L-PI-06-103, Steam Generator Category C-3 Inspection Results 30-Day Report

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Steam Generator Category C-3 Inspection Results 30-Day Report
ML063600314
Person / Time
Site: Prairie Island Xcel Energy icon.png
Issue date: 12/26/2006
From: Thomas J. Palmisano
Nuclear Management Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
L-PI-06-103
Download: ML063600314 (4)


Text

N Committed to Nuclear Excellence Prairie lsland Nuclear Generating Plant Operated by Nuclear Management Company, LLC DEC 2 6 2006 L-PI-06-103 Technical Specification 5.6.7.3 U S Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Prairie lsland Nuclear Generating Plant Unit 2 Docket 50-306 License No. DPR-60 2006 Unit 2 Steam Generator Categorv C-3 lnspection Results 30-Dav Report In accordance with Technical Specification 5.6.7.3 this special report due to Category C-3 inspection results of the Unit 2 steam generator tubing is provided for the information of the NRC Staff.

The results of the inspection of 21 Steam Generator and 22 Steam Generator were classified as Category C-3 in accordance with Technical Specification 5.6.7.3 because more than 1% of the inspected tubes in each Steam Generator were defective. The NRC Staff was informed of the Category C-3 classification by telephone on November 27, 2006. In accordance with Technical Specification 5.6.7.3, the 30-day special report on the Category C-3 steam generator inspection results is provided as Enclosure 1 (Prairie lsland Unit 2 Steam Generator Category C-3 Tube lnspection Special Report) to this letter.

Summar- of Commitments This letter contains no new commitments and no revisions to existing commitments.

~ h o m a sJ. Palmisano Site Vice President, Prairie lsland Nuclear Generating Plant Nuclear Management Company, LLC 1717 Wakonade Drive East Welch, Minnesota 55089-9642 Telephone: 651.388.1121

Document Control Desk Page 2 Enclosure (1) cc: Administrator, Region Ill, USNRC Project Manager, Prairie Island, USNRC Resident Inspector, Prairie Island, USNRC

ENCLOSURE 1 2006 UNIT 2 STEAM GENERATOR CATEGORY C-3 INSPECTION RESULTS 30-DAY REPORT Purpose This report fulfills the special reporting requirements of Prairie Island Technical Specification 5.6.7.3. This report is required whenever the steam generator tube inservice inspection finds more than 10% of the total tubes inspected are degraded tubes or more than 1% of the inspected tubes are defective. This report summarizes the inspection results, the investigation into causes of major tube degradation and corrective measures. Corrective measures to prevent recurrence of Category C-3 inspections are discussed. Steam generator inspection results continue to exceed the category C-3 limits, so corrective measures do not prevent recurrence. However, careful inspections and repairs coupled with chemistry controls and low operating temperature provide assurance of safe and reliable operation of Unit 2 steam generators.

Summary of Inspection Results The inservice inspection for Unit 2 Steam Generators occurred from November 21, 2006 through November 27, 2006. The inservice inspection consisted of inspection of 100% of the full length of tubing with the bobbin coil (except the bend portion of rows 1 through 4 u-bends), 100% of the hot leg tubesheet region, 25% of the hot leg Alloy 690 roll plugs, 100% of the rows 1 through 4 u-bends, 33% of the rows 5 through 8 u-bends and 20% of the cold leg tubesheet region with mechanical rotating probes with +pointTM coil.

As a result of the eddy current inspections, 13.3% (419 of 3146) of the inspected tubes in 21 Steam Generator contained defects requiring repair. Nineteen of these tubes were plugged and the remaining 400 tubes were left in service using the F* and EF*

repair criteria.

As a result of the eddy current inspections, 4.9% (154 of 3130) of the inspected tubes in 22 Steam Generator contained defects requiring repair. Eight of these tubes were plugged and the remaining 146 were left in service using the F* and EF* repair criteria.

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Investigation into Causes of Major Tube Degradation There are two major causes of tube degradation in Unit 2 steam generators:

Secondary side intergranular attack and stress corrosion cracking and Primary water stress corrosion cracking.

Secondary side intergranular attack and stress corrosion cracking (IGAfSCC or ODSCC) is occurring in the hot leg tubesheet crevice region and at the top of the hot leg tubesheet.

This was confirmed by metallurgical examination of three tube samples removed from the original Steam Generator 12 in January 1985. This was also confirmed by examination of a parent tube section removed during the sleeve pulls in the original Steam Generator 12 in 1996. In addition, three tubes were removed from Unit 1 for Generic Letter 95-05 Voltage Based Repair Criteria in 1997 and ODSCC was confirmed at the hot leg tube support plates.

Primary water stress corrosion cracking (PWSCC) at the roll transition region has been confirmed by metallurgical examination of one roll transition zone removed during sleeve pulls in the original Steam Generator 12 in 1996.

Corrective Measures Prairie lsland participates in utility funded research on steam generator related issues.

Corrective measures to reduce andlor prevent tube degradation due to PWSCC and secondary side IGAISCC have been used by the industry with limited success. Prairie lsland corrective measures include:

Chemistry Control: Prairie lsland follows both the original equipment manufacturer's water chemistry guidelines and the Electric Power Research Institute secondary water chemistry guidelines. The PWSCC degradation appears to be relatively independent of chemistry and occurs in regions of high residual stress.

High Hvdrazine Control: Prairie lsland maintains a hydrazine control band of eight times Condensate Oxygen Level to 20 ppb (parts per billion) in the feedwater system.

Feedwater DH Control - Alternate Amine: Prairie lsland maintains MPA (3-methoxypropylamine) at seven to nine ppm (parts per million) in the steam generator blowdown.

Molar ratio control to reduce secondary side corrosion: Molar ratio control has been attempted by adjustments to steam generator blowdown resin ratios.

On-line addition of Boric Acid: Prairie lsland uses on-line addition of boric acid to Unit 2 steam generators.

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