must be considered in Lincoln? How that is to be characterized is not discussed, let alone actually defined. Is it tied to an exceedance probability? If so, what probability is acceptable to the NRC? Then using that magnitude, the analyst is to evaluate the SSC capacity to withstand it. For ash fall on a roof, it might be compared to a snow load. But does the ash fall need to be considered simultaneously with snow or precipitation? And if the SSC is shown to withstand the hazard, is that hazard a design basis for the SSC? Ash fall impacts on mechanical and electrical systems would be even more difficult to compare to existing design bases (the problems with that concept are addressed elsewhere). Once the PRA analyst performs these evaluations, they are to update the simplified model with the product of PE and PH at an appropriate likelihood of the specific demand being exceeded, for which no guidance is provided, and then to once again determine if SSC performance is acceptable. Presumably it would be the total system performance, but once again that term is nebulous.

Then, in Step 7 mitigating actions can be considered in addition to, or instead of, SSC design capabilities. A suitable technical basis is to be developed for the actions, which are to mitigate a hazard of unknown magnitude, and then the PRA analyst is to somehow assign a probability of successfully mitigating that hazard. If the result is still unacceptable performance, then alternative siting may be necessary.

There are additional implementation challenges with this approach. Notably, Step 6 relies on existing design bases. It seems from the context that the DG assumes a design has already been developed and is now being sited within range of a volcanic hazard. While this would be true for a combined license application (COLA) referencing an approved design (Design Certification, Standard Design Approval,

Attachment Page 11 of 11 or Manufacturing License), the guidance is also applicable to a COLA or Part 50 applicant that does not reference a design (i.e., a custom design). Such an applicant would first establish the design bases for the site, then design the plant to address them. Thus, the evaluation of existing SSC capacitieswhich may be necessary in Step 3would be impossible for such an applicant. And an early site permit (ESP) applicant would not be able to use the guidance at all.