L-PI-05-055, 2005 Unit 2 Steam Generator Category C-3 Inspection Results 30-Day Report

2005 Unit 2 Steam Generator Category C-3 Inspection Results 30-Day Report

ML051740093
Person / Time
Site:	Prairie Island
Issue date:	06/22/2005
From:	Solymossy J Nuclear Management Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
L-PI-05-055
Download: ML051740093 (4)

Text

Prairie lsland Nuclear Generating Plant Operated by Nuclear Management Company, LLC 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 2005 Unit 2 Steam Generator Cateqorv 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 5.5.8 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 May 17, 2005.

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 lnspection Results 30-Day Report) to this letter.

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

>kiIqf seph M. Solymos Site Vice President, Prairie lsland Nuclear Generating Plant Nuclear Management Company, LLC Enclosure 1717 Wakonade Drive East Welch, Minnesota 55089-9642 Telephone: 651.388.1 121

Document Control Desk Page 2 cc: Administrator, Region Ill, USNRC Project Manager, Prairie Island, USNRC Resident Inspector, Prairie Island, USNRC Minnesota, Department of Commerce

ENCLOSURE 1 2005 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 May 10, 2005 through May 16, 2005. The inservice inspection consisted of inspection of 100% of the full length of tubing with the bobbin coil (except 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 and 33% of the rows 5 through 8 u-bends with mechanical rotating probes with +pointTMcoil.

As a result of the eddy current inspections, 6.0% (191 of 3158) of the inspected tubes in 21 Steam Generator contained defects requiring repair. 12 of these tubes were plugged and the remaining 179 tubes were left in service using the F* and EF* repair criteria.

As a result of the eddy current inspections, 4.8% (150 of 3138) of the inspected tubes in 22 Steam Generator contained defects requiring repair. Eight of these tubes were plugged and the remaining 142 were left in service using the F* repair criteria.

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 (IGAISCC 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 Page 1 of 2

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 IGAlSCC have been used by the industry with limited success. Prairie lsland corrective measures include:

Chemistrv 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.

Hish 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 pH 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 secondarv 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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