ML20216H009
| ML20216H009 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 03/13/1998 |
| From: | Sorensen J NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML20216H015 | List: |
| References | |
| GL-95-05, GL-95-5, NUDOCS 9803200154 | |
| Download: ML20216H009 (3) | |
Text
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Northern States Power Company i
Prairie Island Nuclear Generating Plant 1717 Wakonade Dr. East Welch, Minnesota 55089 J
i March 13,1998 Generic Letter 95-05 U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Unit 1, Cycle 19 Steam Generntor Tube Support Plate Voltage Based Repair Criteria 90-Day Report During the Unit 1 October 1997 Refueling Outage following operating cycle 18, the Voltage Based Repair Criteria for ODSCC at Tube Support Plates (TSPs) was implemented in accordance with the requirements of NRC Generic Letter (GL) 95-05 and the Safety Evaluation by the Office of Nuclear Reactor Regulation Related to Amendment Nos.133 and 125 to Facility Operating License Nos. DPR-42 and DPR-60 for NSP's Prairia Island Nuclear Generating Plant, dated November 18,1997, " Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking". This SG tube repair criteria allows bobbin coil indications of less than 2 volts at TSPs to remain in service providing the total leakage under postulated Main Steam Line Break (MSLB) conditions is less than 1.0 GPM at room temperature and provided the End of Cycle (EOC) conditional probability of burst is less than 1 X 10 2 Four intersections from three SG tube pulls were tested for leakage, pressurized to burst, and destructively examined. None of the intersections showed leakage at MSLB conditions. Allintemections burst at pressures greater than 10,000 psig and (when normalized to 68.78 ksi flow stress) were above the predicted mean burst pressures.
The results of the destructive examination show that the degradation morphology of all j
the intersections contains intergranular corrosion confined to the TSP crevice region f
which is consistent with the morphology required by GL 95-05 for application of the
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1 USNRC NORTHERN STATES POWER COMPANY March 13,1998 Page 2 voltage based repair criteria. Two of the intersections passed all of the exclusion criteria for addition to the EPRI database for the voltage based repair criteria.
One TSP intersection also included a zone of transgrannular wastage with a maximum depth of 8% separate from the intergranular attack within the crevice region. The eddy current indication for this intersection was characterized as volumetric in nature. Since it was not certain that the volumetric eddy current indications at tube support plates were in compliance with Technical Specification 4.12.D.4.a. all of the tubes with volumetric indications not associated with qualified sizing techniques were plugged.
Since the mixed mode degradation identified by the tube pull from Prairie Island was not explicitly evaluated in GL 95-05, Prairie Island achieved compliance for this outage by examination of all but two of the distorted tube support plate bobbin coil indications (DSis) with rotating coil probes. The two tubes with DSis not inspected with rotating coil probes were plugged. The intersections with volumetric indications are conservatively included in the leakage and burst calculations.
The maximum voltage DSis were 1.6 volts in the 11 SG and 1.74 volts in the 12 SG.
1 The combined population average volta 0e growth rate for cycle 18 was 0.014 volts per EFPY (2.7%).
The predicted leak rates for EOC-19 under Main Steam Line Break conditions are
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0.064 gpm for the 11 SG and 0.055 gpm for the 12 SG. These rates are less than the j
allowable leak rate of 1 gpm. All leak rates are at room temperature.
The predicted conditional probabilities of burst under MSLB conditions are less than 1.90X10~5 for the 11 SG and 2.52X10~5 for the 12 SG. These values are less than the allowable value of 1X10 2,
The analyses performed used the latest database for ODSCC at TSPs which included the data from the 1997 plant A-1 and A-2 pulled tubes. The leakage calculation used the 7/8 inch tubing leak rate to bobbin coil voltage correlation as agreed to by the NRC in January,1998.
During the outage, none of the conditions of Technical Specification 4.12.E.5 (section 6.a of GL 95-05) arose which require notification of the NRC staff prior to returning the steam generators to service. That is:
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- a. the projected end-of-cycle leakage did not exceed the 1 gpm limit,
- b. no circumferential crack-like indications were detected at the tube support plate intersections REPORT. DOC
.in USNRC NORTHERN STATES POWER COMPANY March 13,1998 Page 3
- c. no indications extended beyond the confines of the tube support plai intersections
- d. no indications at the tube support plate elevations were attributable to primary water stress corrosion cracking
- e. the calculated conditional burst probability did not exceed 1X10-2 The Upper Repair Limit of 4.91 volts (established during the outage) used a conservative cycle length of 565.5 EFPD, a growth rate of 30% per EFPY and an NDE measurement uncertainty factor of 20%.
The attached Westinghouse Report SG-98-03-002, dated March 1998, provides the first 90-day report following initialimplementation of the voltage based repair criteria at Prairie Island Unit 1. This report fulfills the requirements of Generic Letter 95-05 Section 6.b.
In this letter we have made no new Nuclear Regulatory Commission commitments.
Please contact Richard Pearson (612-388-1121) if you have any questions related to this letter.
MW Joel P Sorensen Plant Manager Prairie Island Nuclear Generating Plant c: Regional Administrator - Region Ill, NRC Senior Resident inspector, NRC NRR Project Manager, NRC J E Silberg -
Attachments.
- 1. Westinghouse Report SG-98-03-002," Prairie Island Unit 1 Cycle-19 Voltage Based TSP Altemate Repair Criteria 90-Day Report."
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