ML22236A060

Presentation Material for 8/29/22 Pre-Submittal Meeting Reactor Pressure Vessel Head Peening Follow-Up Exam Deferral

ML22236A060
Person / Time
Site:	Millstone
Issue date:	08/29/2022
From:	Dominion Energy Nuclear Connecticut
To:	NRC/NRR/DORL/LPL1
Guzman R
References
EPID L-2022-LRM-0058
Download: ML22236A060 (10)

Text

Millstone Power Station Unit 3 (MPS3)

Alternative Request for Reactor Pressure Vessel Head Penetration Nozzle (RPVHPN)

Peening Follow-up Examination August 29, 2022 1

MPS3 Alternative for RPVHPN Peening Follow-Up Exam AGENDA

• Background

• Proposed Alternative

• Technical Justification

• Conclusions

• Schedule August 29, 2022 2

MPS3 Alternative for RPVHPN Peening Follow-Up Exam Background - Regulatory Requirements

• 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.

• 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:

If all RPVHPNs are free from pre-peening flaws, inspection is performed on each RPVHPN in the second refueling outage (RFO) after peening.

August 29, 2022 3

MPS3 Alternative for RPVHPN Peening Follow-Up Exam Background - Plant-Specific

• Dominion Energy Nuclear Connecticut, Inc. (DENC) implemented an Ultra High Pressure Cavitation Peening (UHPCP) process at MPS3 during the fall 2020 RFO.

• Per MRP-335, Revision 3-A, the follow-up examination is required during the fall 2023 RFO.

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.

• The MPS3 RPV upper head operates at Reactor Cold-leg Temperature (Tcold) and has experienced less than 8 EDY.

• Previous examinations for the MPS3 RPV upper head have identified no indications attributed to primary water stress corrosion cracking (PWSCC).

August 29, 2022 4

MPS3 Alternative for RPVHPN Peening Follow-Up Exam Proposed Alternative

• 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),

which would permit the volumetric examination for the subject nozzles to be deferred from fall 2023 to spring 2025.

• The proposed alternative would not change visual examination frequency.

Bare metal visual examinations will be performed each RFO in accordance with MRP-335, Revision 3-A.

• Deferring this examination to the spring 2025 RFO would:

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.

August 29, 2022 5

MPS3 Alternative for RPVHPN Peening Follow-Up Exam Technical Justification - PWSCC Considerations

• 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.

• The peening application for MPS3 followed MRP-335 Revision 3-A without any deviations.

• The MPS3 peening met the MRP-335, Revision 3-A depth of compression requirements.

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.

• 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.

The reinspection frequency for unmitigated heads is conservative to apply for peened heads, as peening reduces the likelihood of PWSCC occurrence.

August 29, 2022 6

MPS3 Alternative for RPVHPN Peening Follow-Up Exam Technical Justification - Defense In Depth

• Continued bare metal visual examination would provide defense in depth in the unlikely event of pressure boundary leakage due to:

Base metal or J-groove weld cracking, or Small flaws that are too shallow to be reliably detected in the pre-peening examination.

• The MPS3 boric acid corrosion program provides both detection and analysis of leakage of borated water inside containment.

• 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).

• DENC also implements a 0.1 gpm action level on unidentified leakage for MPS3, consistent with WCAP-16465-NP.

August 29, 2022 7

MPS3 Alternative for RPVHPN Peening Follow-Up Exam Conclusions

• The lack of PWSCC for the MPS3 RPV upper head supports performing inspection during the third RFO after peening.

• Since MPS3 peening met the MRP-335, Revision 3-A requirements, PWSCC growth of shallow, pre-existing flaws would likely be arrested.

• A calculation of RIY for an unmitigated head, which is conservative for a peened head, supports deferral of follow-up examination.

• 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.

• 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.

August 29, 2022 8

MPS3 Alternative for RPVHPN Peening Follow-Up Exam Schedule

• Facility Safety Review Committee (FSRC) is targeted for mid September 2022.

• Alternative Request submittal is targeted for the end of September 2022.

• NRC approval will be requested one year after submittal, to support implementation by the fall 2023 outage.

August 29, 2022 9

Questions?

MPS3 Alternative for RPVHPN Peening Follow-Up Exam August 29, 2022 10