NRC-91-0011, Informs NRC That Insp of Number 3 Low Pressure Turbine Rotor Disc Performed During Recent Turbine Outage,Per Licence Condition 2.C.11.Damage to Turbine Blades on Fourth Stage of Number 3 Low Pressure Turbine Also Discussed
| ML20067C413 | |
| Person / Time | |
|---|---|
| Site: | Fermi  |
| Issue date: | 02/07/1991 |
| From: | Orser W DETROIT EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| CON-NRC-91-0011, CON-NRC-91-11 TAC-59614, NUDOCS 9102120060 | |
| Download: ML20067C413 (3) | |
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February 7,1991 NRC-91-0011 U. S. Nuclear Regulatory Commission Attnt Document Centrol Desk Washingt on, D. C.
20555 References
- 1) Fermi 2 NRC Docket No. 50-341 NRC License No. NPF-43
- 2) Detroit Edison letter, EF2-53,823 dated June 22, 1981
- 3) Femi 2 Safety Evaluation Report Section 10.2.2, NUREG-0798, July 1981
- 4) Detroit Edison letter, NRC-90-0135, dated August 17, 1990 Subject Inspection of Low Pressure Turbine Discs (License Condition 2.C.11, TAC 59614)
License Condition 2.C.11 of the Fermi 2 Operating License requires that Detroit Edison' perf orm an inspection of the lov pressure turbine disce during the second refueling outage, including volumetric examination of the disc base using ultrasonic tec' nicues.
It also states that the frequency of subsequent inspectir,t; shall be in accordance with the turbine manuf acturer's reconsteadations. The purpose of this letter is to inform the Nuclear Regulatory Cceimission that the inspection of the number three low pressure tntbine (No. 3 LP) rotor dise was performed during the recent turbine outage, thus meeting the license condition for this turbine. This letter also addresses the damage to the turbine blades on the 4th stage of the No. 3 LP turbine. The blade damage is not associated with the potential concern for which the license condition was established. Mr.
John Stang, NRC Project Manager, requested that the latter topic be addressed.
The purpose of the license condition is to monitor any creck growth at the disc base area. Low pressure turbine discs are subjected to relatively high stresces.
In some turbires, stress corrosion problems have occurred, with creks initiating.it the inner bore area, especially in keyways and other opcoings, and then propagating outward. To prevent disc fe11ure, the turbino discs are forged by processes that minimize flaws and provide adequate f racture toughness 0[
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i USNRC February 7, 1991 Nnc-91-0011 Page 2 to ensure their integrity. Additionally, conservative inspection intervals are established to detect and monitor any crach growth to permit repair or replacement of discs well in advance of a postulated failure.
In Reference 2. Detroit Edison committed to perform a volumetric examination of the low pressure turbines' disc bore area using ultrasonic techniques during the second refueling outage.
In the Fermi 2 Safety Evaluation Report (Reference 3), the NRC indicated they had performed their own crack growth calculations to determine when the turbine should be inspected and concluded that the disco should be inspected during the second refueling outage. The SER also noted that Detroit Edison had committed to perform an inspection during the second refueling outage and that the NRC would include this inspection as a licenne condition. This section concludes that the materials, processes, design, and planned volumetric inspection provide reasonable assurance that the probability of disc f ailure with missile generation is low during normal operation, which includes transients up to design overspeed.
Since the purpcoe of the inspection is to detect any cracks and monitor their growth so that corrective action be raken before failure, inspecting at intervals shorter than originally specified or calculated, as needed, is conservative. Therefore, the ultrasonic examination performed on the No. 3 LP turbine disc bore area during the recent outage fulfills the license condition. No indications of stress corrosion cracking were found on or adjacent to the disc bores or the dowel holes. The same f avorable results were cbtained during the inspection of the No. 2 LP turbine disc bore area, as discussed in Reference 4 The No.1 LP turbice disc remains to be inspected during the second refueling outage, scheduled for spring 1991.
Subsequent inspections for all three low pressure turbines will be performed at a f requency recommended by the turbine manuf acturer to ensure any future cracking will be detected.
During the recent turbine outage, damage was observed to the shroud, blades and blade support area of the 4th stages of the No. 3 LP turbine. A total of 5 blades were broken of f and there were 9 indications on the disc in the blade support area. The indications are repairable by existing conventional means. As a temporary t easure, until new blades can be obtained and installed, the 4th stage T3 blades were removad. A pressure reducing plate was installed in both flow paths for the No. 3 LP turbine in place of the 4th stage. The indications in the blade support area will be repaired when the blades are repisced.
The cause of the blade failures was fatigue, not stress corrosion cracking. The cracks initiated in the roots of the individual blades. Therefore, the integrity and strength of the disc is not reduced. The primary concern of disc failure is the generation of
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6 USNRC February 7, 1991 NRC-91-0011 Page 3 large missiles able to penetrate the turbine casing. The blades that f ailed did not penetrate the turbine casing.
Individual blades are not able to penetrate turbine casings. Therefore, the observed blade failures do not provide a penetrating missile Eeneration risk and have no association with '.. e basis f or the license condition. Actions are being taken in response to the blade failure as were discussed in the December 17, 1990 management meeting.
11 there are any questions or additional inf ormation needed on the results of the No. 3 LP turbine ultrasonic examination of the rotor bore area, please contact Lynne S. Goodman at (313) 586-4211.
Sincerely, av cci A. B. Davis R. W. DeFayette W. G. Rogers J. F. Stang
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