ML19303A025

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Risk-Informed GSI-191 NRC Public Meeting
ML19303A025
Person / Time
Site: Vogtle  Southern Nuclear icon.png
Issue date: 11/04/2019
From:
Southern Nuclear Operating Co
To:
Division of Operating Reactor Licensing
Lamb J, 415-3100
References
Download: ML19303A025 (40)
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Text

Vogtle Risk-Informed GSI-191 NRC Public Meeting November 4, 2019

Meeting Purpose and Agenda Meeting purpose:

  • Identify areas needing additional documentation or justification This meeting will cover the following topics:
  • Resolution of In-Vessel Issue
  • Refueling Water Storage Tank (RWST) Surveillance Requirement (SR) Change
  • Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE
  • Debris Limits and How to Address Future Operability Issues
  • Content of the Proposed Licensing Submittal 2

Resolution of In-Vessel Issue Resolution of In-Vessel Issue

  • Vogtle is bounded by the assumed values in WCAP-17788 for all parameters In-Vessel Effects WCAP-17788 Vogtle Value Comments Parameter Value Maximum Core Inlet Fiber [Proprietary] 46.8 g/FA Max core inlet fiber load for the bounding HLB at tblock, taking for Hot Leg Break (HLB) credit for Alternate Flow Paths (AFPs), is less than the limit.

Maximum Total Core Fiber [Proprietary] 90.6 g/FA Maximum total in-core fiber load for the bounding HLB is for HLB less than the limit.

Minimum Sump 20 minutes 31.9 minutes Later switchover time results in lower decay heat load and, Switchover Time as discussed in the NRC review guidance, the potential for a debris induced core uncover heat-up is greatly reduced.

Minimum Chemical 2.4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> (tblock) 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Complete core blockage due to chemical precipitation would Precipitation Time occur much later than assumed.

Maximum Hot Leg 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> (tchem) 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> Hot leg switchover occurs well before complete core Switchover Time blockage is assumed.

Maximum Rated Thermal 3,658 MWt 3,625.6 MWt Lower thermal power results in a lower decay heat load.

Power Maximum AFP Resistance [Proprietary] [Proprietary] Vogtle has lower AFP resistance than the assumed value, which increases the effectiveness of the AFPs.

Minimum emergency core 8 gpm/FA 15.5 gpm/FA NRC TER indicates that maximum resistance is from the cooling system (ECCS) lowest flow rate. Vogtle flow rate is within the range of flow flow per FA rates evaluated in WCAP-17788. 4

Resolution of the In-Vessel Issue

  • If no alternate flow path (AFP) credit is taken, fiber accumulation at the core inlet (90.6 g/FA) exceeds the WCAP-17788 core inlet limit for the bounding hot leg break
  • NRC staff review guidance states that licensees may justify sufficient long-term core cooling for debris loads that exceed the core inlet limit based on the formation of a non-uniform debris bed at the reactor core inlet 5

Summary of Core Inlet Fiber Accumulation Methodology

  • Generated fiber debris quantities calculated for range of potential break sizes, locations, and orientations using BADGER software
  • Fiber debris size distribution based on the guidance in NEI 04-07 and the ENERCON centroid methodology
  • Debris transport to ECCS and containment spray (CS) strainers based on methodology in NEI 04-07 for refined transport calculations including blowdown, RHR B washdown, pool fill, recirculation, and erosion
  • Time-dependent debris arrival at ECCS CS B and CS strainers calculated based on pool volume and pump flow rates CS A RHR A 6

Summary of Core Inlet Fiber Accumulation Methodology

  • Time-dependent fiber penetration through ECCS and CS strainers calculated based on Vogtle-specific fiber penetration testing and strainer fiber load at the time when incremental fiber arrives
  • Fiber accumulation at the core inlet and within the core calculated based on break side (HLB vs. CLB),

event timing (before or after switchover to hot leg recirculation), and WCAP-17788 methodology

  • Core inlet fiber load for bounding HLB:
  • 46.8 g/FA with AFP credit
  • 90.6 g/FA without AFP credit
  • Methodology for debris transport to reactor vessel was reviewed and accepted in the NRC SE
  • Debris accumulation at the core inlet was not reviewed 7

Excerpt from NRC Staff Review Guidance for In-Vessel Effects Section 6.5 of Volume 1 of WCAP-17788 has a methodology for calculating the amount of fibrous debris arriving at the core following a HLB. This method includes credit for a debris split between the core inlet and AFPs so that some debris is assumed to enter the heated core instead of collecting at the core inlet. The debris split assumes that the debris deposition at the core inlet will be uniform. The NRC staff expects that the debris bed at the core inlet will not be uniform due to the variations in flow velocities at the core inlet.

Therefore, it will take more debris than determined by WCAP-17788 to result in activation of the AFPs and redirection of some flow and debris to the heated core. Other aspects of the Volume 1 method reflect the expected physical behaviors following a LOCA. Because of the non-physical nature of the assumption that a uniform debris bed will form at the core inlet, credit for debris bypassing the core inlet and entering the heated core should not be used.

However, the NRC staff recognizes that AFPs will provide flow to the core if the resistance to flow at the core inlet becomes high enough. It is not the intent of the staff to prevent credit for debris diversion when the debris predicted to be diverted will have no negative effect on flow into the core. Therefore, although the staff does not agree that the core split can be used to calculate an amount of debris that will bypass the core inlet via the AFPs, licensees may justify that a non-uniform debris bed will form at the core inlet allowing adequate flow to assure LTCC, even though the average debris load per FA metric is exceeded.

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Conclusion for In-Vessel Effects

  • Uniform debris loading on the core inlet is not realistic
  • Non-uniform debris deposition at the core inlet results in lower head loss, which means that the core inlet would be able to continue providing sufficient flow for decay heat removal
  • If the flow resistance became high enough to block flow through the core inlet, sufficient flow would pass through the AFPs
  • Vogtle has lower AFP resistance than the value assumed in WCAP-17788, which increases the effectiveness of the AFPs
  • Therefore, the core inlet fiber limit is not a critical parameter, and the total core fiber limit in WCAP-17788 can be used to justify sufficient margin for long term core cooling 9

RWST Surveillance Requirement Change

RWST Surveillance Requirement Change

  • Proposed Tech Spec change to allow 7 days following Mode 4 entry to meet RWST SR 3.5.4.2 requirement of 686,000 gallons
  • Requires 672,193 gallons in RWST to use this allowance (2% of span reduction)

SR 3.5.4.2 ---------------------------NOTE------------------------------ In accordance with Commencing startup from 1R22 and 2R21. Not the Surveillance required to be met until 7 days after entering Frequency Control MODE 4 if RWST borated water volume is Program 672,193 gallons.

Verify RWST borated water volume is 686,000 gallons.

  • This change allows use of RCS borated water during heat-up to be makeup source for RWST
  • Currently required to make up volume to RWST to meet Mode 4 requirements, then process excess RCS volume to rad waste during heat-up 11

RWST Surveillance Requirement Change

  • Risk associated with lower RWST water level was evaluated in a sensitivity study
  • Conditional failure probability (CFP) values calculated in NARWHAL
  • CDF and LERF calculated with PRA model Base Case Reduced Equipment Configuration CFP RWST CFP No Equipment Failures 0.0118 0.0119 1 RHR Pump Failure 0.0679 0.0680 1 RHR Pump + 1 CS Pump Failure 0.1017 0.1017 1 CS Pump Failure 0.0139 0.0141 2 CS Pump Failures 0.0177 0.0177 CDF (yr-1) 2.47E-08 2.49E-08 LERF (yr-1) 3.53E-11 3.56E-11 12

RWST Surveillance Requirement Change

  • Sensitivity study evaluated the equivalent of a permanently lower RWST level
  • Tech Spec change would only allow lower water level for 7 days following Mode 4 entry
  • The incremental core damage probability (risk increase x t) is negligibly small:

ICDP = (2.49 x 10 2.47 x 10 )* = 4 x 10 13

Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE

Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE

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.
  • No action required for this limitation
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.
  • No action required for this limitation 15

Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE

3. Describe in-vessel analysis, establish in-vessel acceptance criteria, and demonstrate the criteria are met.
  • SNC is proposing to use the WCAP-17788 methodology and acceptance criteria with justification for the core inlet fiber quantity as described in this presentation
  • The description of the in-vessel analysis will be provided in the next submittal
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 1.174, Revision 3.
  • The implementation and monitoring program will be described in the LAR submittal 16

Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE

5. Identify key elements of the risk-informed analysis (e.g., methods, approaches, and data) that will be described in the Vogtle UFSAR.
  • Changes to the UFSAR will be described in the LAR submittal
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.
  • Key elements of the risk-informed analysis that would impact the safety analysis will be described in the LAR submittal 17

Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE

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.
  • Enclosure 1, Section 8.0 of the previous submittal (ML18193B165) provided a high level description of the relevant elements of the risk-informed assessment that may need to be periodically updated
  • Clarification on the level of detail expected by the NRC staff would be helpful
  • Risk associated with the effects of debris was calculated using a GSI-191 PRA model based on the Revision 5 Internal Events Model of Record (MOR) with minor modifications to include basic events for strainer and core blockage failures for different equipment configurations
  • The current MOR is a common backbone PRA that includes internal events, internal flood, fire, and seismic hazards
  • GSI-191 basic events are incorporated in the model, but are not currently active (only activated as needed for risk quantification using a flag file)
  • Permanently incorporating the GSI-191 basic events could be done relatively easily, but SNC would like clarification on whether the NRC review and acceptance (documented in the SE) is sufficient basis for modifying the MOR without an additional peer review 18

Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE

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 1.174, Revision 3, have been exceeded.
  • The operability evaluation process described in this presentation will be documented in the LAR submittal 19

Conditions and Limitations from the Vogtle GL 2004-02 Submittal SE

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. In addition:
a. 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.
b. 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 1.174, Revision 3.

Also, the licensee must describe the method of verification.

  • The correction to the coatings error, updated results, and methods of verification will be described in the LAR submittal 20

Debris Limits and How to Address Future Operability Issues

Debris Limits and How to Address Future Operability Issues

  • With approval of the risk-informed GSI-191 license amendment request, the new design basis for Vogtle containment accident generated and transported debris will be that the risk increase due to containment sump failures is within RG 1.174 Region III (i.e., a CDF less than 1E-06 yr-1)
  • The current NRC guidance does not allow the use of risk to address operability issues (even for a plant with a risk-informed design basis)
  • To address this, debris limits must be defined that will ensure that the plant stays within its design basis and can be used for operability determinations
  • Based on the NUREG-1829 LOCA frequencies, failures for all breaks larger than 8.5 inches would result a CDF of approximately 1E-06 yr-1
  • Strainer debris limits have been defined based on strainer testing
  • Strainer debris accumulation is dependent on the number of strainers in operation:
  • Single RHR pump failures would result in up to twice as much debris accumulating on the operating RHR strainer 22

Debris Limits and How to Address Future Operability Issues

  • Probability of a single RHR pump failure (due to non-debris related causes) is less than 2%
  • Conservatively assume that single RHR train operation will be available 5% of the time and two train operation will be available 95% of the time 23

Debris Limits and How to Address Future Operability Issues

  • Risk will still be in Region III if all breaks larger than 10 inches fail for two train operation and all breaks larger than 6 inches fail for single train operation CDF = 0.95
  • 6.56 x 10 0.05
  • 2.85 x 10 = 7.7 x 10 24

Debris Limits and How to Address Future Operability Issues

  • Must consider breaks 10 inches for 2 RHR train operation
  • Must consider breaks 6 inches for 1 RHR train operation 25

Debris Limits and How to Address Future Operability Issues

  • Tested fiber debris limit of 107.7 ft3 gives at least 90.4 ft3 of margin 26

Debris Limits and How to Address Future Operability Issues

  • Tested particulate debris limit of 17.8 ft3 gives at least 1.6 ft3 of margin 27

Debris Limits and How to Address Future Operability Issues Acceptable Debris Type Limit Fiber Debris 95.2 ft3*

Qualified and Unqualified Coatings 16.8 ft3 Fire Barrier 284.7 lbm Latent Debris 200 lbm Miscellaneous Debris (Tags, Labels, etc.) 50 ft2 Submerged Aluminum Surface Area 926.6 ft2 Unsubmerged Aluminum Surface Area 348.4 ft2

  • Based on an equivalent test quantity of 107.7 ft3 fiber minus 12.5 ft3 of latent fiber, which has a separate limit.

Based on an equivalent test quantity of 17.8 ft3 particulate minus 1.0 ft3 of latent dirt/dust, which has a separate limit.

Using the NRC accepted split of 85% dirt/dust and 15% fiber, along with the accepted fiber density of 2.4 lbm/ft3 and dirt/dust density of 169 lbm/ft3, this is equivalent to 12.5 ft3 fiber and 1.0 ft3 dirt/dust.

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Debris Limits and How to Address Future Operability Issues Identify unanalyzed debris source Estimate additional Estimate additional debris that could debris that could be generated for be generated for breaks 10 inches breaks 6 inches Compare against available margin in design basis calculation(s)

Remove debris Are Debris No Sump is source during next Limits Operable outage or update Exceeded?

margin calculation Yes Next Slide 29

Debris Limits and How to Address Future Operability Issues Previous Slide Enter TS 3.6.9 Sump is Condition A: Restore Inoperable sump to operable status within 90 days Re-quantify risk associated with GSI-191 failures Remove debris Is CDF Yes source during next less than outage or modify 1.0E-06?

design basis analysis No Shut down to remove or mitigate debris source 30

Debris Limits and How to Address Future Operability Issues

  • Other operability issues related to GSI-191 could be addressed in a similar manner by evaluating breaks up to 10 inches for two train operation and 6 inches for single train operation
  • This could include unanticipated changes for a range of different parameters, e.g.:
  • Pump flow rates
  • Pool volume/level
  • Containment temperature
  • Sump pool temperature

Content of the Proposed License Submittal

Content of the Proposed License Submittal

  • Proposed LAR submittal will include three enclosures (order of enclosures may change):
  • Enclosure 1: Request for Exemptions
  • Enclosure 2: License Amendment Request
  • The requirements of 10 CFR 50.12 will be addressed for each requested exemption
  • Environmental considerations will be addressed for each requested exemption 33

Content of the Proposed License Submittal

  • Enclosure 2 will provide a proposed amendment to revise Vogtles licensing basis to (a) allow the use of a risk-informed approach to address safety issues discussed in GSI-191 and GL 2004-02, (b) adopt TSTF-567 for the containment sump, and (c) revise SR 3.5.4.2 for the RWST level following Mode 4 entry
  • UFSAR changes will be provided
  • Tech Spec changes will be provided
  • The implementation and monitoring program will be described along with the change control and reporting process for key elements of the risk-informed methodology
  • The approach for addressing operability issues will be described
  • Enclosure 3 will update the GL 2004-02 response to include the following information:
  • Correction of the coatings error identified in the previous submittal (ML18193B165) and update to the risk quantification results
  • Description of the approach used to address in-vessel effects 34

Backup Slides Incorporation of GSI-191 Failures in PRA Model of Record 37

Incorporation of GSI-191 Failures in PRA Model of Record 38

Incorporation of GSI-191 Failures in PRA Model of Record 39

Incorporation of GSI-191 Failures in PRA Model of Record 40

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