ML20238B980: Difference between revisions

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{{#Wiki_filter:Significance Determination Process for FLEX Findings Michelle Kichline              John Hanna Senior Reliability and Risk Analyst Senior Reactor Analyst Office of Nuclear Reactor Regulation      Region III
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FLEX Significance Determination Process IMC 0609 Appendix A Screening The Significance Determination Procedure            Process for Findings At-Power FLEX Screening          Green                  Detailed Risk Question Results                              Evaluation (DRE)
Detailed Risk Green          White          Yellow    Red Evaluation Results 2
 
SDP Findings/Violations Involving FLEX FLEX was Subject of Finding/Violation                FLEX was Credited in SDP, but not Subject of Finding/Violation St. Lucie external flooding Davis Besse ALL FLEX Findings TDAFW pump have been Green                                                  Clinton EDG air start valves Robinson Tainter Gate
 
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.
* The performance deficiency was evaluated using the Phase 1 screening guidance in IMC 0609 Appendix A, Exhibit 2, Mitigating Systems, Section E for FLEX.
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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.
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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.
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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.
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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.
* The performance deficiency was evaluated using the Phase 1 screening guidance in IMC 0609 Appendix A, Exhibit 2, Mitigating Systems, Section E for FLEX.
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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.
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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.
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FLEX Screening Question 3 11
 
How can a FLEX finding be greater than Green?
CDF old baseline risk prior to FLEX 2E-5 1E-5 new baseline risk with FLEX credited Exposure Time 12
 
How can a FLEX finding be greater than Green?
CDF Delta CDF = (2E 1E-5) = 1E-5/year Exposure time to reach Greater than Green = (1E-6/1E-5) x 12 months/year
      = 1.2 months old baseline risk prior to FLEX 2E-5 1E-5 new baseline risk with FLEX credited Exposure Time 13
 
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) 14
 
Backup Slides Design/Licensing Basis -
Perception                              B.5.b Internal Events          (Aircraft Impacts)
LOIA LLOCA                                  External Events LOOP IS-LOCA TRANS            SBO        FLEX (Mitig.
LOMFW                      LOHS Strategies)
LODC Bus 16
 
Design/Licensing Basis -
Reality Internal Events                      B.5.b (Aircraft LOIA                            Impacts)
LLOCA External Events LOOP IS-LOCA TRANS        SBO        FLEX (Mitig.
LOMFW                  LOHS Strategies)
LODC Bus 17}}

Revision as of 22:40, 24 September 2020

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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