ML21085A490

Risk-informed Resolution of Generic Letter 2004-02

ML21085A490
Person / Time
Site:	Vogtle
Issue date:	05/19/2021
From:	John Lamb Plant Licensing Branch II
To:
Lamb J
References
GL-04-002
Download: ML21085A490 (48)

Text

Vogtle Unit 1 and 2 Risk-informed Resolution of Generic Letter 2004-02 ACRS Subcommittee Meeting May 19, 2021

Presentation Outline

• Background - Generic Safety Issue (GSI) - 191, Generic Letter (GL) 2004-02

• Actions: strainer and reactor vessel

• Vogtle and South Texas Project (STP) approach comparison

• Staff Evaluation of Vogtle GL 2004-02 analysis

- Technical Report

- Limitations and Conditions

- License Amendment Request 2

ECCS Debris Effects Background

• In the early 1990s, NRC identified that sump blockage due to loss-of-coolant accident (LOCA) generated debris could challenge emergency core cooling system (ECCS) and containment spray (CS)

• GSI-191 for PWRs issued in 1996 to address strainer performance

• GL 2004-02 requested plant-specific evaluation of debris to demonstrate compliance with 10 CFR 50.46 3

General Licensee Actions to Address Debris Effects on Long Term Core Cooling (LTCC)

• Installed larger strainers

• Removed unnecessary debris sources

• Removed/replaced some problematic materials, or reinforced with banding and jacketing

• Implemented administrative controls

- Control materials in containment

• Implemented procedural/operational changes

• Improved chemical effects attributes 4

Overall Status of Compliance

• 3 Staff Requirements Memorandum options from SECY-12-0093

- Option 1 - Closeout using existing guidance

- Option 2 - Perform additional testing and analysis, including risk-informed resolution

- Option 3 - Treat strainer deterministically and in-vessel using risk analysis

• 19 units demonstrated compliance using Option 1

• Remainder of plants chose Option 2 5

Option 2 Plant Compliance

• 2A - Deterministic and Transition Break Size

- 29 Units likely to use typical deterministic methods

- 5 Units likely to use NEI 04-07, Section 6 evaluation (TBS)

- Staff currently reviewing those in-house and waiting on the remainder

• 2B - Full Risk-Informed

- STP has demonstrated compliance

- Vogtle review discussed in this meeting

- Wolf Creek and Point Beach have not yet submitted a LAR. A LAR was recently received from Callaway.

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Types of strainers

• Uniform Flow PCI stack disk strainer

• Non-Uniform Flow AECL* & GE stacked discs/plates Enercon Top hat CCI pocket strainers 7

In-Vessel Downstream Effects Status (IVDE)

• 19 units resolved issue based on low fiber loading at the core

- Basis is WCAP-16793, Rev. 2 and staff SE

• Two STP units used plant-specific thermal-hydraulic (TH) analysis

• Remainder of licensees using recent staff guidance

• Staff review guidance (ML19228A011) based on

- Industry testing and analysis

- NRC analysis

- Safety Significance

- Discussed with ACRS in April and September 2019 8

IVDE - WCAP-17788 Summary

• Plant-specific evaluations with event timing considerations

• Higher debris limits

• Address NRC staff and ACRS comments from WCAP-16793

• Effects of debris, boric acid precipitation (BAP)

• Performed fuel assembly (FA) testing, TH analysis, chemical effects (autoclave/filtration) testing 9

Methodology Comparison - Vogtle and South Texas Project (STP)

• RoverD

• CAD for debris generation

• Casa Grande vs. NARWHAL

• Vogtle used more refined headloss model

• Very small change in risk (RG 1.174)

• Difference in break evaluation 360 vs 8

• In-vessel - STP used TH, Vogtle used staff guidance

• Deterministic staff approved methods for some aspects 10

Risk over Deterministic Methodology (RoverD)

Deterministic Test Data Debris Limit Established Risk Informed Analysis Calculate Debris for Individual Breaks R

over D

Scenario Deterministically Debris Acceptable No Calculated Testing Limit Yes 11

Evaluation of Technical Report

• Most review of technical areas was completed during review of the technical report (Staff Evaluation ML19120A469)

• Open technical issues were identified as limitations and conditions (L&Cs)

• Staff evaluated the 5 key principles of risk-informed regulation

• Used approved guidance to evaluate risk and deterministic aspects 12

Evaluation of LAR

• More focused on evaluation of licensing issues

- TS changes

- Exemption request

- L&Cs from the staff evaluation of the licensees plant-specific Technical Report

• The staff evaluation of the Technical Report was for Vogtle-specific only and not for generic use

• Staff evaluated the 9 limitations and conditions in the technical report 13

References

• NEI 04-07 and Staff SE

• Staff guidance on coatings, chemical effects, and headloss

• RG 1.174

• RG 1.82

• Draft RG 1.229

• NRC staff review of STP Risk-Informed LAR

• WCAP-16530 Chemical Effects

• WCAP-16406 Ex-vessel Downstream Effects

• In-vessel staff review guidance 14

Staff Methodology Five Key Principles of Risk-Informed Regulation

2. Change is consistent with defense-in-depth philosophy.

1. The proposed licensing basis change meets the current 3. Maintain sufficient regulations or an safety margins.

exemption is requested. Integrated Decisionmaking

5. Impact of proposed

4. Proposed increases change should be in risk should be small monitored using and consistent with the performance Commissions Safety measurement Goal Policy Statement.

strategies.

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Principle 1 Meet Regulations or Request Exemption

• 10 CFR 50.46c rulemaking status

• Exemptions requested from use of deterministic analysis method

- Acceptance Criteria for ECCS

- Did not request exemption to GDC like STP

• GDC 35, 38, 41 16

Principles 2 and 3 Safety Margins and Defense-In-Depth

• Licensee met guidance of RG 1.174 and provided significant safety margins and Defense-in-depth (DiD)

• Safety margins include construction and inspection per industry codes and the use of licensing basis values when assigning strainer failure criteria.

• DiD includes actions identified that are taken in response to the loss of the normal ECCS function. DiD also includes verification that balance is maintained among prevention and mitigation, redundancy is maintained, barrier independence is maintained, etc.

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Principle 4 Deterministic Inputs to Risk Analysis

• Debris Source Term

- Used NRC approved guidance for all areas

- Calculations performed in BADGER

- Differences from typical deterministic evaluations

• For partial breaks, all weld locations evaluated for multiple orientations instead of focusing on the limiting large break

- 8 orientations vs. 360 for STP

• Double-ended guillotine break (DEGB) source term uses the same method as typical deterministic calculations

- Source term and transport calculated for each break and compared against tested amount

- The most conservative orientation was selected for partial breaks at each weld location

- Assumptions and calculations verified independently by SwRI

- Extensive Review of BADGER/CAD debris generation software 18

31 inch 29 inch 20 inch 12 inch 6 inch 19

Minimum Average Maximum Main Loop, 56 Welds 10-16 inch, 39 welds 6-8 inch, 77 welds 2.5-4 inch, 122 welds

< /=2 inch, 119 welds 20

Principle 4 Deterministic Inputs to Risk Analysis

• Extensive review of NARWHAL that implemented transport, headloss, failure criteria, conditional failure probability, and sensitivity calculations

• Debris Transport - Strainer Evaluation

- Used NRC approved guidance implemented via NARWHAL

- Correction of coatings transport was the subject of an L&C that resulted in small changes in overall risk. Correction was documented in the LAR.

• Debris Transport - In-Vessel Effects

- Fiber penetration determined via testing

- Used conservative bypass values from testing

- Calculated fiber amounts arriving at the core for hot-leg breaks using conservative plant state (pump combinations)

- Evaluated fiber amount reaching the core against limits using NRC staff guidance 21

Principle 4 Deterministic Inputs to Risk Analysis

• Impact of Debris - Strainer

- Strainer evaluated at tested debris loads/dPs for effects on net positive suction head, structural, deaeration, partial submergence failure, vortexing, and flashing

• If a transported debris amount of any type exceeded the tested amount, the scenario was assumed to fail

- Testing and evaluations were performed using staff approved guidance

- Testing showed that increasing fiber amounts result in greater head losses

• Majority of breaks bounded by test results

• Some breaks generate much larger debris amounts 22

Principle 4 Deterministic Inputs to Risk Analysis

• Impact of Debris - In-vessel

- This area was not addressed in the technical report and was the subject of an L&C. It was addressed in the LAR.

- Debris amounts low enough to permit adequate cooling flow to the core based on WCAP-17788 findings and NRC Staff Review Guidance

- Evaluated core inlet fiber amount that was > WCAP limit, but < WCAP total in-vessel limit for Vogtle RCS design

• Non-uniform debris distribution at core inlet

• HLSO time < 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, Chemical effects time > 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />

• Sump switchover time is 32 minutes at Vogtle vs. 20 in the WCAP

- Other WCAP parameters bound Vogtle values 23

Summary of Systematic Risk Assessment

• Used existing acceptance guidelines in RG 1.174 to address Principle 4

• Vogtle determined only LBLOCAs (> 12 inches) contribute to strainer failure

• CDF (LERF) = CDF with debris - CDF no debris, compared to RG 1.174, Region III acceptance guideline 24

Summary of Systematic Risk Assessment

• Sensitivity and Uncertainty Analyses (focused on CDF)

• Performed consistent with NUREG-1855 guidance

• Parametric sensitivity and uncertainty analysis

• All cases fall within Region III

• Model uncertainty analysis

• Key assumptions and sources of uncertainty

• Strainer testing used conservative NRC staff guidance

• Completeness uncertainty analysis

• Results fall in Region III acceptance guidelines in RG 1.174, for very small risk increase

• CDF < 10-6

• LERF < 10-7

• Corrected debris loading in LAR does not change conclusions 25

Staff Review of Systematic Risk Assessment

• Acceptability of base PRA Model

• Scope, level of detail, technical adequacy

• Acceptability of approach to focus on ISI welds in un-isolable portion of Class-I pressure boundary piping and SSBIs

• Acceptability of Vogtles focus on high likelihood configurations of ECCS pumps:

• Accidents that need recirc through ECCS strainers

• High likelihood equipment configurations

• Low likelihood configuration can be bounded

• Acceptability of using CFP from NARWHAL for breaks that impact strainer (CFP = 0, for breaks that didnt fail the strainer) 26

Staff Review of Systematic Risk Assessment

• Vogtles disposition of key assumptions and sources of uncertainty

• Geometric and arithmetic mean aggregation schemes

• LOCA frequency allocation (top down and hybrid)

• Partial (continuum) and complete (DEGB) break consideration

• Discretization of LBLOCA frequency

• NRC Staff Confirmatory Calculations

• Minimum transition break size assumed to be 12 inches

• Conservatively assumed failure for breaks larger than minimum break (CFP = 1)

• Used Vogtles high likelihood configuration

• Results do not challenge Region III of the RG 1.174 27

Summary of Systematic Risk Assessment

• Results fall in Region III acceptance guidelines in RG 1.174, for very small risk increase

• CDF < 10-6

• LERF < 10-7 Maximum of staffs confirmatory calculations Vogtles base assessment 28

Staff Review of Systematic Risk Assessment CONCLUSION:

• The licensee used a PRA of the appropriate scope, level of detail, and technical elements and plant representation.

• The risk-informed approach used by the licensee to address the effects of debris on LTCC is acceptable.

• Alternative assumptions were considered as sensitivities for each key assumption employing non-consensus approaches.

• The increase in risk meets the risk acceptance guidelines in RG 1.174, Revision 2.

• Principle 4 of integrated risk-informed decision-making is addressed.

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Principle 5

• Performance monitoring addressed in LAR review

• Vogtle identified existing programs and processes

- Ensures key inputs and assumptions remain valid

- Ensures corrective action, if necessary

• Staff review determined Principle 5 of monitoring using performance measurement strategies is addressed.

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Review of LAR

• Evaluated TS changes

- New Sump TS per TSTF-567

• Reviewed FSAR changes

• Evaluated exemption request

• Evaluated each L&C from the staff evaluation of the licensees plant-specific Technical Report 31

Overall Summary

• Vogtle acceptably evaluated the impact of debris.

• Vogtle appropriately considered both risk and deterministic aspects in the submittal.

• Most break scenarios are addressed using conservative deterministic methods.

• Vogtles LTCC evaluation method and simulations are conservative and meet acceptance criteria.

• Vogtles debris analyses meet the key principles of risk-informed regulation.

• Vogtles PRA results show that the change in risk is very small.

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Questions?

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Primary References

• (1) ML18193B162 - SNC Technical Report Submittal dated July 10, 2018.

• (2) ML19120A469 - NRC Staff Evaluation of the Technical Report dated September 30, 2019.

• ML21068A109 - Amendment Package for Vogtle GSI-191.

• ML21071A050 - Exemption Package for Vogtle GSI-191.

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5 Principles - RG 1.174

• (1) The proposed licensing basis change meets the current regulations unless it is explicitly related to a requested exemption (i.e., a specific exemption under 10 CFR 50.12).

• (2) The proposed licensing basis change is consistent with the defense-in-depth philosophy.

• (3) The proposed licensing basis change maintains sufficient safety margins.

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5 Principles - RG 1.174

• (4) When proposed licensing basis changes result in an increase in risk, the increases should be small and consistent with the intent of the Commissions policy statement on safety goals for the operations of nuclear power plants.

• (5) The impact of the proposed licensing basis change should be monitored using performance measurement strategies.

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Limitations & Conditions

• (1) The applicability of the NRC's acceptance is limited to the structures, systems, and components; plant configurations; and operations described in Enclosures 2, 3, and 4 of SNC's letter dated July 10, 2018 and the strainer design described in the Section entitled, "16-Disk ECCS Suction Strainer Summary," of Enclosure 2.

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Limitations & Conditions

• (2) The applicability of the NRC's acceptance is limited to the Vogtle assessment of risk attributable to debris described in Enclosures 1 and 3 of SNC's letter dated July 10, 2018.

• (3) Describe in-vessel analysis, establish in-vessel acceptance criteria, and demonstrate the criteria are met.

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Limitations & Conditions

• (4) Address Key Principle 1 (i.e., the proposed licensing basis change meets the current regulations unless it is explicitly related to a requested exemption) and Key Principle 5 (i.e.,

the impact of the proposed licensing basis change should be monitored using performance measurement strategies) in RG 1.17 4, Revision 3.

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Limitations & Conditions

• (5) Identify key elements of the risk-informed analysis (e.g., methods, approaches, and data) that will be described in the Vogtle UFSAR.

• (6) Identify key elements of the risk-informed analysis and corresponding methods, approaches, and data that, if changed, would constitute a departure from the method used in the safety analysis as defined by 10 CFR 50.59.

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Limitations & Conditions

• (7) Identify the relevant elements of the risk-informed assessment that may need to be periodically updated. The licensee must describe the program or controls that will be used to ensure relevant elements of the risk-informed assessment are periodically updated.

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Limitations & Conditions

• (8) Describe a reporting and corrective action strategy for addressing situations in which an update to the risk-informed assessment reveals that the acceptance guidelines described in Section 2.4 of RG 1.17 4, Revision 3, have been exceeded.

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Limitations & Conditions

• (9) Correct the error concerning the evaluation of transported coatings debris loads described in SNC's letter dated December 4, 2018. Specifically, provide corrected coating debris volumes and describe how coating debris loads on the strainers are determined.

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Limitations & Conditions

• (9a) Verify that the use of the corrected coating debris volumes has a limited impact on strainer head loss and the head loss is acceptable. Also, the licensee must describe the method of verification.

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Limitations & Conditions

• (9b) Verify that the use of the corrected coating debris volumes has a limited impact on CDF and does not result in exceeding the acceptance guidelines for very small change in risk, as described in Section 2.4 of RG 1.17 4, Revision 3. Also, the licensee must describe the method of verification.

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Limitations & Conditions

• Modify RHR strainer height and maintain plant configuration consistent with the Technical Report

• Maintain evaluation consistent with TR

• Evaluate in-vessel effects

• Address Key Principle 1 - change meets regulations

• Address Key Principle 5 - monitoring of impact of change

• Define key elements of the analysis in the FSAR

• Define elements of the analysis that would require NRC review prior to change

• Approach to ensure periodic update of risk-informed assessment

• Reporting and corrective action strategy in case the acceptance criteria of RG 1.174 are exceeded

• Correct coatings transport error and evaluate effect on risk 46

Limitations and Conditions LAR RESOLUTION OF LIMITATION AND CONDITIONS:

• L&C 4b: Performance Monitoring

• Staff found Vogtle identified a set of existing programs and procedures to monitor the performance of the sump strainers in containment and assess the impact on the risk-informed assessment

• L&C 8: Condition Reporting and Corrective Action

• Staff found Vogtles proposed to use appropriate plant programs and procedures for its corrective action strategy

• Staff found Vogtles reporting strategy is consistent with the existing requirements in 10 CFR 50.72 and 10 CFR 50.73, as applicable

• L&C 9b: Debris Loading Error Results

• Staff found CDF and LERF increased slightly but remained well within the RG 1.174 range for very small changes (Region III) 47

Plant LOCA Response Spray nozzles Fuel Assemblies Tank RWST Heat exchanger Valve Strainers CSS SIS Sump Safety Injection Containment Spray System (CSS)

(SIS) 48