ML23076A282

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Licensee Presentation on Defense-in-Depth Process
ML23076A282
Person / Time
Site: Limerick  Constellation icon.png
Issue date: 02/23/2023
From:
PWR Owners Group
To: Rhoades D
Constellation Energy Generation
Sreenivas V, NRR/DORL/LPLI, 415-2597
References
EPID L-2021-LLA-0042
Download: ML23076A282 (52)
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Text

Global Expertise

Agenda Alternate Defense-in-Depth and PWROG-20015-NP

  • Objective of Presentation
  • Discuss each of the NRC concerns on the process that have been provided to Constellation
  • Questions
  • Discuss methodology, insights, and bases of the alternate defense-in-depth approach and why we believe it is an improvement to the defense-in-depth process.

Objective of Presentation Alternate Defense-in-Depth and PWROG-20015-NP

  • Cutset screened in means that the associated SSCs are candidate HSS for the alternate core damage defense-in-depth assessment for this cutset.
  • Cutset screened out means that the associated SSCs are candidate LSS for the alternate core damage defense-in-depth assessment for this cutset.

Definition Alternate Defense-in-Depth and PWROG-20015-NP

  • Firsta high level overview of the comparison between the two approaches.
  • Note that there are other portions of the analysis beyond core damage defense-in-depth, including (not comprehensive list):
  • Review of Risk Information (now required to be completed by engineering team with this alternate process)
  • Review Defense-in-Depth Implications (now required to be completed by engineering team with this alternate process)
  • Containment defense-in-depth
  • Quantitative PRA assessments
  • Pressure boundary assessments
  • No impact on how the levels of defense-in-depth are achieved from design perspective.
  • Neither the NEI 00-04 or alternate categorization process changes the design requirements so no impact on defense-in-depth from a design perspective.

Comparison Between Processes Alternate Defense-in-Depth and PWROG-20015-NP

  • Identify the design basis events for the function required.

Comparison Between Processes Alternate Defense-in-Depth and PWROG-20015-NP

  • Alternate Process
  • The PRA model identifies design basis and beyond design basis events to be modeled consistent with the ASME/ANS PRA Standard along with being peer reviewed.
  • Initial plant-wide cutset review that is further examined at the system categorization level (e.g.,

qualitative considerations).

  • Identify other systems and trains that can support the function /

provide alternate success path to avoid core damage.

  • NEI 00-04, Section 6.1 core damage defense-in-depth discusses internally initiated design basis events in the safety analysis report and level of defense-in-depth available based on the success criteria in the PRA.

Comparison Between Processes Alternate Defense-in-Depth and PWROG-20015-NP

  • Alternate Process
  • Uses the core damage cutsets and risk insights to identify levels of defense-in-depth.
  • Prescriptive process to identify levels of defense-in-depth.
  • Allows credit for redundancy within system in specific scenarios.
  • Takeaway: New process considers more failure pathways and is a robust, repeatable process.
  • Uses a table to identify candidate HSS / candidate LSS determination based on frequency and level of defense-in-depth.
  • Can credit candidate LSS SSCs outside of the system being categorized for defense-in-depth.

Comparison Between Processes Alternate Defense-in-Depth and PWROG-20015-NP

  • Alternate Process
  • Cutset screened in as candidate HSS or screened out as candidate LSS based on criteria:
  • IEF 1E-03/yr two basic events or fewer in cutset results in screened in cutset / candidate HSS components
  • 1E-03/yr > IEF 1E-04/yr one basic event or fewer in cutset results in screened in cutset /

candidate HSS components

  • 1E-04/yr > IEF screened out cutset / candidate LSS components
  • Alternate Process Initiating Event Frequency (IEF) 3 or more basic events in cutset 2 basic events in cutset 1 or fewer basic events in cutset IEF 1E-03/yr Candidate LSS Candidate HSS Candidate HSS 1E-03/yr > IEF 1E-04/yr Candidate LSS Candidate LSS Candidate HSS 1E-04/yr > IEF Candidate LSS Candidate LSS Candidate LSS Basic events within a cutset are examined if they should be counted towards level of defense-in-depth. More information is provided in PWROG-20015-NP, Revision 3.

Only CCF basic events with 4+ components failing are screened out / candidate LSS due to redundancy. More information provided later in this presentation.

  • Alternate Process Three basic events would require 3 diverse failures. This could include 3 CCF*

basic events which is similar to 2 redundant systems outside of the system assessed in NEI 00-04 core damage DID.

  • (CCF of 4 or more discussed later)

Initiating Event Frequency (IEF) 3 or more basic events in cutset 2 basic events in cutset 1 or fewer basic events in cutset IEF 1E-03/yr Candidate LSS Candidate HSS Candidate HSS 1E-03/yr > IEF 1E-04/yr Candidate LSS Candidate LSS Candidate HSS 1E-04/yr > IEF Candidate LSS Candidate LSS Candidate LSS Basic events within a cutset are examined if they should be counted towards level of defense-in-depth. More information is provided in PWROG-20015-NP, Revision 3.

Only CCF basic events with 4+ components failing are screened out / candidate LSS due to redundancy. More information provided later in this presentation.

  • Alternate Process Two basic events would require 2 diverse failures. This could include 2 CCF* basic events which is similar to 1 redundant automatic system outside of the system assessed in NEI 00-04 core damage DID.
  • (CCF of 4 or more discussed later)

Initiating Event Frequency (IEF) 3 or more basic events in cutset 2 basic events in cutset 1 or fewer basic events in cutset IEF 1E-03/yr Candidate LSS Candidate HSS Candidate HSS 1E-03/yr > IEF 1E-04/yr Candidate LSS Candidate LSS Candidate HSS 1E-04/yr > IEF Candidate LSS Candidate LSS Candidate LSS Basic events within a cutset are examined if they should be counted towards level of defense-in-depth. More information is provided in PWROG-20015-NP, Revision 3.

Only CCF basic events with 4+ components failing are screened out / candidate LSS due to redundancy. More information provided later in this presentation.

  • Alternate Process Only becomes candidate LSS once the threshold is below 1E-03/yr for initiating event frequency, similar to the 1 redundant automatic system evaluation in NEI 00-04.

Initiating Event Frequency (IEF) 3 or more basic events in cutset 2 basic events in cutset 1 or fewer basic events in cutset IEF 1E-03/yr Candidate LSS Candidate HSS Candidate HSS 1E-03/yr > IEF 1E-04/yr Candidate LSS Candidate LSS Candidate HSS 1E-04/yr > IEF Candidate LSS Candidate LSS Candidate LSS Basic events within a cutset are examined if they should be counted towards level of defense-in-depth. More information is provided in PWROG-20015-NP, Revision 3.

Only CCF basic events with 4+ components failing are screened out / candidate LSS due to redundancy. More information provided later in this presentation.

  • Alternate Process This scenario is screened as candidate HSS if one basic event or fewer can result in core damage. IEF below 1E-04/yr are screened from analysis due to low frequency of occurrence (other portions of 50.69 categorization, for example review of defense-in-depth implications, are still evaluated).

Initiating Event Frequency (IEF) 3 or more basic events in cutset 2 basic events in cutset 1 or fewer basic events in cutset IEF 1E-03/yr Candidate LSS Candidate HSS Candidate HSS 1E-03/yr > IEF 1E-04/yr Candidate LSS Candidate LSS Candidate HSS 1E-04/yr > IEF Candidate LSS Candidate LSS Candidate LSS Basic events within a cutset are examined if they should be counted towards level of defense-in-depth. More information is provided in PWROG-20015-NP, Revision 3.

Only CCF basic events with 4+ components failing are screened out / candidate LSS due to redundancy. More information provided later in this presentation.

  • Summary
  • The tables show similarities with how both processes identify candidate HSS /

candidate LSS.

  • Have taken into account realistic redundancy shown within the PRA in the alternate defense-in-depth process.
  • The alternate defense-in-depth process continues to use the PRA structure to support evaluation of defense-in-depth.
  • Prescriptive, objective, and repeatable approach is now taken which removes a lot of the engineering judgement in the analysis.
  • Easily reproducible results
  • Uses peer reviewed PRA rather than just an engineering evaluation during the categorization.
  • Only when sufficient redundancy is present within the system will common cause failure allow screening out a cutset / candidate LSS SSCs.
  • Will be discussed more later in the presentation.

Comparison Between Processes Alternate Defense-in-Depth and PWROG-20015-NP

NRC Issue 1 Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Current Process
  • Core damage defense-in-depth in NEI 00-04, Section 6.1 uses success criteria in the internal events PRA model for design basis events and the engineering team identifies the level of defense-in-depth available.
  • NEI 00-04, Section 6.1 core damage defense-in-depth: This figure depicts the internally initiated design basis events considered in the licensee's safety analysis report (i.e., the events that were used to identify an SSC as safety-related) and considers the level of defense-in-depth available, based on the success criteria used in the PRA.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Current Process
  • Defense-in-depth is also evaluated in other aspects of the NEI 00-04 process including:
  • Review of Risk Information
  • Review of Defense-in-Depth Implications
  • Containment defense-in-depth NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP
  • Origins of Use of Internal Events PRA
  • Figure 6-1 in NEI 00-04 provides guidance to assess design-basis DID based on the likelihood of the design-basis internal event initiating event and the number of redundant and diverse trains nominally available to mitigate the initiating event.
  • Our origin for using the internal events PRA are these statements since the PRA is already used in the core damage defense-in-depth process.
  • Pressure boundary evaluations are evaluated in a separate analysis.
  • Core damage defense-in-depth still requires evaluation of pressure boundary failure initiating events and basic events that impact the FPIE CDF PRA model through non-flooding scenarios. Flooding scenario evaluations are completed in a separate evaluation.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Goal of Development
  • Leverage work done on PRA modeling to gain efficiencies.
  • Not intended to have significant differences from current process.
  • Alternate process does take into account significant redundancy within systems.
  • Reflect the as-build, as-operated plant.
  • Have been built to the ASME/ANS PRA Standard.
  • Have gone through a peer review process.
  • Have an F&O closure process for closing findings.
  • Components modeled in a PRA provide for a realistic interpretation of risk scenarios at the plant.
  • Allows for proper identification of SSCs that are impacting the safety of the plant.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Goal of Development
  • Goal of the alternate defense-in-depth process is to identify levels of defense-in-depth by continuing to use the PRA model but with less engineering judgement in the analysis.
  • Specific criteria outlined in process to screen out a cutset / candidate LSS SSC.
  • Goal of alternate defense-in-depth is to have a prescriptive screening process.
  • Creates standard and repeatable process.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Goal of Development
  • Alternate core damage defense-in-depth process only provides an alternative for the core damage defense-in-depth outlined in NEI 00-04, Section 6.1 and makes additional requirements for analyzing NEI 00-04, Section 9.2.2 (Review of Risk Information and Review Defense-in-Depth Implications).
  • Containment defense-in-depth remains
  • Quantitative evaluations remain
  • Other analyses remain
  • During development and review of process, there were concerns with overreliance on PRA. Therefore, additional considerations were added for a confirmation check on the process.
  • Will be discussed on next slides.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Additional Considerations
  • Also evaluated during periodic reviews.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Additional Considerations
  • Engineering team now required to evaluate NEI 00-04, Section 9.2.2 Review of Risk Information (7 Considerations) and Review Defense-in-Depth Implications.
  • Now a requirement for engineering team to evaluate beforehand.
  • Allows IDP to focus on reviews and confirm results.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Additional Considerations
  • Confirmatory check of PRA model for system to use this process; otherwise defaults to NEI 00-04, Section 6.1 core damage defense-in-depth process:
9. In addition to the IDP review and approval of the initiating event frequencies identified in Step 7.b.4), Step 7.b.5), and Step 7.b.6) (and sub-steps), the IDP should review the alternate defense-in-depth assessment consistent with NEI 00-04, Section 9:

IDP Review and Approval, while evaluating the system categorization, including the Review of Risk Information and Review Defense-in-Depth Implication considerations identified in NEI 00-04, Section 9.2.2. Additionally, the IDP should examine the following considerations for each system prior to system categorization approval. These considerations are initially examined by the engineering team during the system categorization with an initial evaluation provided to the IDP prior to IDP review and approval. If confirmation of these two criteria are not met, the NEI 00-04, Section 6.1 process should be used for the system categorization. It should be confirmed that:

a. The system modeled within the FPIE CDF PRA is modeled with sufficient detail to identify core damage defense-in-depth via the alternate core damage defense-in-depth method.
b. The assumptions identified in the FPIE CDF PRA model do not prevent identification of failure pathways that would impact the identification of core damage defense-in-depth via the alternate core damage defense-in-depth method.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

  • Summary
  • Internal events PRA model continues to be used for core damage defense-in-depth.
  • Rigorous and repeatable process.
  • Reduces engineering judgements from engineering team by use of a prescriptive process.
  • Substantial controls on quality of work by use of PRA.
  • PRA is peer reviewed.
  • Components modeled in a PRA provide for a realistic interpretation of risk scenarios at the plant.

NRC Issue 1 Discussion - Inconsistent or lack of correlation to layers of DID Alternate Defense-in-Depth and PWROG-20015-NP

NRC Issue 2 Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Current Process
  • NEI 00-04, Section 6.1 core damage defense-in-depth process requires the engineering team to evaluate multiple things including:
  • Design basis events the function is required for.
  • Other systems and trains that support the function.
  • Examination of the success criteria of the PRA for those design basis events.
  • Determines evaluation of candidate HSS / candidate LSS based on a table that requires significant engineering judgement.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Origins of Alternate Process
  • Elimination of reliance on engineering team determining evaluation of level of defense-in-depth in alternate process.
  • Use of the PRA model based on the number of basic events in the cutset to evaluate screening.
  • Allows for a consistent, prescriptive process to be implemented.
  • PRA model has been:
  • Developed in accordance with the PRA Standard.
  • Peer reviewed.
  • Accepted for use in the license amendment for 50.69.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Origins of Alternate Process
  • NRC concern there wasnt additional verification of the results from the PRA model structure.
  • Additional engineering team and IDP evaluations were added compared to current approach to examine acceptability.
  • The internal events PRA model has already been peer reviewed and been accepted for use in 10 CFR 50.69 applications. This is intended to provide one additional confirmatory check on the process.
  • Alternate seismic approach allows for IDP to review seismic insights.
  • Similar concept here with review of core damage defense-in-depth.
  • IDP also provides for a confirmatory check of the engineering team evaluations.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • The alternate approach allows for evaluation of all initiating events that are within the internal events PRA model rather than those that are just within the licensees safety analysis report.
  • This would include beyond design basis events if modeled in the PRA.
  • Additional Consideration
  • Evaluated by engineering team and reviewed by the IDP.
  • Allows proper examination on whether initiating events are appropriately modeled to evaluate system categorization.
  • Provides additional check on PRA model acceptability beyond the peer review process.
  • Evaluation requires engineering team to provide basis on whether it impacts core damage DID or not, and is reviewed and approved by IDP.
  • Evaluated during periodic reviews as well.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Additional Considerations
  • Engineering team now required to evaluate:
  • Review of Risk Information 7 Considerations (NEI 00-04, Section 9.2.2)
  • Review Defense-in-Depth Implications (NEI 00-04, Section 9.2.2)
  • Now a requirement for engineering team to evaluate beforehand.
  • IDP has to review and confirm.
  • This provides for a requirement that the engineering team examines these qualitative considerations in the process.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Additional Considerations
  • Confirmatory check of PRA model for system to use this process; otherwise NEI 00-04, Section 6.1 core damage defense-in-depth process:
9. In addition to the IDP review and approval of the initiating event frequencies identified in Step 7.b.4), Step 7.b.5), and Step 7.b.6) (and sub-steps), the IDP should review the alternate defense-in-depth assessment consistent with NEI 00-04, Section 9:

IDP Review and Approval, while evaluating the system categorization, including the Review of Risk Information and Review Defense-in-Depth Implication considerations identified in NEI 00-04, Section 9.2.2. Additionally, the IDP should examine the following considerations for each system prior to system categorization approval. These considerations are initially examined by the engineering team during the system categorization with an initial evaluation provided to the IDP prior to IDP review and approval. If confirmation of these two criteria are not met, the NEI 00-04, Section 6.1 process should be used for the system categorization. It should be confirmed that:

a. The system modeled within the FPIE CDF PRA is modeled with sufficient detail to identify core damage defense-in-depth via the alternate core damage defense-in-depth method.
b. The assumptions identified in the FPIE CDF PRA model do not prevent identification of failure pathways that would impact the identification of core damage defense-in-depth via the alternate core damage defense-in-depth method.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Additional Considerations
  • The system modeled within the FPIE CDF PRA is modeled with sufficient detail to identify core damage defense-in-depth via the alternate core damage defense-in-depth method.
  • This is intended to provide an additional confirmatory check; the PRA model has already been peer reviewed and accepted for use in 10 CFR 50.69 applications.
  • This consideration is evaluated to verify that there is sufficient detail to identify core damage defense-in-depth via this process. The engineering team will complete their evaluation with engineering judgement similar to how they use engineering judgement in the NEI 00-04, Section 6.1 process on the level of defense-in-depth.
  • If the engineering team or IDP do not believe there is sufficient detail, defaults to the conservative NEI 00-04, Section 6.1 core damage defense-in-depth process.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Additional Considerations
  • The assumptions identified in the FPIE CDF PRA model do not prevent identification of failure pathways that would impact the identification of core damage defense-in-depth via the alternate core damage defense-in-depth method.
  • This is intended to provide an additional confirmatory check; the PRA model has already been peer reviewed and accepted for use in 10 CFR 50.69 applications.
  • This requires examination of the PRA model for this system to properly examine if failure pathway identification for the system are identifiable that would impact core damage defense-in-depth.
  • If the engineering team or IDP do not believe there is sufficient detail, defaults to the conservative NEI 00-04, Section 6.1 core damage defense-in-depth process.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

  • Summary
  • PRA model has been developed consistent with the PRA Standard, has been peer reviewed, and has been approved for use in 10 CFR 50.69 license amendment.
  • Additional considerations allow for further evaluation beyond the prescriptive approach as an additional confirmatory check on the PRA.
  • IDP is a multi-discipline panel of experts that provides for a confirmatory check on the 10 CFR 50.69 process, including core damage defense-in-depth.
  • Engineering team has also been required to evaluate the Review of Risk Information and Review Defense-in-Depth Implication considerations to reduce burden on IDP and to allow for the IDP to focus on confirmatory checks.

NRC Issue 2 Discussion - Inappropriate compensation for reduced rigor Alternate Defense-in-Depth and PWROG-20015-NP

NRC Issue 3 Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP

  • Current Process
  • Functions / SSCs that are LSS outside of the system being categorized can be credited towards core damage defense-in-depth in the NEI 00-04, Section 6.1 process.
  • Currently, any identical, redundant components cannot be credited in the NEI 00-04, Section 6.1 core damage defense-in-depth process.
  • This does not allow for taking credit for realistic defense-in-depth that is present within systems.
  • Example: Core spray pumps NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP
  • NEI 00-04, Section 6.1 Core Damage Defense-in-Depth is not the sole area where CCF is identified:
  • The PRA quantitative evaluation in NEI 00-04, Section 5 examines CCF basic events for quantitative impacts.
  • The Review of Risk Information and Review Defense-in-Depth Implications considerations in NEI 00-04, Section 9.2.2 also provides a confirmatory examination of the overall categorization process including Review Defense-in-Depth Implications #4:
  • Potential for common cause failures is taken into account in the risk analysis categorization.

NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP

  • Basis for screening out of 4 or more CCF:
  • Taking no credit for reliance within the system prevents proper identification of defense-in-depth within the system itself and has led to situations where realistic DID cannot be credited.
  • 2 or 3 components CCF in a cutset would not be screened out due to the limited number of components common cause failing.
  • Only cutsets that have at least 4 or more components having a common cause failure are allowed to be screened out / candidate LSS for the core damage defense-in-depth analysis.
  • Does not mean that components will not be screened in / candidate HSS from other cutsets or from other portions of the 10 CFR 50.69 analysis.
  • Additional quantitative evaluation in NEI 00-04, Section 5.

NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP

  • Basis for Screening out of 4 or more CCF
  • Generic Example with 4 components in a CCCG.

NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP Initiating Event 1 - 5E-04/yr Indep. Comp. 1 Fails Indep. Comp. 2 Fails Indep. Comp. 3 Fails Cutset Screened Out since 3 BEs / Candidate LSS Initiating Event 1 - 5E-04/yr CCF Comp. 1 & 2 Indep. Comp 4.

Cutset Screened Out since 2 BEs / Candidate LSS Initiating Event 1 - 5E-04/yr CCF Comp. 1 & 2 & 3 & 4 Cutset Screened Out since CCF of 4 or more /

Candidate LSS Initiating Event 1 - 5E-04/yr CCF Comp. 1 & 2 & 3 Cutset Screened In /

Candidate HSS

Initiating Event 1 - 5E-04/yr CCF Comp. 1 & 2 & 4 Cutset Screened In /

Candidate HSS

  • Basis for Screening out of 4 or more CCF
  • As shown in generic example, since the cutset occurs with 3 components, the CCF of 3 components will screen in and cause all of the components to be candidate HSS even though the CCCG is a group of 4.
  • Therefore, even though the cutset with CCF of 4 components is screened out / candidate LSS, the components still end up as candidate HSS due to other cutsets.

NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP

  • The current alternate guidance and RAI response describes use of NEI 00-04, Section 10.2 as one basis for screening of CCF for defense-in-depth. We want to clarify that although NEI 00-04, Section 10.2 (detailed categorization) can only be used to classify components as LSS for other aspects of categorization outside of core damage defense-in-depth, we believe it is realistic to have a prescriptive detailed categorization process in core damage defense-in-depth, as outlined in the alternate defense-in-depth process, where credit can be taken for redundancy in core damage defense-in-depth (other evaluations may classify the components as candidate HSS) if a common cause event of 4 or more components leads to core damage. Common cause events of 2 or 3 components still can result in candidate HSS for those SSCs in the core damage defense-in-depth evaluation.

NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP

  • Summary
  • Credit for in-system redundancy.
  • Does not screen out / candidate LSS from a common cause failure basic event unless 4 or more components are in the CCF basic event. (can still be screened out from other requirements in the alternate DID).
  • The Review of Risk Information and Review Defense-in-Depth Implications considerations in NEI 00-04, Section 9.2.2 also provides a confirmatory examination including Review Defense-in-Depth Implications #4:
  • Potential for common cause failures is taken into account in the risk analysis categorization.
  • Additional evaluations are completed for 10 CFR 50.69 evaluations.

NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP

NRC Issue 4 Unjustified mixed use of DID categorization methodologies Alternate Defense-in-Depth and PWROG-20015-NP

  • Origins of Use of Multiple Methods
  • The alternate process identifies a realistic interpretation of defense-in-depth by a prescriptive process with credit for redundancy.
  • NEI 00-04 approach is still valid, although not as realistic of an interpretation of defense-in-depth.
  • System categorizations have already been completed with the NEI 00-04 process, at Limerick and other plants.
  • Allowing continued use of the NEI 00-04, Section 6.1 core damage defense-in-depth approach since it is a conservative evaluation.
  • NEI 00-04, Section 6.1 core damage defense-in-depth is also used in the scenario the engineering team or IDP believes NEI 00-04, Section 6.1 should be used for a system based on qualitative considerations (refer to next slide).

NRC Issue 4 Discussion - Unjustified mixed use of DID categorization methodologies Alternate Defense-in-Depth and PWROG-20015-NP

  • Origins of Use of Multiple Methods
  • NRC concern on whether PRA could identify every system with respect to identifying levels of defense-in-depth.
  • The PRA model follows the PRA Standard and is peer reviewed; but additional confirmatory checks were added to the process to address this concern.
  • Considerations were added for evaluation of PRA acceptability when using the alternate method for a system evaluation.
  • Current NEI 00-04 process just requires examination of defense-in-depth via success criteria of the PRA, the new defense-in-depth process evaluates beyond this and includes these additional qualitative confirmatory checks.
  • If the considerations are not met, the NEI 00-04, Section 6.1 core damage defense-in-depth process would be used.
  • These confirmatory checks allow for engineering judgement to be used even though the PRA model has already been peer reviewed and been accepted for use in 10 CFR 50.69 applications. These considerations are intended to provide one additional confirmatory check in the process.

NRC Issue 4 Discussion - Unjustified mixed use of DID categorization methodologies Alternate Defense-in-Depth and PWROG-20015-NP

  • Origins of Use of Multiple Methods
  • Other alternate processes have allowed continued use of the original method.
  • The alternate seismic approach proposed by Constellation allows for either the current seismic method or the alternate seismic method at Limerick.
  • Used at other plants as well.
  • Section 1 of the Harris SE for use of alternate seismic approach (ML21316A248):
  • Specifically, the proposed amendment would revise the license condition to allow the use of an alternative seismic approach in addition to the use of the seismic margin assessment to evaluate seismic risk for categorization of SSCs under the licensees previously approved 10 CFR 50.69 program, and also delete the license condition that required the completion of certain implementation items prior to the implementation of 10 CFR 50.69.

NRC Issue 4 Discussion - Unjustified mixed use of DID categorization methodologies Alternate Defense-in-Depth and PWROG-20015-NP

  • The alternate core damage DID method remains evaluated at the system level.
  • In alternate approach, an initial plant-wide cutset review is completed but further examination occurs at the system categorization level (e.g., qualitative considerations).

NRC Issue 4 Discussion - Unjustified mixed use of DID categorization methodologies Alternate Defense-in-Depth and PWROG-20015-NP

  • Summary
  • Alternate process allows for a more realistic evaluation of defense-in-depth that is prescriptive.
  • The NEI 00-04, Section 6.1 core damage defense-in-depth process has already been used in system categorized and is considered conservative.
  • Evaluations were added for confirmatory check on alternate process with a backstop to use the NEI 00-04, Section 6.1 process if determined by the engineering team or IDP.
  • Alternate seismic allows for continued use of original approach.

NRC Issue 4 Discussion - Unjustified mixed use of DID categorization methodologies Alternate Defense-in-Depth and PWROG-20015-NP

Backup Slides Alternate Defense-in-Depth and PWROG-20015-NP

  • Example 1 under alternate process

Compressors or Gas From The Nitrogen Bottles to the ADS Main Steam Safety Relief Valves (MSRVs).

  • The manual reactor trip initiating event frequency is greater that 1E-03 thus requiring at least 3 additional levels of Defense in Depth to screen as LSS. As this cutset does not have 3 levels of Defense in Depth, it screens in as HSS. The remaining basic events in this cutset represents a pipe rupture failure. Basic Event APPALLHFI represents the failure of the PCIG system as a result of the pipe rupture and not the impacts of any resulting flood. Basic Event ZTUHRXCWI represents the common cause failure of the HPCI and RCIC systems.

Backup Slide NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP Cut Value BE Value BE Description 3.43E-11 1.13E+00

%TMS MANUAL REACTOR TRIP 1.20E-07 APPALLHFI PCIG PIPE RUPTURES AND ACCUMULATOR RUPTURES 2.53E-04 ZTUHRXCWI COMMON CAUSE FAIL OF HPCI/RCIC PUMPS, TURBINES, OR VALVES

  • Example 2 under alternate process
  • This cutset identifies the D RHR pump and the DIV 1, DIV 2 ad DIV 3 battery chargers as candidate HSS. The loss of condenser vacuum initiating event frequency is greater that 1E-03 thus requiring at least 3 additional levels of Defense in Depth to screen as LSS. As this cutset does not have 3 levels of Defense in Depth, it screens in as HSS. The common cause failure of the battery chargers would drive the battery chargers to HSS however this system was not part of the pilot study. The DPM02DTM basic event represents the unavailability of the D RHR pump due to maintenance. This basic event drives the PRA Suppression Pool cooling function the HSS.

Backup Slide NRC Issue 3 Discussion - Inadequate basis for treatment of common cause failures Alternate Defense-in-Depth and PWROG-20015-NP Cut Value BE Value BE Description 4.31E-11 7.06E-02

%TCV LOSS OF CONDENSER VACUUM 5.69E-03 DPM02DTM RHR PUMP 1DP202 IN MAINTENANCE 1.07E-07 EBC123CWI CCF OF DIV 1, DIV 2, AND DIV 3 CHARGERS

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