ML20192A321

Review of the Fall 2019 Steam Generator Tube Inspection Report

ML20192A321
Person / Time
Site:	Oconee
Issue date:	07/13/2020
From:	Shawn Williams Plant Licensing Branch II
To:	Burchfield J Duke Energy Carolinas
Williams S
References
EPID L-2020-LRO-0005
Download: ML20192A321 (4)

Text

July 13, 2020 Mr. J. Ed Burchfield, Jr.

Site Vice President Oconee Nuclear Station Duke Energy Carolinas, LLC 7800 Rochester Highway Seneca, SC 29672-0752

SUBJECT:

OCONEE NUCLEAR STATION, UNIT 2 - REVIEW OF THE FALL 2019 STEAM GENERATOR TUBE INSPECTION REPORT (EPID: L-2020-LRO-0005)

Dear Mr. Burchfield:

By letter dated March 10, 2020, Duke Energy (the licensee) submitted information summarizing the results of the fall 2019 steam generator inspections performed at Oconee Nuclear Station, Unit 2. The inspections were performed during refueling outage 29 (O2R29). The licensee provided additional information concerning the inspections in a letter dated June 18, 2020.

The NRC staff has completed its review of the report and concludes that the licensee provided the information required by their technical specifications and that no additional follow-up is required at this time. The NRC staffs review is enclosed.

If you have any questions, please contact me at 301-415-1009 or via e-mail at Shawn.Williams@nrc.gov.

Sincerely,

/RA/

Shawn A. Williams, Senior Project Manager Plant Licensing Branch II-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-270

Enclosure:

As Stated cc: Listserv

OCONEE NUCLEAR STATION, UNIT 2 NRC STAFF REVIEW OF THE FALL 2019 STEAM GENERATOR TUBE INSPECTION REPORT DOCKET NO. 50-270 By letter dated March 10, 2020 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML20070H575), Duke Energy (the licensee) submitted information summarizing the results of the fall 2019 steam generator (SG) inspections performed at Oconee Nuclear Station, Unit 2. The inspections were performed during refueling outage 29 (O2R29).

The licensee provided additional information concerning the inspections in a letter dated June 18, 2020 (ADAMS Accession No. ML20170A475).

Oconee, Unit 2, has two replacement once-through steam generators (ROTSGs) designed and fabricated by Babcock and Wilcox International. Each ROTSG has 15,631 thermally-treated Alloy 690 tubes with a nominal outside diameter of 0.625 inches and a nominal wall thickness of 0.038 inches. The tubes were hydraulically expanded for 13 inches from the tube end into the 22-inch thick tubesheet. Tube support is provided by 15 Type 410 stainless steel tube support plates (TSPs) of trifoil broach design; however, there are some round drilled openings at the 14th TSP.

The licensee provided the scope, extent, methods, and results of the SG tube inspections in the letters referenced above. In addition, the licensee described corrective actions (e.g., tube plugging) taken in response to the inspection findings.

Based on the review of the information provided, the U.S. Nuclear Regulatory Commission (NRC) staff has the following observations:

During O2R29, the licensee implemented two changes affecting the depths and quantities of TSP wear indications reported. Specifically, the depth reporting threshold was changed from 5 percent through-wall to 8 percent through-wall to eliminate reporting signals due to tube noise or mix residual as opposed to true tube wear, and a fixed curve for bobbin depth sizing was implemented. The licensee reported that some calibration groups were either overestimating or underestimating growth rates and this variation was due to using different calibration standards from the previous inspection. In an effort to normalize the variation, a fixed calibration curve was developed by obtaining a normalization for each standard.

Depths were assigned to the various voltage readings to develop a depth versus voltage curve. The sizing calibration curve was established using three points from the depth versus voltage curve.

During O2R29, the licensee assigned a Historical No Change (HNC) code to 31 and 20 historical tube to tube wear (TTW) indications in the 2A and 2B SGs, respectively, because they had been previously inspected at least three times with array probe and there was no change in the bobbin probe signal. The licensee compared the current outage bobbin data to the first outage bobbin data, (the first outage the indication was reported), or the pre-service inspection data to determine if the bobbin probe signal had changed. Specifically, any indication that exhibited a change by > 0.50 volts or > minus 10 degrees was Enclosure

considered to have changed. The licensee reported that the 31 and 20 HNC indications in the 2A and 2B SGs, respectively, were not depth sized because they previously had ample margin to the structural integrity performance criteria and there has been no change in the bobbin probe signals, and that the largest array probe depth from the 2017 data was 17 percent through-wall. The 25 TTW indications that did not meet the previously stated criteria to be assigned an HNC code were depth sized with array probe and the maximum depth was reported as 12 percent through-wall.

The licensee clarified that tube R91C115 in the 2A SG was incorrectly reported in the table of tubes plugged during O2R29. Tube R91C126 in the 2A SG was plugged due to TSP wear of 41 percent through-wall.

Based on a review of the information provided, the NRC staff concludes that the licensee provided the information required by their technical specifications. In addition, the NRC staff concludes that there are no technical issues that warrant additional follow-up action at this time since the inspections appear to be consistent with the objective of detecting potential tube degradation and the inspection results appear to be consistent with industry operating experience at similarly designed and operated units.

ML20192A321 *via email OFFICE DORL/LPL2-1/PM DORL/LPL2-1/LA* DNRL/NCSG/BC* DORL/LPL2-1/BC*

NAME SWilliams KGoldstein SBloom MMarkley (EMiller for)

DATE 7/9/2020 07/13/2020 7/7/2020 7/13/2020 OFFICE DORL/LPL2-1/PM NAME SWilliams DATE 7/13/2020