ML22271A913
| ML22271A913 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 09/28/2022 |
| From: | James Holloway Dominion Energy Nuclear Connecticut |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 22-277 | |
| Download: ML22271A913 (1) | |
Text
Serial No.22-277 Docket No. 50-423 ATTACHMENT ALTERNATIVE REQUEST IR-4-11 PROPOSED ALTERNATIVE REQUEST FOR EXTENSION OF EXAMINATION INTERVAL FOR REACTOR PRESSURE VESSEL HEAD PENETRATION NOZZLES WITH MITIGATED ALLOY 600/82/182 PEENED SURFACES MILLSTONE POWER STATION UNIT 3 DOMINION ENERGY NUCLEAR CONNECTICUT, INC. (DENC)
Serial No.22-277 Docket No. 50-423 Attachment, Page 1 of 7 Alternative Request IR-4-11 Proposed Alternative Request for Extension of Examination Interval for Reactor Pressure Vessel Head Penetration Nozzles with Mitigated Alloy 600/82/182 Peened Surfaces Pursuant to 10 CFR 50.55a (z)(2)
--Hardship Without a Compensating Increase in Quality and Safety
- 1. ASME Code Component(s) Affected ASME Code Code Class 1 Class Reference MRP-335, Revision 3-A [4] per 10 CFR 50.55a (g)(6)(ii)(D)(5)
Examination Reactor Pressure Vessel Head Penetration Nozzles Category (RPVHPNs) with Effective Degradation Years (EDY) < 8 years Components
- Control Rod Drive Mechanisms (CRDMs) and Instrumentation Devices (RPVHPN Numbers 1 through 78)
- Vent Pipe Assembly Penetration Nozzle Description
- CRDMs and Instrumentation Devices: The CRDMs and instrumentation devices interface with the reactor vessel closure head (RVCH) using head adaptors. These head adaptors consist of an Alloy 600 (SB-167) tubular member (penetration nozzle) and a stainless steel (SA-182, F304)
CRDM housing flange, joined via a full penetration Alloy 82/182 weld. The head adaptor penetration nozzles are attached by Alloy 82/182 partial penetration welds to the underside of the closure head.
- Vent Pipe Assembly Penetration Nozzle: The vent pipe assembly consists of an Alloy 600 (SB-167) tubular member (penetration nozzle), a 90º elbow, and piping all joined via Alloy 82/182 full penetration welds. The vent assembly penetration nozzle is attached by an Alloy 82/182 partial penetration weld to the underside of the closure head.
Serial No.22-277 Docket No. 50-423 Attachment, Page 2 of 7
2. Applicable Code Edition and Addenda
The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, 2013 Edition [1].
3. Applicable Code Requirement
Per 10 CFR 50.55a (g)(6)(ii)(D)(5) [2]:
Peening. In lieu of inspection requirements of Table 1, Items B4.50 and B4.60, and all other requirements in ASME BPV Code Case N-729-6 [3] pertaining to peening, in order for a RPV [reactor pressure vessel] upper head with nozzles and associated J-groove welds mitigated by peening to obtain examination relief from the requirements of Table 1 for unmitigated heads, peening must meet the performance criteria, qualification, and examination requirements stated in MRP-335, Revision 3-A [4], with the exception that a plant-specific alternative request is not required and NRC condition 5.4 of MRP-335, Revision 3-A does not apply.
The second bullet in MRP-335, Revision 3-A, Section 4.3.3, provides the Follow-Up Inspection requirement below:
For plants where RVPHPNs [sic] and associated J-groove welds in a reactor vessel closure head have experienced EDY< 8, if all RPVHPNs in the reactor vessel closure head are free from pre-peening flaws, inspections shall be performed on each RPVHPN in the second refueling outage subsequent to peening.
4. Reason for Request
Dominion Energy Nuclear Connecticut, Inc. (DENC) implemented an Ultra High Pressure Cavitation Peening (UHPCP) process at MPS3 during the fall 2020 RFO.
In accordance with MRP-335, Revision 3-A [4], the RPVHPN follow-up volumetric examination is currently scheduled for fall 2023 RFO. However, the warranty from the vendor that performed the UHPCP application specifies that a follow-up volumetric examination be conducted in the third RFO after peening.
To eliminate performance of equivalent examinations in two sequential RFOs, DENC is requesting deferral of the MPS3 RPVHPN post-peening follow-up volumetric examination scheduled for the fall 2023 RFO until the spring 2025 RFO. There will be no deviation in visual examination requirements, as bare metal visual examinations will be performed each RFO in accordance with MRP-335, Revision 3-A, Section 4.3.4.
Serial No.22-277 Docket No. 50-423 Attachment, Page 3 of 7 Performing volumetric examinations during two sequential RFOs would add unnecessary dose for support personnel and would be contrary to As-Low-As Reasonably Achievable (ALARA) program practices. The RPVHPNs identified in Section 1 are located in a Locked High Radiation Area (LHRA) inside containment.
The volumetric examination evolution results in personnel radiological exposure while setting up the equipment, conducting the examination, and demobilizing the equipment. Based on historical data at MPS3, the estimated increase in occupational dose due to performing RPVPHN volumetric examination in two separate outages would be approximately 396 mRem. Also, an additional estimated 149 mRem would be attributed to set-up and demobilization of equipment. A higher dose would be expected if tool breakdowns or issues requiring additional LHRA entry were to occur.
Conducting two volumetric examinations would also increase the potential for industrial safety issues and contamination exposure. Therefore, DENC concludes that performing RPVHPN volumetric examinations during the second and third RFOs after peening would present a hardship without commensurate safety benefit.
5. Proposed Alternative and Basis for Use
5.1 Proposed Alternative Pursuant to 10 CFR 50.55a(z)(2), DENC is requesting a one-time alternative to the requirements of 10 CFR 50.55a(g)(6)(ii)(D), in which the post-peening follow-up volumetric examination interval for the nozzles identified in Section 1 is extended by one fuel cycle (i.e., from the fall 2023 RFO to spring 2025 RFO). This alternative corresponds to a nominal 4.5 calendar years since the last volumetric examination for these nozzles. An interval of 4.5 calendar years is still less than the RIY
[reinspection years] of 2.25 from Code Case N-729-6 for an unmitigated RPV head with no indications of cracking attributed to PWSCC. This corresponds to 6.0 calendar years for the MPS3 RVCH operating temperature of 559 °F, when conservatively assuming a 98% capacity factor for all projected cycles.
5.2 Primary Water Stress Corrosion Cracking (PWSCC) Considerations The primary degradation mechanism addressed by 10 CFR 50.55a(g)(6)(ii)(D) is PWSCC. This degradation mechanism occurs when a susceptible material is exposed to a primary water environment, elevated tensile stress levels and elevated operating temperatures. Previous examinations of the MPS3 RPV upper head have identified no indications of cracking attributed to PWSCC. The MPS3 RPV upper head was peened as a pre-emptive mitigation measure during the fall 2020 RFO, and the UHPCP application followed the process of MRP-335, Revision 3-A without any deviations. A pre-peening volumetric examination was conducted during this RFO in accordance with Section 2.5.1 of MRP-335, Revision 3-A. This pre-peening
Serial No.22-277 Docket No. 50-423 Attachment, Page 4 of 7 inspection also did not identify any indications of cracking attributed to PWSCC for the MPS3 RPV upper head.
When applicable MRP-335 performance criteria are met, peening mitigation prevents initiation of PWSCC. The MPS3 peening application met or exceeded the MRP-335 depth of compression requirements. 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 [4]. The deterministic and probabilistic analyses in MRP-335 (Sections 5.2 and 5.3, respectively) also show that it is not necessary for growth of shallow pre-existing flaws to be arrested by the post-peening stress field. Pre-existing flaws are effectively addressed by the combination of pre-peening and follow-up inspections.
The likelihood that a pre-existing flaw exists below the depth of peening application is low since there have been no discoveries of PWSCC for the MPS3 upper head. In addition, the peening mitigation does not cause significantly increased crack growth rates for any pre-existing flaws deeper than the peening compressive residual stress zone, as the balancing tensile residual stress tends to be spread out over the remaining wall thickness [4, 5].
The effectiveness of follow-up examinations in the second RFO (N+2) compared to the third RFO (N+3) was evaluated in Dominion Engineering, Inc. Technical Note TN-4069-00-01, Revision 0 [8], which was submitted as Attachment 1 of Byron Station Unit 2, Relief Request I4R-16, dated August 30, 2018 [7]. Specifically, TN-4069-00-01 investigated inspection timing for RPV upper heads operating at reactor cold-leg temperature (Tcold) with a nominal 18 month fuel cycle to prevent through-wall cracking and pressure boundary leakage, based on the crack growth rate analysis already documented in MRP-335, Revision 3-A (which included various hypothetical initial crack sizes). The deterministic cases in the matrix for RPVHPNs operating at Tcold show that both the N+2 and N+3 inspection schedules result in an identically low fraction of cases with through-wall cracking, which demonstrates how the N+3 timing would ensure a similarly low likelihood of leakage. Thus, TN-4069-00-01, Revision 0 justifies that a N+3 post-peening follow-up examination is as effective as a N+2 follow-up examination in providing timely identification of previously undetected PWSCC flaws. DENC has reviewed and confirmed that this conclusion is also applicable to MPS3, which uses a nominal 18 month fuel cycle, and operates its RPV upper head at Tcold conditions.
ASME Code Case N-729-6 Item B4.20 [3] provides inspection requirements for PWR Reactor Vessel Upper Heads with Alloy 600 nozzles and Alloy 82/182 partial-penetration welds that have not been mitigated by peening. It bases the frequency of inspection, in part, on calculated EDY and RIY of the head. Each of these parameters is a function of the time and temperature history of the head.
Susceptibility to crack initiation is represented by the EDY parameter, and potential for crack propagation is represented by the RIY parameter. DENC performed a calculation of RIY and EDY for an unmitigated head using the methods outlined in Code Case N-729-6. DENCs EDY calculation for the MPS3 RPV upper head, which
Serial No.22-277 Docket No. 50-423 Attachment, Page 5 of 7 operates at Reactor Cold-leg Temperature (Tcold), produced a result below 8 EDY.
DENCs RIY calculation 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 possibility of PWSCC occurrence.
5.3 Maintaining Defense In Depth Under the proposed alternative, the required bare metal visual examination would still be performed each RFO including the fall 2023 outage. This examination for evidence of pressure boundary leakage would provide defense in depth in the unlikely event that leakage was to occur due to base metal or J-groove weld cracking or due to small flaws that are too shallow to be reliably detected in the pre-peening examination. The visual examination frequency ensures timely identification of through-wall cracking, before the development of conditions that may lead to substantial boric acid corrosion of the low-alloy steel RPV upper head.
The boric acid corrosion program for MPS3 uses visual inspections to detect the boric acid leakage source, path, and any targets of the leakage. This program ensures that boric acid corrosion is consistently identified, documented, evaluated, trended, and effectively repaired. The boric acid corrosion program provides both detection and analysis of leakage of borated water inside containment.
The Containment Atmosphere Particulate Radioactivity Monitoring System and the Containment Drain Sump Monitoring System are required to be available during the operating cycle to detect substantial leaks or increases in radiation levels within the MPS3 containment. If an unidentified reactor coolant system (RCS) leak is greater than 1 gpm, MPS3 Technical Specification (TS) 3.4.6.2 ("Reactor Coolant System Operational LEAKAGE) outlines the timely actions required to maintain safe unit operation. These detection methods and TS requirements provide early leak detection of the RCS and minimize the consequences associated with RCS leakage.
MPS3 also implements a 0.1 gpm action level on unidentified leakage, consistent with WCAP-16465-NP, Pressurized Water Reactor Owners Group Standard RCS Leakage Action Levels and Response Guidelines for Pressurized Water Reactors
[6].
5.4 Conclusions The lack of PWSCC indications for the MPS3 RPV upper head supports performing the post peening follow-up examination during the third RFO after peening. Since the peening application for MPS3 met or exceeded the MRP-335, Revision 3-A requirements, PWSCC growth of shallow, pre-existing flaws would likely be arrested.
In addition, peening mitigation does not cause significantly increased crack growth rates for any pre-existing flaws deeper than the peening compressive residual stress zone. On the basis of previously documented crack growth rate analysis, a follow-up UT examination in the third RFO post-peening is as effective as a follow-up
Serial No.22-277 Docket No. 50-423 Attachment, Page 6 of 7 examination in the second RFO post-peening in providing timely identification of previously undetected PWSCC flaws. The calculated reinspection frequency for unmitigated heads, which is conservative to apply for peened heads, supports deferral of the follow-up examination. Defense in depth is maintained through continued performance of the bare metal visual examination each refueling outage, boric acid corrosion control practices, and the plants online leak detection capabilities. Therefore, DENC concludes that performing RPVHPN volumetric examinations during the second and third RFOs after peening would present a hardship without a compensating increase in quality and safety and performing a single RPVHPN volumetric examination during the third RFO after peening would provide an acceptable alternative.
6. Duration of Proposed Alternative
The proposed one-time alternative is applicable to MPS3s fourth 10-year Inservice Inspection (ISI) interval, which began on February 23, 2019, and ends on February 22, 2029. After the volumetric examination is performed during the spring 2025 RFO, the approved frequency of in-service inspections per 10 CFR 50.55a(g)(6)(ii)(D) and MRP-335, Revision 3-A would resume.
7. Precedents
Plant Relief Request NRC Safety Approval Evaluation Date Byron Station Unit 2 ML18248A060 ML19035A294 02/25/2019 Braidwood Station Unit 2 ML17360A173 ML18162A184 06/04/2018 Byron Station Unit 2 ML20199M304 ML20245E506 09/20/2020 Braidwood Station Unit 1 ML21025A417 ML21063A016 03/08/2021
- 8. References
- 1. ASME Boiler and Pressure Vessel Code,Section XI, "Rules for lnservice Inspection of Nuclear Power Plant Components," 2013 Edition.
- 2. 10 CFR 50.55a Codes and Standards.
- 3. ASME Code Case N-729-6, "Alternative Examination Requirements for PWR Reactor Vessel Upper Heads With Nozzles Having Pressure-Retaining Partial-Penetration Welds,Section XI, Division. 1," approved March 3, 2016.
- 4. Materials Reliability Program: Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement (MRP-335, Revision 3-A),
EPRI, Palo Alto, CA: 2016, 3002009241.
- 5. Materials Reliability Program: Technical Basis for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement (MRP-267 Revision 2), EPRI, Palo Alto, CA: 2016, 3002008083.
Serial No.22-277 Docket No. 50-423 Attachment, Page 7 of 7
- 6. WCAP-16465-NP, Revision 0, "Pressurized Water Reactor Owners Group Standard RCS Leakage Action Levels and Responses Guidelines for Pressurized Water Reactors," September 2006. [NRC ADAMS Accession No. ML070310082].
- 7. Exelon Generation Company, LLC, Byron Station Unit 2, Relief Request I4R-16, Request for Alternative Follow-up Inspection for Reactor Pressure Vessel Head Penetration Nozzles with Mitigated Alloy 600/82/182 Peened Surfaces in Accordance with 10 CFR 50.55a(z)(2), dated August 30, 2018. [NRC ADAMS Accession No. ML18248A060].
- 8. Technical Note TN-4069-00-01, Revision 0, "MRP-335 R3-A Matrix of Deterministic Crack Growth Calculations for Tcold Reactor Vessel Top Head Nozzles Evaluated for Alternative Peening Follow-up Volumetric Examination Timing," Dominion Engineering, Inc., Reston VA, August 2018. (Attachment 1 of Byron Station Unit 2, Relief Request I4R-16).