ML20005B819

Requests Util Commitment to Use Westinghouse Criteria in Encl Safety Evaluation to Determine Safe Insp & Reinsp Frequencies of Low Pressure Turbine Discs

ML20005B819
Person / Time
Site:	Zion  File:ZionSolutions icon.png
Issue date:	08/19/1981
From:	Varga S Office of Nuclear Reactor Regulation
To:	Delgeorge L COMMONWEALTH EDISON CO.
References
NUDOCS 8109020306
Download: ML20005B819 (3)

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DISTRIBUTION Dockets NRC PDR L PDR

'AUG 191981 TERA Docket Mos. 50-295 NSIC ORB #1 Rdg and 50-304 g g g

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/((f IE-3 tir. Louis DelGeorge J

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SVarga Commonwealth Edison Company 8-u g,,%% 8 Post Office Box 767 DWigginton Chicago, Illinois 60690 CParrish Gray File

Dear Mr. DelGeorge:

g ui By letter dated February 25, 1980 we infomed all licensee / users of Westing-house inw-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 perforn 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 terperature and naterial characteristics. Westinghouse is continuing 1

i to evaluate the effect of other manufacturing and operational variables.

Unti: a satisfactory solution can be found we believe that it would be pru-dent for you to continue inspecting your low-pressure turbine discs on a schedule designed to minimize the probability that a crack will fom and grow to a depth that would cause a disc to rupture. Westinghouse has deve-loped a nethod to detemine safe inspection and re-inspection frequencies and has submitted this infomation in f tenorandun MSiG-1-P, June 1981 (Pro-prietary) for review by the HRC staff.

Our appraisal of the Westinghouse approach is presented in the enclosed Safety Evaluation. He cnnclude that inspection schedules based on the reconnendations in the Westinghouse Menorandum will provide an acceptably o om

@5 high degree of assurance that discs will be inspected before cracks can j

grow to a size that could cause disc failure at speeds up to design speed.

In our Safety Evaluation we list four criteria for an acceptable inspection gm n8 schedule.

I request that you connit to use these criteria for future disc G

inspections. We believe that such a commitnent will reduce the probability for a safety problem to such a degree that the NRC staff would no longer o$

need to nonitor your turS!ne inspections except through the normal activi-ties of our Office of Inspection and Enforcenent. Your connitment would

'M-also eliminate the need for you to report these inspection results to the EU staff or to transmit the computerized disc data sheets that are prepared sv moinnhmmn.

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Ifr. Louis De1 George 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 transmitted two proprietary reports related to turbine missiles for NRC staff review and evaluation. We have been advised by Westinghouse that the methodology dese.ribed in these reports was used to provide its customers with estinates of the probability of disc rupture from stress corrosion cracking and with analyses of potential missile energies. Wo.

shall provide Westinghouse with our evaluation of this nethodology as soon as our resources permit.

Sincerely, gginal signed by:

S. A. Varga Steven A. Varga, Chief Operating Reactors Branch #1 Divison of Licensing

Enclosure:

Safety Evaluation cc w/ enclosure See next page

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Mr. Louis C. De1 George Commonwealth Edison Company cc:

Robert J. Vollen, Esquire 109 North Dearborn Street Chicago, Illinois 60602 Dr. Cecil Lue-Hing Director of Research and Development Metropolitan Sanitary District of Greater Chicago 100 East Erie Street Chicago, Illinois 60611 Zion-Benton Public Library District 2600 Emmaus Avenue Zion, Illinois 60099 Mr. Phillip P. Steptoe Ishan, Lincoln and Beale Counselors at Law One First National Plaza 42nd Floor Chicago, Illinois 60603 Susan N. Sekuler, Esquire Assistant Attorney General Environmental Control Divistor 188 ' Jest Randolph Street, Suite 2315 Chicago, Illinois 60601 U. S. Nuclear Regulatory Ccomission Resident Inspectors Office 105 Shiloh Blvd.

Zion, Illinois 60099 i

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SAFETY EVALUATICN REPORT Criteria For Low Pressure Nuclear Turbine Disc Inspection Westinghouse has pmpared a proprietary report covering their investisation and analysis of turbine disc cracking. This report includes a statistical analysis of all turbine disc cracks found to date and recomends criteria for scheduling disc inspections that provide a ve..i lcw probability of disc failum prior to inspection.

We have evaluated each of the criteria presented in the report and are in agreemnt with either the Westinghouse position or with one of the positions in those cases where they s 6 gest alternatives. These criteria and our evalua '

tion of each is described b91ow.

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 Allcwed.

The Westinghouse criterion for establishing each of these factors and our evalua-I tion is discussed below.

A.

Crack Growth Rate I

Westinghouse has performed statistical studies using the field data on crack sizes and shapes as related to temperature of operation, location (bore or ~

keyway}, material strength, and envi ronment. They have selected a conservative C

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upper bound basis and developed equations that define a conservative crack growth rate for each disc. We have rcviewed the Westinghouse methodology and find the growth-rate equations to be acceptable.

B.

Critical Crack Sire Llestinghouse 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 values used in the calculations are determined from actual charpy V tests on each disc, using the common Rolf-Novak correlation. Westinghouse also pre-sented test results, obtained from both fracture mechanics specimens and a spin test, to show that this correlation yields over-conservatively low values of the toughness related to actual disc cracks because the cracks are irregular and branched. We therefore prefer the alternative proposed; i.e., to increase the estimate of fracture toughnest 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 very 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-volves a very conservative critical crack size calculation using the Rolf-

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Novak value of toughness, and then permitting operation until a crack grows to a predicted maximum of 75% of the critical size. An alternative approach is to use the more realistic (but still conservative) augmented toughness value (discussed in B above) that gives a larger and more realistic crack l

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size and then permitting operation until a crack grows to a predicted maximum of 50% of critical crack size. We prefer this latter approach.

A grcwth limit of 50% of critical crack size has been the NRC criterion; consequently, an acceptable inspection schedule criterir. is maintained as follows:

1] New discs should be in'spected 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 criterien as for new discs, calculating crack growth frcm 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.

41 These inspection schedules may be varied to coincide with scheduled outages. Westinghouse recon =endations in this regard should be followed.

Surmiary and Conclusions A.

We agree that tho Westinghouse crsck 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 23% above the Rolf-Novak value.

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C.

We will retain a criterion of relating allowable running time befort inspections to the time to reach 1/2 of the critical crack depth.

D.

The NRC staff will no longer monitor each turbine inspection except through the normal activities of the Office of Inspection and Enforce-t ment.

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