ML22250A427

EPZ TR ACRS Subcommittee Slides - Open Session

ML22250A427
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Issue date:	09/08/2022
From:	Office of Nuclear Reactor Regulation
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TR-0915-17772, Rev 3
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Staff Presentation to the ACRS Sub-Committee NuScale Licensing Topical Report Methodology for Establishing the Technical Basis for Plume Exposure Emergency Planning Zones at NuScale Small Modular Reactor Plant Sites (TR-0915-17772, Revision 3)

Open Session SEPTEMBER 8, 2022

2 Topical Report Review Chronology NuScale submitted topical report TR-0915-17772, Revision 2, Methodology for Establishing the Technical Basis for Plume Exposure Emergency Planning Zones, on August 4, 2020 NRC issued requests for additional information (RAIs) 9828 and 9830 in April 2021 Initially, TR was applicable to light-water small modular reactors (SMRs) and non-light water reactors (non-LWRs). In May 2021, in response to RAI 9830, NuScale removed applicability to non-LWRs.

NRC performed a regulatory audit as part of its review of the methodology in Revision 2, from November 2021 to April 2022 NuScale provided response to RAI 9828 and submitted Revision 3 of the TR on June 10, 2022, titled Methodology for Establishing the Technical Basis for Plume Exposure Emergency Planning Zones at NuScale Small Modular Reactor Plant Sites, applicable only to the NuScale design and its derivatives and incorporating the RAI responses NRC issued the advanced safety evaluation on August 8, 2022

3 NRC Staff Reviewers:

Marie Pohida, Sr. Reliability and Risk Analyst, NRR/DRA/APLC Shilp Vasavada, Sr. Reliability and Risk Analyst, NRR/DRA/APLC Elijah Dickson, Sr. Reliability and Risk Analyst, NRR/DRA/ARCB Raymond Hoffman, Emergency Preparedness Specialist, NSIR/DPR/RLB Todd Smith, Sr. Level Advisor for Emergency Preparedness, NSIR/DPR Project Managers:

Alina Schiller, TR Project Manager, NRR/DNRL/NRLB Getachew Tesfaye, Sr. Project Manager, NRR/DNRL/NRLB

Outline Regulatory and technical basis for staffs evaluation Overview of TR methodology Dose-distance criteria and figures-of-merit (FoMs)

Source term and consequence assessment Screening of Probabilistic Risk Assessment (PRA) sequences Treatment of uncertainty Conditions of use 4

Regulatory and Technical Basis for Staffs Evaluation 10 CFR 50.47, Appendix E to Part 50, 10 CFR Part 20 Probabilistic Risk Assessment (PRA) Policy Statement SRM-SECY-98-144 RG 1.174 RG 1.200 NUREG-0396 (1978) 5

NUREG-0396 (1978) 6

• Identifies the purpose of the EPZ

- Area within which prompt protective actions may be necessary to provide dose savings in the event of a radiological release

• Technical basis for current prescribed plume exposure pathway EPZ radius of 10 miles

• Based on a review of licensed design-basis events and a spectrum of accident sequences from WASH-1400

7 Staffs Method of Review

• Consistent with technical basis in NUREG-0396

• Consistent with risk-informed decision making and use of PRA in risk-informed applications

• Supported by regulatory audit

• Included example calculations implementing nearly entire methodology

8 Staffs Review of Applicability of NuScales EPZ TR Methodology

• Only applicable to NuScale design and its derivatives, including Standard Design Approval

• TR Section 2.5.1 provides high-level design characteristics that determine applicability

• All characteristics must be met

• Applicability limitations are acceptable

• High-level design characteristics capable of identifying NuScale designs with the features and risk profile considered by the staff in its review

9

• Considered recent research efforts to re-assess current 10-mile EPZ technical bases in NUREG-0396

• Considered updated information from guidance, methods, models, and analytical tools

• Assessed TR methodology consistency with analyses, assumptions and considerations of NUREG-0396 and current information

• Important topic areas:

- Radiological consequence assessment modeling approaches

- Dosimetric criteria and dose-distance curves

- Considerations derived from assessments Staffs Review Approach for Dose-Distance Criteria and FoMs

10 Dose-Distance Criteria and FoMs1 Criterion Description Source Term Figure of Merit and Acceptance Criteria a

Encompass those areas in which projected dose from DBAs could exceed the early phase PAGs NUREG-0800 Chapter 15 analysis 1 rem and 5 rem TEDE over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br />, consistent with the EPA PAG Manual early phase PAGs b

Encompass those areas in which consequences of less severe accident (containment intact) sequences could exceed the early phase PAGs PRA-specific accident sequences with MELCOR analyses utilizing mitigation design features and operator actions 1 rem and 5 rem TEDE over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br />, consistent with the EPA PAG Manual early phase PAGs c

Sufficient size to provide for substantial reduction in early severe health effects in the event of more severe accident sequences PRA-specific accident sequences with MELCOR analyses and separate effects models utilizing additional mitigation design features and operator actions 200 rem red marrow dose, within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, is less than the TR specified acceptance criteria, consistent with NUREG-0396 1 - See staff report, Use of MACCS Dose Coefficient Files to Compute Total Effective Dose Equivalent, which provides information on how to compute TEDE and red marrow FOMs using the MACCS computer code. (ADAMS Accession No. ML21211A584)

11

• For source term and radiological consequence analyses, the TR:

- Recommends computational codes such as MELCOR and RELAP to develop design-specific source terms

- Requires the MACCS code to perform radiological consequence analyses

• The staff finds these computer codes to be appropriate to perform these analyses Staffs Review of Source Term and Radiological Consequence Analysis

12 Fundamentals of Event Selection for EPZ Sizing Consistent with NUREG-0396

• Spectrum of accidents licensing and design against QHOs

• Spectrum of accidents for EPZ sizing includes characteristics of radiological consequences from DBAs and BDBAs

- Includes some of the key characteristics of more severe accidents (with large releases)

- Ensures capability to reduce early severe health effects (injuries or deaths) in the event of the most severe BDBAs

• Selection of spectrum of accidents for EPZ sizing does not alter the design or operation

13 Fundamentals of Event Selection for EPZ Sizing Consistent with NUREG-0396 (contd)

• Purpose of EPZ is dose savings and not dose avoidance

- NUREG 0396: while the EPZ should not be solely dependent on the most severe and most improbable BDBEs, the determination of the EPZ size needs to include some of the key characteristics of very large releases.

• Seismic risk expected to dominate NuScale risk profile and EPZ sizing

- Design reduces the risk from internal events

- Timing of seismically-initiated sequences different compared to non-seismic events

- Inclusion in spectrum of accidents maintains consistency with NUREG-0396

14 Seismic Risk - Potential Dominant Contributor for the NuScale Design NuScale Core Damage Frequency (per year)

Profile Without Seismic Contribution Estimated NuScale Core Damage Frequency (per year) Profile for Site Within NuScale Certified Design Response Spectrum 3E-10 6E-11 1E-09 9E-10 7E-10 9E-10 9E-08 Internal Events Internal Floods Internal Fires High Winds (Tornado)

High Winds (Hurricane)

External Floods Module Drop 3E-10 6E-11 1E-09 9E-10 7E-10 9E-10 9E-08 9E-07 Internal Events Internal Floods Internal Fires High Winds (Tornado)

High Winds (Hurricane)

External Floods Module Drop Representative Seismic

15 Staff Review of Non-Seismic Single Module Sequence Screening

• TR sequence screening of 1E-7 per reactor year CDF would have screened in all the WASH-1400 release sequences

- None of these release sequences had a frequency less than 1E-7 per reactor year

16 Staff Review of Consideration of Multi-Module Impacts for Non-Seismic Hazards

• WASH 1400 did not explicitly model common mode mechanisms such as fire, flood, and tornadoes (which have the potential to impact multiple modules) in the event trees and fault trees

• Appendix IV of WASH 1400: common mode mechanisms were assessed as impacts on the system fault trees

• Core damage sequence screening of 1E-7 per reactor year was also applied to multi-module core damage sequences

- None of WASH 1400 release sequences had a frequency less than 1E-7 per reactor year

17 Staff Review of Screening Threshold for Seismic Hazard in TR, Revision 2

• Proposed threshold was 1E-5 per year initiating event frequency

• Proposed threshold did not provide spectrum of accidents consistent with NUREG-0396

- Negligible beyond design basis seismic risk

- Excluded key characteristics of more severe seismic accidents

- Risk gap in EPZ sizing insights

• Proposed threshold was inconsistent with technical basis in NUREG-0396 and consideration of seismic events in WASH-1400

18 Staff Review of Screening Threshold for Seismic Hazard in TR, Revision 3

• TR identified a proprietary screening threshold for seismic hazard

• Risk gap approach developed by staff for evaluation

- Determine appropriateness of screening threshold for seismic hazard consistent with basis in NUREG-0396 Beyond Design Basis Seismic Risk 0%

100%

1E-5/year initiating event frequency

>2.0g acceleration

19 Staff Review of Screening Threshold for Seismic Hazard in TR, Revision 3 (contd)

• Overview of risk gap approach

- Step 1: Collect ensemble of hazard curves representing different sites within the certified seismic design response spectrum

- Step 2: Identify plant-level fragility and spectral ratios

- Step 3: Convolve hazard curves with plant-level fragility

- Step 4: Calculate simple average absolute and relative risk gap below screening threshold value

20 Staff Review of Screening Threshold for Seismic Hazard in TR, Revision 3 (contd)

• Staff evaluation used risk gap approach with 9 operating reactor sites

- Within NuScale Certified Seismic Design Response Spectra

- Using NuScale high confidence of low probability of failure (HCLPF) plant-level fragility

- Threshold captures 20% - 50% of seismic risk for the NuScale design across majority of 9 sites

21 Staff Review of Screening Threshold for Seismic Hazard in TR, Revision 3 (contd)

Figure 1 in NRC staffs safety evaluation (ML22118A760)

22 Staff Review of Screening Threshold for Seismic Hazard in TR, Revision 3 (contd)

• Achieves consistency with NUREG-0396 and WASH-1400 for seismic events

- Results in a complete spectrum of accidents

- Avoids extremely unlikely seismic accelerations

- Does not penalize NuScale for its risk profile

• Addresses multi-module risk (details in subsequent slide)

• Uses quantified seismic sequences because they can be dominant contributor to plant risk and EPZ sizing

23 Technology-Inclusive Applicability of Risk Gap Approach for Risk-Informed EPZ Sizing

• Provides design-and site-specific screening threshold value(s) consistent with technical basis in NUREG-0396

- Single value across technologies and sites is difficult to defend

• Uses accessible design-specific information

• Compatible with risk metrics other than CDF or LERF

• This approach provides regulatory stability

24 Staff Review of Treatment of Uncertainty in Screening Threshold for Seismic Hazard

• Sufficient to identify new insights

- Addresses uncertainty in context of identification of spectrum of accidents for EPZ sizing

- Addresses potential for cliff edge effect (i.e., large change in EPZ size decision with small change in event screening)

• New sequences included in remaining steps of methodology

25 Staff Review of Consideration of Multi-Module Impacts for Seismic Hazard

• Methodology appropriately identifies and includes multi-module impacts from seismic events

• Considerations in staff evaluation:

- Seismic events are a common-mode initiator

- Screening threshold results in inclusion of accelerations with potential multi-module impacts

- Dominant contributors to NuScales seismic large release frequency impact multiple modules

- Impact of multi-module sequences reflected in source term for those sequences

26 Information at Various Licensing Stages Under 10 CFR 52 Design Certification Combined License Fuel Load Part 52:

EPZ TR:

PRA Based Seismic Margins Analysis (SMA) Allowed Not Applicable PRA Based SMA Allowed Site-Specific Seismic PRA Site-Specific Seismic PRA Site-Specific Seismic PRA

27 Staffs Review of Treatment of Modeling Uncertainty for Seismic and Non-Seismic Hazards and Staff Review

• TR contains broad discussion of consideration of assumptions and sources of uncertainty in the underlying PRA

• Staff determined that consistency with established guidance applied for voluntary risk-informed applications is important

- RG 1.200, Revision 3 and NUREG-1855, Revision 1

• Includes a corresponding Condition of Use

28 Staff Review of Consideration of Defense-In-Depth

• Methodology identifies key plant design and operational characteristics necessary for achieving defense-in-depth

- Includes consideration of severe accident management strategies and diverse and flexible coping strategies

• Consistent with PRA Policy Statement and RG 1.174, Revision 3

• Five levels of defense in INSAG-10 adds confidence in consideration of defense-in-depth

29 Staffs Conditions of Use

• Ensure user addresses key inputs for methodology and staff findings

• Eight (8) conditions of use:

A.

Consistency in use between applicant for operating license application under Part 50 and COL under Part 52 B.

Consistency of PRA technical acceptability with established guidance C.

Treatment of PRA key assumptions and sources of uncertainty consistent with established guidance D.

Limit seismic event screening threshold to sites with GMRS bounded by NuScales CSDRS E.

Limit seismic event screening threshold to NuScales HCLPF plant-level fragility F.

Demonstration of GMRS and HCLPF plant-level fragility at application and prior to fuel-load G.

Monotonic decrease of 200 rem dose exceedance curve H.

Periodic evaluation to ensure (1) E is met AND (2) conclusion on EPZ size remain valid. Periodicity consistent with 10 CFR 50.71(h)(2)

30 Staffs Conclusion

• TR methodology is generally consistent with technical basis of current 10-mile EPZ prescribed in 10 CFR 50.47 (i.e., NUREG-0396)

• Reasonable assurance that methodology is adequate for assessing plume exposure pathway EPZ sizing

• Applicants using the methodology as the technical basis for assessing plume exposure pathway EPZ sizing need to meet :

- Scope of applicability in Section 2.5 of the TR

- Conditions of Use listed in Section 5.0 of staffs SE

31 Staffs Conclusion (contd)

• NuScale EPZ TR is risk-informed and reflective of NUREG-0396 approach

- Uses risk information from design-and site-specific PRAs, with explicit consideration of uncertainty

- Explicitly addresses defense-in-depth

- Staff evaluated impact on safety margins

- Includes performance monitoring (based on corresponding Condition of Use)

32 Caveats on Scope of Staffs Findings

• Only applicable to this TR for EPZ sizing

- Not extended to design and operation for licensing

- Quantitative Health Objectives applicable for design and operation for licensing

• Screening threshold, especially for the seismic hazard, not applicable to PRA development

- Follow applicable endorsed PRA Standard

• Does not define credible events for any hazard

33 Acronyms BDBA/E beyond design basis accident/event CDF core damage frequency CSDRS certified seismic design response spectrum DBA design basis accident EPZ emergency planning zone GMRS ground motion response spectrum HCLPF high confidence of low probability of failure LERF large early release frequency LRF large release frequency LWR light water reactor Non-LWR non-light water reactor ONT other nuclear technology PRA probabilistic risk assessment QHO quantitative health objective RG regulatory guide SMR small modular reactor WASH-1400 Nuclear Regulatory Commission Reactor Safety Study, 1975

34 References 10 CFR 50.47 Emergency Plans 10 CFR Part 50 Appendix E Emergency Planning and Preparedness 10 CFR Part 20 Standards for Protection Against Radiation NUREG-0396, Planning Basis For The Development Of State And Local Government Radiological Emergency Response Plans In Support Of Light Water Nuclear Power Plants (ML051390356)

Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory, 60 FR 42622 SRM-SECY 98-144, White Paper Risk Informed and Performance-Based Regulation (ML003753601)

Regulatory Guide (RG) 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, Revision 3 (ML17317A256)

RG 1.200, Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities, Revision 3 (ML20238B871)