ML20238B980

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FLEX Summit Session 4 - IMC 0609 App a Tabletop
ML20238B980
Person / Time
Issue date: 08/25/2020
From: John Hanna, Michelle Kichline
NRC/NRR/DRA/APOB, NRC/RGN-III
To:
References
Download: ML20238B980 (17)
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Text

Significance Determination Process for FLEX Findings Michelle Kichline Senior Reliability and Risk Analyst Office of Nuclear Reactor Regulation John Hanna Senior Reactor Analyst Region III

2 FLEX Significance Determination Process Detailed Risk Evaluation Results FLEX Screening Question Results Screening Procedure IMC 0609 Appendix A The Significance Determination Process for Findings At-Power Detailed Risk Evaluation (DRE)

Green White Yellow Red Green

SDP Findings/Violations Involving FLEX FLEX was Subject of Finding/Violation FLEX was Credited in SDP, but not Subject of Finding/Violation ALL FLEX Findings have been Green Robinson Tainter Gate Davis Besse TDAFW pump St. Lucie external flooding Clinton EDG air start valves

4 Example 1 - Description

  • During surveillance testing, a FLEX diesel generator (DG) failed to start due to a failed temperature switch. The FLEX DG that failed to start was one of two DGs that could be deployed to provide AC power for the suppression pool cooling (SPC) pumps to provide core cooling after an ELAP.
  • The FLEX strategy to provide core cooling involves repowering the SPC pumps to take a suction on the suppression pool and return the water to the reactor after being cooled in the SPC HXs. This core cooling strategy also requires the use of different diesel-driven FLEX pumps to provide cooling water to the SPC HXs.
  • The licensee performed an extent of condition review and identified that the other FLEX DG that would be used to provide power for the SPC pumps also failed to start.

5 Example 1 - Screening Question 1 This finding is not associated with spent fuel pool instrumentation or containment venting, so we continue to the next question.

6 Example 1 - Screening Question 2

  • The FLEX function to provide AC power for the SPC pumps using a FLEX DG was completely failed because both DGs that could be used to accomplish this function were failed.
  • The Phase 1 strategy to use RCIC for continued RPV injection to provide core cooling after an ELAP remained available.

7 Example 1 - Conclusion

  • The Phase 1 strategy to use RCIC to provide core cooling cannot be continued indefinitely. As RCIC operates, the suppression pool will heat up and must be cooled to maintain both core cooling and containment parameters.
  • The Phase 1, 2, and 3 strategies are dependent on each other and each strategy must succeed as planned in order to ensure that core cooling and/or containment capabilities are maintained.
  • It is not appropriate to use another FLEX strategy or a non-FLEX strategy as a backup for a failed FLEX strategy when answering the screening questions.
  • The best answer to Exhibit 2, Question E.2 for this situation is YES and a DRE should be performed.

8 Example 2 - Description

  • During phase rotation checks, a licensee identified that the electrical connections on one of its FLEX DGs and the connections to in-plant equipment were reversed.
  • The affected FLEX DG provides AC power to re-energize the DC battery chargers and several AC motor operated valves (MOVs) after an ELAP.
  • The phase rotation issue would not affect operation of the battery chargers, but it would cause any valves operated during the time the phase rotation issue existed to travel in the wrong direction, potentially damaging the valves and preventing them from being repositioned either electrically or manually.

9 Example 2 - Screening

  • The phase rotation issue itself would not immediately result in a complete loss of the ability to maintain or restore core cooling or containment capabilities.
  • However, if the FLEX DG were connected to in-plant equipment, it would have caused phase-dependent loads to rotate backwards and have the potential to damage in-plant equipment.

10 Example 2 - Conclusion

  • Since the licensees primary and alternate FLEX strategies use equipment from different divisions of AC power, the licensee would be able to identify the phase rotation issue and install the alternate FLEX DG to supply power to the other division of AC valves in order to complete the FLEX function.
  • However, because the phase rotation issue would not be identified until the primary FLEX DG was connected and valves had been operated unsuccessfully, installation and connection of the alternate FLEX DG could not be accomplished within the time allotted in the licensees FIP.
  • The best answer to Exhibit 2, Question E.2 for this situation is YES because the phase rotation issue failed the ability to restore AC power within the time described in the FIP, and a DRE should be performed.

11 FLEX Screening Question 3

12 How can a FLEX finding be greater than Green?

CDF Exposure Time old baseline risk prior to FLEX 1E-5 2E-5 new baseline risk with FLEX credited

13 How can a FLEX finding be greater than Green?

CDF Exposure Time old baseline risk prior to FLEX 1E-5 2E-5 new baseline risk with FLEX credited Delta CDF = (2E-5 - 1E-5) = 1E-5/year Exposure time to reach Greater than Green = (1E-6/1E-5) x 12 months/year

= 1.2 months

14 Where does FLEX make the most difference?

Single unit sites Fewer EDGs and/or no SBO EDG No crosstie capability at multi-unit sites Absence of low leakage RCP seals Small DC batteries Higher LOOP likelihood Higher (LOOP/SBO/Electrical Bus) risk from internal events (Fire)

Backup Slides

16 Design/Licensing Basis -

Perception LLOCA IS-LOCA B.5.b (Aircraft Impacts)

LOMFW TRANS SBO LOOP LOHS LODC Bus LOIA FLEX (Mitig.

Strategies)

Internal Events External Events

17 LLOCA IS-LOCA B.5.b (Aircraft Impacts)

LOMFW TRANS SBO LOOP LOHS LODC Bus LOIA FLEX (Mitig.

Strategies)

Internal Events External Events Design/Licensing Basis -

Reality

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