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| {{#Wiki_filter:Millstone Power Station Unit 3 (MPS3) | | {{#Wiki_filter:}} |
| Alternative Request for Reactor Pressure Vessel Head Penetration Nozzle (RPVHPN)
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| Peening Follow-up Examination August 29, 2022 1
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam AGENDA
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| * Background
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| * Proposed Alternative
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| * Technical Justification
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| * Conclusions
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| * Schedule 2 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Background - Regulatory Requirements
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| * 10 CFR 50.55a(g)(6)(ii)(D)(5) states, in part, that a reactor pressure vessel (RPV) upper head with nozzles and associated J-groove welds mitigated by peening can obtain examination relief from the requirements of Code Case N-729-6, Table 1 if the peening meets the performance criteria, qualification and examination requirements of MRP-335, Revision 3-A.
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| * For RPVHPNs with effective degradation years (EDY) < 8 years, Section 4.3.3 of MRP-335, Revision 3-A, provides the Follow-Up Examination requirement below:
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| If all RPVHPNs are free from pre-peening flaws, inspection is performed on each RPVHPN in the second refueling outage (RFO) after peening.
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| 3 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Background - Plant-Specific
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| * Dominion Energy Nuclear Connecticut, Inc. (DENC) implemented an Ultra High Pressure Cavitation Peening (UHPCP) process at MPS3 during the fall 2020 RFO.
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| * Per MRP-335, Revision 3-A, the follow-up examination is required during the fall 2023 RFO.
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| The examination would be performed on the control rod drive mechanism (CRDM) nozzles and vent pipe assembly, which use Alloy 82/182 welds to join to Alloy 600 members.
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| * The MPS3 RPV upper head operates at Reactor Cold-leg Temperature (Tcold) and has experienced less than 8 EDY.
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| * Previous examinations for the MPS3 RPV upper head have identified no indications attributed to primary water stress corrosion cracking (PWSCC).
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| 4 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Proposed Alternative
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| * Pursuant to 10 CFR 50.55a(z)(1), DENC is planning to request a one-time alternative to the requirements of 10 CFR 50.55a(g)(6)(ii)(D),
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| which would permit the volumetric examination for the subject nozzles to be deferred from fall 2023 to spring 2025.
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| * The proposed alternative would not change visual examination frequency.
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| Bare metal visual examinations will be performed each RFO in accordance with MRP-335, Revision 3-A.
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| * Deferring this examination to the spring 2025 RFO would:
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| Alleviate scheduling and resource conflicts with the RPV hot leg dissimilar metal weld post-peening follow-up examination, and Align with the RPVHPN post-peening follow-up examination frequency required by the vendor warranty.
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| 5 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Technical Justification - PWSCC Considerations
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| * A pre-peening volumetric examination was conducted in accordance with MRP-335 Revision 3-A and did not identify any indications of cracking attributed to PWSCC.
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| * The peening application for MPS3 followed MRP-335 Revision 3-A without any deviations.
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| * The MPS3 peening met the MRP-335, Revision 3-A depth of compression requirements.
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| Per MRP-335, Revision 3-A, in cases when a shallow pre-existing flaw is located within a region of compressive residual plus operating stress, PWSCC growth of the pre-existing flaw would likely be arrested.
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| * A calculation of RIY [reinspection years] for an unmitigated head (per Code Case N-729-6) concluded that the next volumetric examination would not be required until the fall of 2026.
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| The reinspection frequency for unmitigated heads is conservative to apply for peened heads, as peening reduces the likelihood of PWSCC occurrence.
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| 6 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Technical Justification - Defense In Depth
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| * Continued bare metal visual examination would provide defense in depth in the unlikely event of pressure boundary leakage due to:
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| Base metal or J-groove weld cracking, or Small flaws that are too shallow to be reliably detected in the pre-peening examination.
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| * The MPS3 boric acid corrosion program provides both detection and analysis of leakage of borated water inside containment.
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| * MPS3 Technical Specification (TS) 3.4.6.2 requires online leakage detection and outlines the timely actions required to maintain safe operability for Reactor Coolant System leakage greater than 1 gallon per minute (gpm).
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| * DENC also implements a 0.1 gpm action level on unidentified leakage for MPS3, consistent with WCAP-16465-NP.
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| 7 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Conclusions
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| * The lack of PWSCC for the MPS3 RPV upper head supports performing inspection during the third RFO after peening.
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| * Since MPS3 peening met the MRP-335, Revision 3-A requirements, PWSCC growth of shallow, pre-existing flaws would likely be arrested.
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| * A calculation of RIY for an unmitigated head, which is conservative for a peened head, supports deferral of follow-up examination.
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| * Defense in depth is maintained through continued performance of the bare metal visual examination each RFO, boric acid corrosion control practices, and the plants online leak detection capabilities.
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| * Therefore, deferring the follow-up examination from the fall 2023 RFO to the spring 2025 RFO would maintain an acceptable level of quality and safety.
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| 8 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Schedule
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| * Facility Safety Review Committee (FSRC) is targeted for mid September 2022.
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| * Alternative Request submittal is targeted for the end of September 2022.
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| * NRC approval will be requested one year after submittal, to support implementation by the fall 2023 outage.
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| 9 August 29, 2022
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| MPS3 Alternative for RPVHPN Peening Follow-Up Exam Questions?
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| 10 August 29, 2022}}
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