ML20010E862
| ML20010E862 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 08/24/1981 |
| From: | Varga S Office of Nuclear Reactor Regulation |
| To: | Clayton F ALABAMA POWER CO. |
| References | |
| NUDOCS 8109080393 | |
| Download: ML20010E862 (7) | |
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DISTRIBUTION Dockets NRC PDR AUS 2 419gy L PDR TERA NSIV 1
Docket Nos. 50-348 2
ORB #1 Rdg and 50-364 g
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J!!eltemes ftr. F. L. Clayton, Jr.
SVarga Senior Vice President h,[g Alabama Power Company g-
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EReeves Biming' ham, Alabana 35291 t
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Dear Mr. Clayton:
CParrish l
Gray File RE: Turbine Disc Cracking Joseph H. Farley Huclear Plant Unit Nos. I and 2
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By letter dated February 25, 1980 we infomed all licensec/ users of Westing-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 dis', failure we requested that you perform ultrasonic inspections on your low-pressure discs and justify that your plant could continue to operate safely.
All Westinghouse 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, operating experience indicates that crack initiation and growth are related to disc terperc?ure and naterial characteristics. Westinghouse is continuing to evaluate the effect of other raanufacturing and operational variables.
Until a satisfactory solution can be found we believe that it would be pru-dent for you to continue inspecting your lowmressure turbine discs on a schedule designed to mininize the probability that a crack will fom and grow to a depth that would cause a disc to rupture. License condition 2.C.(19)(d) for Unit No. 2 requires the first inspection to be done prior to resuming power operation following the first refueling. Westinghouse has developed a rethod to detemine safe inspection and re-inspection frequen-i cies and has submitted this infomation in Perorandum MSTG-1-P, June 1981 (Proprietary) for review by the NRC staff.
4 Our appraisal of the Vestinghouse approach is presenteo in the enclosed Safety Evaluation. We conclude that inspection schedules based on the recormndations in the Westinghouse Memorandum will provide an acceptably high cegree of assurance that discs will be inspected before cracks can grow to a size that could cause disc failure at speeds up to design speed.
In our Safety Evaluation we list foi.r criteria for an acceptable inspection schedule.
I request that you corr'it to use these criteria for future disc in - tinnt Un Enifavn that w h a enmitnont will roAce tha nenh3hility omen t
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Mr. F. L. Clayton t l
l for a safety problen to such a degree that the flRC staff would no longer need to monitor your turbine inspections.except through the-cornal activi-ties of our Office of Inspection and Enforcement. Your comitnent would also eliminate the need for you to report these inspection results to the staff or to transmit the computerized disc data sheets that are prepared by Westinghouse.
Your response to this request should be submitted within 30 days of re-ceipt of this letter.
I also take this opportunity to advise you that on June 11,1981 Westing-house transnitted two proprietary reports related to turbine missiles for NRC s'aff review and evaluation. We have been advised by Westinghouse that the rethodology described in these reports was used to provide its customrs with estimates of the probability of disc rupture from stress corrosion cracking and with analyses of potential wssila energies. We shall provide Westinghouse with our evaluation of this methodology as soon as our resources pemit.
Sincerely, Steven A. Varga, Chief Operating Reactors Branch #1 Division of Licensing
Enclosure:
Safety Evaluation cc w/ enclosure See next page l
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Mr. F. L. Clayton, Jr.
Alabama Power Company l
cc: Mr. W.-0. Whitt L +.
Executive Vice President l.
Alabama Power Company -
Post Office Box 2641 l
Birmingham, Alabama 35291 f
l Ruble A. Thomas, Vice President l
Southern Company Services, Inc.
Post Office Box 2625 Birmingham, Alabama 35202 j
George F. Trowbridge, E' squire Shaw, Pittman, Potts and Trowbridge l
1800 M Street, N.W.
Washington,'D. C.
20036 Mr. Robert A. Buettner, Esquire Balch, Bingham,' Baker, Hawthorne,
- ' Williams and Ward Post Of fice Box 306 -
Birmingham, Alabama 35201 George S. Houston Memorial Library 212 W. Burdeshaw Street Dothan,' Alabama 36303 Resident Inspector V. S. Nuclear Regulatory Commission Post Office Box 24-Route 2 Columbia, Alabama 36319 i
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I SAFETY EVALUATIch REPORT Criteria For low Pressure Nuclear Turbine Disc Inspection Westinghouse has prepared a proprietary report covering their investigation and analy-is of turbine disc cracking. This report includes a statistical analysis of all turbine disc cracks found to date and recorm: ends criteria for scheduling disc inspections that provide a very low probability of disc failure prior.to inspectfon.
We have evaluated each of the criteria presented in the report and are in agreement with either the Westinghouse position or with one of the positions in those cases where they suggest alternatives. These criteria and our evalua-!
i tion of ear.h is described below.
There are several major criteria involved in setting inspection schedules.
Basically, the approach used is to make a conservative prediction of how fast a presumed or actual crack will grow and then schedule an inspection prior to the time the crack grows large enough to be of concern. Analytic components of this approach are:
A. Crack Growth Rate B. Critical Crack Size C. Fraction of Critical Crack Size Allowed.
The Westinghouse criterion for estaMishing each of these factors and our evalua-tion is-discussed below.
A.
Crack Growth Rate Westinghouse has performed statistical studies using the field data on crack si:es and shapes as related to temperature of operation, location (bore or keyway), material strength, and environment. They have selected a conservittive w
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upper bound basis and developed equations that define a conservative crack growth rate for each disc. We have reviewed the Westinghouse methodology and find the growth-rate equatious to be acceptable.
B.
Critical Crack Size Westinghouse has used the usual LEFM model to calculate critical crack.
size, taking into consideration effects of crack shapes expected in dif-ferent locations (bore or keyway). The fracture toughness salues used in the calculations are determined from actual charpy V tests on each disc, using the common Rolf-Novak correlation. Westinghouse also pre-f sented test results, obtained from both fracture mechanics specimens and a spin test, to show that this correlation yields over-censervatively low values of the toughness related to actual disc cracks because the cracks are irregular and branched. We therefore prefer the alternative 1
proposed; i.e., to increase the estimate of. fracture toughness derived from the Rolf-Novak expression by 20% to reflect the effect of the irreg-ular nature of actual service-induced disc cracks. This 20% increase is still veiy conservative, as all of the test data show even larger increases.
C.
Fraction of Critical Size Allowed, Westinghouse has proposed two methods for applying this factor.
One in-i volves a very conservative critical crack size calculation using the Rolf-i Novak value of toughness, and then permitting operation until a crack grows to a predicted maximum of 75t of the critical size. An alternative approach is to use the more realistic (but still conservative) augmented toughness value (dir.ussed in B above) that gives a larger and more realistic crack a
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size and then pennitting operation until a crack grows to a predicted maximum of 50t of critical crack size. We prefer this latter approach.
A growth limit of 50". of critical crack size us been the NRC criterion; consequently, an acceptable inspection schedule criteria is maintained as follows:
l} New discs should be inspected at the first refueling outage, or be-fore any postulated crack would grow to more than 1/2 the critical.
depth.
- 2) Discs previously inspected and found to be free of cracks or that have been repaired to eliminate all indications should be reinspected using the same criterion as for new discs, calculating crack growth from the time of the last inspection.
- 3) Discs operating with known and measured cracks should be reinspected before 1/2 the time calculated for any crack to grow to 1/2 the criti-cal crack depth.
4} These inspection schedules may be varied to coincide with scheduled outa;es. Westinghouse recorr.endations in this regard should be followed.
Sumary and Conclusions A.
We agree that the tiestinghouse crack growth rate equations for bore and keyway cracks are acceptable.
3.
We agree with the alternattve Westinghouse critical crack size calculational method, using a value of fracture toughness increase of 20" above the Rolf-Novak value.
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. C.
- le will retain a criterion of relating allcwable running time before inspections to the time to reach 1/2 of the critical crack depth.
O.
The NRC staff will no longer monitor each turbine inspection except through the normal activities of the Office of Inspection and Enforce-ment.
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