ML12128A415

Summary of Conference Telephone Call Regarding the 2012 Steam Generator Tube Inspections

ML12128A415
Person / Time
Site:	Prairie Island
Issue date:	05/14/2012
From:	Thomas Wengert Plant Licensing Branch III
To:	Schimmel M Northern States Power Co
Wengert T
References
TAC ME7371
Download: ML12128A415 (4)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 May 14, 2012 Mr. Mark A. Schimmel Site Vice President Prairie Island Nuclear Generating Plant Northern States Power Company - Minnesota 1717 Wakonade Drive East Welch, MN 55089-9642

SUBJECT:

PRAIRIE ISLAND NUCLEAR GENERATING PLANT, UNIT 2 -

SUMMARY

OF CONFERENCE TELEPHONE CALL REGARDING THE 2012 STEAM GENERATOR TUBE INSPECTIONS (TAC NO. ME7371)

Dear Mr. Schimmel:

On March 14,2012, the U.S. Nuclear Regulatory Commission (NRC) staff participated in a conference call with Prairie Island Nuclear Generating Plant, Unit No. 2 representatives regarding the ongoing steam generator (SG) tube inspection activities conducted during their 2012 outage. The NRC follows the results of the industry's SG inspections in order to maintain an awareness of the condition of the SGs and the types of tube degradation mechanisms that are active.

The enclosed documentation of the phone call is provided to Northern States Power Company Minnesota (NSPM) for information. The slides provided by NSPM in support of this discussion (Agencywide Documents Access and Management System (ADAMS) Accession No. ML120790479) are available to the public.

Based on the information provided during the conference call, the NRC staff did not identify any issues that warranted additional follow-up action at this time. If there are any questions, please contact me at 301-415-4037.

Sincerely, Thomas J. Wengert, Senior Plant Licensing Branch 111-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-306

Enclosure:

Conference Call Summary cc w/encl: Distribution via ListServ

SUMMARY

OF CONFERENCE CALL WITH NORTHERN STATES POWER COMPANY - MINNESOTA REGARDING THE SPRING 2012 STEAM GENERATOR TUBE INSPECTION RESULTS PRAIRIE ISLAND NUCLEAR GENERATING PLANT UNIT 2 DOCKET NUMBER 50-306 On March 14,2012, the U.S. Nuclear regulatory Commission (NRC) staff participated in a conference call with representatives of Northern States Power Company, a Minnesota corporation (the licensee), regarding the ongoing steam generator (SG) tube inspection activities at Prairie Island Nuclear Generating Plant (PINGP) Unit 2. In support of the conference call, the licensee provided a document summarizing some of the inspection results.

This document is located in the NRC's (Agencywide Documents Access and Management System (ADAMS) at Accession No. ML120790479).

PINGP Unit 2 has two Westinghouse model 51 SGs. Each SG contains 3388 mill-annealed Alloy 600 tubes. Each tube has a nominal outside diameter of 0.875 inches and a nominal wall thickness of 0.050 inches. The tubes were roll-expanded into the tubesheet at both ends for approximately 2.75 inches, i.e., they are expanded for only a fraction of the tubesheet thickness and are considered partial depth hard-rolled tubes. The tubes are supported by a number of carbon steel tube support plates. The original anti-vibration bars were removed and replaced.

The row one and two tubes were subjected to in-situ thermal stress relief in May 2000. Many tubes have been roll-expanded into the tubesheet above the original factory roll expansion to permit defects below these re-rolled locations and allow the tubes to remain in service.

In addition to the depth-based tube repair criteria, the licensee is also authorized to apply the voltage-based tube repair criteria for predominantly aXially-oriented outside diameter stress corrosion cracking (ODSCC) at the tube support plate elevations. Although authorized to implement the voltage-based repair criteria, the licensee has not found it necessary to implement these criteria since few, if any, indications subject to this repair criteria have been identified at Unit 2. In addition, the licensee is authorized to leave flaws within the tubesheet region in service provided they satisfy the F*/EF* repair criterion. The major cause of degradation within the tubesheet region is primary water stress corrosion cracking at the roll transition zones. Secondary side intergranular attack and ODSCC have also been observed at this location.

Information provided by the licensee is summarized below:

• The licensee stated that the primary-to-secondary leakage was slightly elevated during the last operating cycle, as compared to leakage over the last three cycles. The licensee reported that SG 21 and SG 22 had steady state leakage that ranged from 0.5 gallons per day (gpd) to 3.5 gpd and from 0.5 gpd to 4.0 gpd, respectively.

• The licensee planned to perform a secondary side pressure test due to the elevated primary-to-secondary leakage. At the time of the call this test had not been performed.

Enclosure

-2

• The licensee identified a circumferential outer diameter stress corrosion crack at a location 0.25 inches below the top-of-tubesheet. This indication is located near a dent with amplitude of 11.26 volts.

• At the time of the call the licensee did not plan to perform any tube pulls or in-situ pressure testing.

The following abbreviations are used in the document provided by the licensee:

=

ADR Absolute Drift

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CUD Copper Deposit

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DEP Deposit

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DNI Dent with Indication

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TSP Tube Support Plate

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DNT Dent

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DRI Distorted Roll transition with Indication

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DSI Distorted Support signal with Indication

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DTI Distorted Tubesheet signal with Indication

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INR Indication Not Reportable

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MBM Manufacturing Burnish Mark

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NQI Non-Quantifiable Indication PLP = Possible Loose Part

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MRPC Motorized Rotating Probe Coil

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PSI Possible Support Indication (not in the tube, in the support plate)

The NRC staff did not identify any issues that required follow-up action at this time, however, the staff asked to be notified in the event that any unusual conditions were detected during the remainder of the outage.

ML12128A415 Slides Accession No. ML120790479 *Bsy memo dated 5/3112 NRR 106 OFFICE NRR/LPL3-1/PM NRR/LPL3-1/LA NRR/DE/ESGB/BC NRR/LPL3-1/BC (A)

NAME TWengert BTuily GKulesa* IFrankllPTam for DATE 05/10/12 05/10/12 05/03/12 05/14/12