BYRON 2022-0030, Materials Reliability Program: Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement (MRP-335, Revision 3-A) Report

Materials Reliability Program: Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement (MRP-335, Revision 3-A) Report

ML22123A219
Person / Time
Site:	Byron
Issue date:	05/03/2022
From:	Kowalski J Constellation Energy Generation
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
BYRON 2022-0030
Download: ML22123A219 (5)

Text

Constellation Energy Generation. LLC (CEG)

Byron Station Constellation . 4450 N. German Church Road Byron. IL 61010-9794 www.constellationenergy.com May 3, 2022 LTR: BYRON 2022-0030 File: 1.10.0101 (1 D.101)

United States Nuclear Regulatory Commission ATTN : Document Control Desk Washington, DC 20555-0001 Byron Station, Unit 2 Renewed Facility Operating License No. NPF-66 NRC Docket No. STN 50-455

Subject:

Materials Reliability Program: Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement (MRP-335, Revision 3-A) report

Reference:

Event Notification 55857: Ultrasonic Examination Results - Reactor Vessel Head Penetration, Event Date April 23, 2022 In accordance with MRP-335, Revision 3-A, "Materials Reliability Program: Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement,"

Constellation Energy Generation, LLC (CEG) is providing the report required per section 5.2 for Byron Station, Unit 2 (Byron).

While performing volumetric inspections required by ASME Code Case N-729-6, a rejectable indication on Reactor Vessel Head Penetration 75 Core Exit Thermocouple (CETC) was identified. The indication is located inboard of the J-groove weld and is outside diameter-initiated in an area that was not surface stress mitigated (peened) as documented in the Reference above. MRP-335, revision 3-A section 5.2 requires the following for this indication:

If a wetted surface-connected flaw, an unacceptable flaw based on the ASME Code,Section XI, or unacceptable flaw growth is observed in a peened DMW, RPVHPN, or J-groove weld, (a) ,a report summarizing the evaluation, including (nputs, methodologies, assumptions, extent of conditions, and causes of the new flaw, unacceptable flaw, or flaw growth, must be submitted to the NRC prior to the plant entering into Mode 4.

LTR: Byron 2022-0030 May 3, 2022 U.S . Nuclear Regulatory Commission Page 2 The attached report will provide the above information.

There are no regulatory commitments contained in this letter. Should you have any questions concerning this letter, please contact Ms. Zoe Cox, Regulatory Assurance Manager at (815) 406-2800.

John J. Kowalski Site Vice President Byron Generating Station JJK/PM/ZC/sg

Attachment:

MRP-335, revision 3-A Report cc: NRC Regional Administrator, Region Ill NRC Senior Resident Inspector, Byron Station

Attachment:

MRP-335, revision 3-A Report Report Summarizing the evaluation, including inputs, methodologies, assumptions, extent of conditions, and causes of the new flaw, unacceptable flaw, or flaw growth pursuant of the Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement (MRP-335, Revision 3-A) Report.

During the spring 2022 refueling outage at Byron Station Unit 2 (B2R23), the in-service inspection of the Alloy 600 reactor vessel penetration nozzles such as Control Rod Drive Mechanism (CROM) and Core Exit Thermocouple (CETC) nozzle penetrations has revealed one indication at Penetration 75, which is a CETC nozzle as shown in Figure 1-1. This indication is located at 184 degrees with a length of 0.197" with a depth of 0.141" from the outer diameter (OD) surface of the CETC penetration and is outside of the peened area. The indication extends from 1.498" to 1.695" from the end of the nozzle and exhibits characteristics of typical PWSCC. The indication is axially oriented and is at the location of one of the funnel fillet welds. The cracking mechanism has been attributed to primary water stress corrosion cracking (PWSCC).

Constellation elected to evaluate the flaw rather than attempt flaw removal during B2R23. A flaw growth evaluation was conducted in accordance with Section -3132.3 of Code Case N-729-6 and IWB-3660 of ASME Section XI, 2007 Edition with 2008 Addenda. The evaluation methodology used was based on the PWSCC crack-growth formula given in MRP-420 Rev 1. The stress intensity factor solution methodology used was Framatome's proprietary weight function method. Flaw growth was projected in both the axial and circumferential directions. PWSCC crack growth due to residual and normal operating hoop and axial stresses were considered. The residual stresses were conservatively taken as those determined for the j-groove weld region prior to Ultra-High Pressure Cavitation peening.

Hydrogen concentration was conservatively assumed to be 25 cc/kg. Fatigue was assumed to contribute insignificantly to crack growth since this portion of the nozzle is exposed to RCS water on both sides which prevents significant transient thermal stress from developing during RCS temperature transients.

Using this methodology, flaw growth was calculated iteratively using a time step of one week. The result of this process was that the flaw grows to a length of 1.531 inches and a depth of 0.625 inch (100% wall thickness) in 5.63 effective full power years (EFPY). The evaluation was stopped at this point because Table IWB-3663-1 limits flaw depth to 100% of the nozzle wall thickness. At this point, the crack would still be 4.1 inches below the j-groove weld.

Constellation performed a full volumetric inspection consistent with Code Case N-729-6 during B2R23, no additional indications were found, and the extent of condition was limited to Penetration 75.

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