ML24047A347

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Presentation for February 22, 2024 ACRS Digital I&C Subcommittee Meeting
ML24047A347
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Issue date: 02/22/2024
From: Marshall M
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Advisory Committee on Reactor Safeguards Digital Instrumentation & Controls Briefing February 22, 2024 SRM-SECY-22-0076 Implementation:

Branch Technical Position 7-19, Revision 9

Opening Remarks

Presentation Outline

  • Background
  • Timeline
  • SRM-SECY-22-0076 Direction and Staff Response
  • Changes from Revision 8 to Revision 9
  • Changes since the September 7, 2023, ACRS DI&C SC Briefing
  • Key Messages and Next Steps
  • Closing Remarks 3

Recent Activities 01/25/2021 Revision 8 of BTP 7-19 issued 09/07/23 ACRS DI&C Subcommittee briefing 05/24/24 Revision 9 of BTP 7-19 issued 02/22/24 ACRS DI&C Subcommittee briefing 10/24/23 -

11/24/23 Public comment period 01/23/23 Supplement to SECY-22-0076 submitted to Commission 03/06/24 ACRS Full Committee briefing 08/10/2022 SECY-22-0076 submitted to Commission 05/25/23 SRM-SECY-22-0076 issued 4

SRM-SECY-22-0076

  • The Commission approved the staffs recommendation to expand the existing policy for digital I&C CCFs to allow the use of risk-informed approaches to demonstrate the appropriate level of defense-in-depth, subject to the edits provided
  • The Commission directed the staff to clarify, in the implementing guidance, that the new policy is independent of the licensing pathway selected by the reactor licensees and applicants
  • The Commission directed the staff to complete the final implementing guidance within a year from the date of the SRM (May 24, 2024) 5

Staff Response to Meet the SRM

  • Drafted Rev. 9 to SRP BTP 7-19
  • Allows the staff to review risk-informed applications
  • May result in use of design techniques other than diversity
  • Focused the revisions on implementing the expanded policy
  • Staff briefed the ACRS DI&C Subcommittee on September 7, 2023
  • Staff received and dispositioned public comments 6

Substantive Changes to BTP 7-19 (Rev. 8 - Rev. 9)

  • Revised Section B.1.1 to reflect the updated four points in SRM-SECY-22-0076
  • Revised Section B.1.2 for clarification of critical safety functions
  • Revised Section B.3.1.3 for evaluation of alternative approaches
  • Added Section B.3.4 for evaluation of risk-informed D3 assessment
  • Revised Section B.4 for evaluation of different approaches for meeting Point 4
  • Added five flowcharts to facilitate the review
  • Added language from RG 1.152 to address a prior commitment to ACRS regarding communication independence and control of access 7

Overview of BTP 7-19, Revision 9 8

Point 2 Detailed D3 Assessment:

Risk-Informed Approaches (Sections B.3.4.1, B.3.4.2)

Point 2 Detailed D3 Assessment:

Best-Estimate Methods (Section B.3.2)

Point 3 Addressing, Mitigating, or Accepting the Consequences of Each CCF Using Design Techniques or Mitigation Measures Other than Diversity (Sections B.3.4.3, B.3.4.4)

Deterministic Path Risk-Informed Path Point 1 Need for a Detailed D3 Assessment (Sections B.2, B.3.1)

Point 3 Addressing, Mitigating, or Accepting the Consequences of Each CCF Using Diverse Means (Sections B.3.2, B.3.3)

Point 4 Independent and Diverse Displays and Manual Controls (Section B.4)

Changes to Sections B.1.1 and B.1.2

  • Updated Four Points of the Policy (Section B.1.1)
  • Replaced the four SRM-SECY-93-087 points with the SRM-SECY-22-0076 points and updated the explanation of the points
  • Critical Safety Functions (Section B.1.2)
  • Clarified the term critical safety functions and that the list of these functions in SECY-22-0076 are examples representative of operating light water reactors
  • Clarified that other types of reactors may have different critical safety functions based on the reactor design safety analysis
  • The identification of such functions may be risk-informed 9

Alternative Approaches (Section B.3.1.3)

Two Pathways for the evaluation of alternative approaches other than diversity and testing to eliminate the potential for CCF from further consideration

  • Previous endorsement or approval
  • Ensure it is applicable
  • Ensure it is followed
  • Justify any deviations
  • A new approach proposed as part of an application
  • Use the acceptance criteria in BTP 7-19
  • Review description of vulnerability being addressed
  • Review description of alternative approach and justification 10

Risk-Informed D3 Assessment Process (Section B.3.4) 11 Identify each postulated CCF Address the CCF using a risk-informed approach Model the CCF in the PRA (Section B.3.4.2)

Determine the risk significance of the CCF (Section B.3.4.3)

Determine appropriate means to address the CCF (Section B.3.4.4)

Determine consistency with NRC policy and guidance on RIDM (Section B.3.4.1)

Address the CCF deterministically Justify alternative approaches

Risk-Informed D3 Assessment Determine Consistency with NRC Policy and Guidance on RIDM

  • Review applications that use risk-informed approaches for consistency with established NRC policy and guidance on RIDM Model the CCF in the PRA
  • Determine if the base PRA meets PRA acceptability guidance identified in the application
  • Evaluate how the CCF is modeled in the PRA and the justification that the modeling adequately captures the impact of the CCF on the plant 12

Risk-Informed D3 Assessment Determine the Risk Significance of the CCF

  • The risk significance of a CCF can be determined using a bounding sensitivity analysis or a conservative sensitivity analysis
  • A bounding sensitivity analysis assumes the CCF occurs
  • A conservative sensitivity analysis assumes a probability less than 1
  • Provides a technical basis for a conservative probability of the CCF
  • Demonstrates that all principles of RIDM are addressed
  • Addresses the impact of this assumption on PRA uncertainty
  • A CCF is not risk significant if the following criteria are met:
  • The increase in CDF is less than 1 x 10-6 per year
  • The increase in LERF is less than 1 x 10-7 per year 13

Risk-Informed D3 Assessment 14

Approaches for Meeting Point 4 (Section B.4) 15

  • Section B.4 provides six acceptance criteria for independent and diverse main control room displays and controls for manual actuation of critical safety functions
  • Applications that propose a different approach (i.e., one that does not meet all the acceptance criteria in B.4) provide appropriate justification
  • Clarifications made throughout the BTP to address:
  • Public comments
  • Discussions during the September 7, 2023, ACRS DI&C SC briefing
  • Comments from Member Brown and Member Roberts (attachment to transcript)
  • No substantive changes made to analysis methodologies or acceptance criteria 16 Changes to BTP Since Previous ACRS Briefing

General Changes to the BTP

  • Revised the BTP to consistently use the term digital I&C system instead of the multiple variations of the term (e.g., digital safety system, I&C equipment, I&C systems, digital I&C system or component, digital technology, etc.)
  • This also ensures the BTP uses language consistent with SRM-SECY-22-0076 (NEI 1)
  • Replaced point X of the policy with point X of SRM-SECY-22-0076 to clarify which point is being referred to (NEI 18)
  • Revised the BTP to consistently use the term defense in depth and diversity (NEI 24) 17

Changes to Section A. Background

  • Added historical information at the beginning of the section
  • Restored the sentence on latent design defects in the design of the DI&C system (ACRS Member Comment 8a)
  • Added footnote 3 to provide clarification to the NRC staff on the Commission direction
  • Removed the sentences regarding NUREG/CR-6303 because they did not add value to the discussion
  • Added segmentation to the list of design technique examples (NEI 26)
  • Removed references to other guidance documents which are not explicitly used in the BTP (NEI 1) 18

Changes to Section A. Background

  • Removed references to regulations from the Regulatory Basis section that are not specifically called for in the BTP criteria
  • Added a reference to NUREG-2122 in the Relevant Guidance section (ACRS Member Comment 3)
  • Removed references to SRP chapters or sections that are not used or are already referenced in specific parts of the BTP
  • Clarified that the BTP is intended to provide review guidance to the NRC staff for ensuring an application meets the policy and applicable regulations (i.e., it is not intended as guidance to applicants for developing a D3 assessment) (NEI 2) 19

Changes to Section B.1

  • Added a new figure at the end of the document depicting the applicable BTP sections for addressing each of the four points in SRM-SECY-22-0076 (NEI 2)
  • Clarified the discussion on Points 3 and 4 of SRM-SECY-22-0076 (NEI 1, 10, and 11)
  • Clarified the discussion on critical safety functions in Section B.1.2 (NEI 12) 20

Changes to Section B.3.1.1

  • Removed references to NUREG/CR-6303 and NUREG/CR-7007 because they may be interpreted as review guidance, which is not the staffs intent (NEI 30)
  • Reworded acceptance criterion c. to use language consistent with SECY-18-0090 21

Changes to Section B.3.1.3

  • Removed language that was added on risk-significance of the CCF and the pointer to B.3.4 (discussions during ACRS DI&C SC briefing)
  • Provided a well-designed watchdog timer as an example of an alternative approach (NEI 16)
  • Not dependent on the platform software
  • Puts the actuators in a safe (i.e., actuated) state
  • Clarified acceptance criterion a. for identification of CCF vulnerabilities using a hazards analysis technique (NEI 3) 22

Changes to Section B.3.1.4

  • Added a footnote to clarify that SRM-SECY-22-0076 did not modify the reference to RIS 2002-22, Supplement 1, in SECY-18-0090 23

Changes to Section B.3.2

  • Clarified the term diverse (NEI 17)
  • Removed references to NUREG/CR-6303 and NUREG/CR-7007 because they may be interpreted as review guidance, which is not the staffs intent (NEI 30)
  • Removed references to 10 CFR 50.69 and GL 85-06 to avoid potential confusion with different safety significance categorization schemes
  • Added a sentence on manual control connections (ACRS Member Comment 5a)
  • Added a clarification that displays and manual controls credited as the diverse means for Point 3 may also be credited for Point 4 (NEI 32)

Changes to Section B.3.4

  • Clarified the language to address concerns associated with references to SRP Chapter 19 (NEI 4, 22)
  • Included a discussion of the base PRA model (NEI 6)
  • Added reference to previously approved risk-informed applications
  • Clarified the language to address concerns regarding the need to consider intersystem CCFs of DI&C
  • Removed terminology not typically used in PRA (NEI 19)
  • Clarified modeling the impact on multiple systems (NEI 5) 25

Changes to Section B.3.4

  • Clarified acceptance criteria for risk involving operator actions (NEI 8)
  • Provided specific acceptance criteria for determining the appropriate means to address the CCF instead of referencing the criteria in Section B.3.1.3 (discussions during ACRS DI&C SC briefing) 26

Changes to Section B.4

  • Various edits made to improve the clarity of the Point 4 discussion and ensure consistency with SRM-SECY-22-0076 (NEI 9, 34, and 35)
  • Removed reference to RG 1.62 as it is not intended to address Point 4 (NEI 20)
  • Removed paragraph on long-term management of critical safety functions because it did not contain related acceptance criteria (NEI 21)
  • Replaced risk-informed critical safety functions with critical safety functions (which may have been determined using risk information)

(discussions during ACRS DI&C SC briefing) 27

Key Messages

  • BTP 7-19 revised to incorporate SRM-SECY-22-0076
  • Changes made after September 2023 ACRS DI&C Subcommittee briefing in response to public comments and ACRS member feedback
  • Clarifications made throughout the BTP
  • No substantive changes made to analysis methodologies or acceptance criteria 28

Next Steps

  • ACRS Full Committee briefing scheduled for March 6, 2024
  • The staff is planning to issue the final BTP 7-19, Rev. 9 in May 2024 29

Closing Remarks

Acronyms ACRS Advisory Committee on Reactor Safeguards BTP Branch Technical Position CCF Common Cause Failure D3 Defense-in-Depth and Diversity DI&C Digital Instrumentation and Control I&C Instrumentation and Control NEI Nuclear Energy Institute NRC Nuclear Regulatory Commission PRA Probabilistic Risk Assessment RG Regulatory Guide SECY Commission Paper SRM Staff Requirements Memorandum SRP Standard Review Plan

References

  • Transcript of September 7, 2023, ACRS DI&C Subcommittee Briefing and attachment with comments provided by Member Charles Brown and Member Thomas Roberts (ML23264A865)
  • NEI Comments on Draft BTP 7-19, Revision 9, dated November 21, 2023 (ML23326A117)
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