ML20005B755
| ML20005B755 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 08/19/1981 |
| From: | Stolz J Office of Nuclear Reactor Regulation |
| To: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| Shared Package | |
| ML17212A650 | List: |
| References | |
| NUDOCS 8109020083 | |
| Download: ML20005B755 (2) | |
Text
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DISTRIBUTION:
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NRC' PDR
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Dear ffr. flattiroe:
SUBJECT:
TURBINE DISC CRACKI!!G By letter dated February 25, 1980 we infomed all ifcensee/ users of Meri.ing-house low-pressure turbines that stress corrosion cracks were being found in the keyway and bore areas of low-pressure discs. Because these cracks were considered to increase the probability of disc failure we requested that you perfom ultrasonic inspections on your low-pressure discs and justify that yone plant could continue to operate safely.
All Westinqhouse low-pressure turbines at operating nuclear power plants have now been inspected, at least once, for keyway and bore cracks.
Indica-tion of one or both types of these cracks has been found at 20 plants. Al-though all factors related to cracking have not been positively established, onorating experience indicates that crack initiation and growth are related to disc te'perature and ratorial characteristics. Westinghouse is continuing to evaluate the effect of other ranufacturing and operational variables.
Until a satisfactory solution can be found we believe that it would be pru-dent for you to continue inspccting your low-pressure turbine discs on a schedule designed to minimize the probability that a crack will form and l
grow to a depth that would cause a disc to rupture. Westinghouse has deve-loped a rethod to detemine safe inspection and re-inspection frequencies and has submitted this information in Memorandum MSTG-1-7, June 1981 (Pro-prietary) for review by the NRC staff.
Our appraisal of the Hestinghouse aoproach is presented in the enclosed l
Safety Evaluation. Ve conclude that inspection schedules based on the reco-rnendations in the Mestinghouse Memorandum will provide an acceptably high degree of assurance that discs will be inspected before cracks can i
grow to a size that could cause disc failure at speeds up to design speed.
l In our Safety Evaluation we list four criteria for an acceptable inspection schedule.
I request that you commit to use these criteria for future disc inspections. We believe that such a comitment will reduce the probability for a safety problen to such a degree that the !!P.C staff would no longer need to ronitor.vour turbine insnections except throuqh the normal activi-ties of our Office of Inspection and En forcer.'e n t.
Your commitment would l
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fir. J. J. Mattiroe
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Your response to this request should be submitted within 30 da,s of re-cefot of this letter.
I also take this opportunity to advise you that on June 11, 1981 Westing-house transmitted two proprietary reports related to turbine missiles for flRC staff review and evaluation.- We have been advised by Westinghouse that the nothodology described in these reports was used to provide its customers with estimates of the probability of disc rupture fron stress corrosion cracking and with analyses of pntential aissile energies. We shall provide Westinghouse with our evaluation of this methodology as soon as our resources pemit.
Sincerely, 80RIGINAL SIGED M JOIDIF. STOLTZ' John F. Stolz, Chief Operating Reactors Branch #4 Division of Licensing
Enclosure:
Safety Evaluation cc w/ enclosure:
See next page
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