ML20010E367
| ML20010E367 | |
| Person / Time | |
|---|---|
| Site: | Beaver Valley |
| Issue date: | 08/19/1981 |
| From: | Varga S Office of Nuclear Reactor Regulation |
| To: | Carey J DUQUESNE LIGHT CO. |
| References | |
| 12598, NUDOCS 8109040013 | |
| Download: ML20010E367 (4) | |
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Docket NRC PDR
> AU6 191981 hE Docket No. 50-334 NSIC 9
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IE-3 Mr. J. J. Carey, Vice President 1
Nuclear Division
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Duquesne Light Company ACRS-10 cy.f h
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Post Office Box 4 8
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CParrish MI Grey File
Dear Mr. Carey:
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By letter dated February 25, 1980 we informed al1 licensee / users of Westing-E use low-pressure turbines that Gress 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 perfonn ultrasonic inspections on your low-pressure discs and justify that your plant could continue to operate safely.
All Westinghouse low-pcessure turbines at operating nuclear pcwer plants nave now been inspected, at least once, for
/way and bore cracks.
Indica-tion of oe.e or both types of these cracks has been found at 20 plants. Al-though all factors related to cracking have not been positively established, 4
operating experience indicates that crack initiation and growth are related to disc temperature and naterial characteristics. Westinghouse is continuing to evaluate the effect of other manufacturing and operational variables.
l Until a satisfactory solution can be found we believe that it would be pru-1 dent for you to continue inspecting ynur low-pressure turbine discs on a i
schedule designed to ninimize the probability that a crack will form and grow to a depth that would cause a disc to rupture. Westinghouse has deve-i loped a cethod to determine safe inspection and re-inspection frequencies i
and has sub nitted this infornation in Mer orandun MSTG-1-P, June 1931 (Pro-o om jg prietary) for review by the MRC staff.
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- 2 Our appraisal of the Vestinghouse approach is presented in the enclosed k8 Safety Evaluation. We cone'ude that inspection schedules based on the
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recomendations in the Westinghouse Fenorandum will provide an acceptably i
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high degree of assurance that discs will be inspected before cracks can 9
grow to a size that could cause disc failure at speeds up to design speed.
Oo In our Safety Evaluation we list four c;iteria for an acceptable inspection n03 schedule.
I request that you comit to use these criteria for future disc
@M4 insnections. We believe that such a committrent will reduce the probability for a safety problem to such a degree that the NRC staff would no longer need to nonitor your turbine inspections except through the normal activi-ties of our Office of Inspection and Enforcenent. Your commitment would also eliminate the need for you to report these inspection results to the j
staff or to trans91t '.he computerized disc data sheets that are prepared
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s DISTRIBUTION Docket j
' AUG 191981 TERA Docket No. 50-334 NSIC sn I
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OELD fp % EF j
IE-3 Mr. J. J. Carey, Vice President C
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ACRS-10 Duquesne Light Company D "5 4
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?:uclear Division
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Post Office Box 4 1-45 d
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NI Gray File
Dear Mr. Carey:
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By letter dated February 25, 1980 we informed al1 1icensee/ 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 disc failure we requested that you perform ultrasonic inspections on your low-pressure discs and justify that your plant could continue to operate safely.
f All Westinghouse low-pressure turbines at operating nuclear pcwer plants have new 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-i though all factors related to cracking have not been positively established, operating experience indicates that crack initiation and growth cre related l
to disc te perature and r'aterial characteristics. Westinghouse is continuing to evaluate the effect of other manufacturing and operational variables.
i Until 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 1
sche 41e designed to ninimize the probability that a crack will fann and crew to a depth that would cause a disc to rupture. Westinghouse has deve-1 j
loped a cethod to determine safe inspection and re-inspection frequencies and has submitted this infornation in Memorandun MSTG-1-P, June 1981 (Pro-o om gg prietary) for review by the t :10 staff.
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Our appraisal of the Vestinghouse approach is presented in the enclosed k@
Safety Evaluation. We conclude that inspection schedules based on the recomndations in the Westinghouse Menorandum will provide an acceptably U
o high degree of assurance that discs will be inspected before cracks can 9
grow te a size that could cause disc failure at speeds up to design speed.
8o In our Safety Evaluation we list four criteria for an acceptable inspec; 203 schedule.
1 request that you comit to use these criteria for future di, t @f inspections. We believe that such a commitrent will reduce the probability for a safety problem to such a decree that the HRC staff would no longer need to nonitor your turbine inspections except through the nornal activi-ties of'our Office of Inspection and Enforcement. Your commitnent would also eliminate the need for you to report these inspection results to the staff or to transmit the coguterized disc data sheets that are prepared
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Ifr. J. J. Carey !
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 nissiles for HRC staff review and evaluation. We have been advised by Westinghouse that the nethodology described in these reports was used to provide its customers with eetinates of the probability of disc rupture fron stress corrosion cracking and with analyses of potential missile energies. We shall provide Westinghouse with our evaluation of this methodology as soon as our resources permit.
Sincerely, Orir at signedby:
S. A. Varsa Steven A. Varga, Chief Operating Reactors 3 ranch #1 Divison of Licensing
Enclosure:
d Safety Evaluation cc w/ enclosure See next page 3
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o Mr. J. J. Carey Dugyesne Light Company Gerald Charnof'f, Esquire Mr. R. J. Washabaurgh, Qf ianager Jay E. Silberg, Esquire cc:
Duquesne Light Comapny Shaw, Pittman, P,otts and Trowbridge Quality Assurance Depa. nent 1800 M Street, N.W.
Post Office Box 4 Shippingport, Pennsylvania 15077 Washington, D. C.
20036 Karin Carter, Esquire Mr. J. A. Werling Special Assistant Attorney General Station Superintendent Bureau of Administrative Enforcement Duquesne Light Company 5th Floor, Executive House Beaver Valley Power Station Harrisburg, Pennsylvania 17120 Post Office Box 4 Shippingport, Pennsylvania 15077 Mr. Roger Tappan Stone & Webster Engineering Corporation Mr. T. D. Jones, Manager P.O. Box 2325 Nuclear Operations Boston, Massachusetts 02107 Duquesne Light Company Post Office Box 4 Shippingport, Pennsylvania 15077 Mr. F. Noun R & D Center Westinghouse Electric Corporation Mr. F. J. Bissert, Manager Nuclear Support Services Building 7-303 Pittsburgh, Pennsylvania 15230 Duquesne Light Company Nuclear Division Marvin Fein Post Office Box 4 Shippingport, Pennsylvania 15077 Utility Counsel City of Pittsburgh B. F. Jones Memorial Library 313 City-County Building Pittsburgh, Pennsylvania 15219 663 Franklin Avenue Aliquippa, Pennsylvania 15001 Mr. John A. Levin Public Utility Commission Mr. R. M. Mafrice, Nuclear Engineer P.O. Box 3265 Duquesne Light Company Harrisburg, Pennsylvania 17120 435 Sixth Avenue Pittsburgh, Pennsylvania 15219 Irwin A. Popowsky: Esquire Office of Consumer Advocate Mr. R. E. Martin 1425 Strawberry Square l
Duquesne Light Company Harrisburg, Pennsylvania 17108 435 Sixth Avenue Pittsburgh, Pennsylvania 15219 Charles A. Thomas, Esquire Thomas and Thomas Mr. N. R. Tonet, Manager 212 Locust Street Muclear Engineering Box 999 Duquesne Light Company Harrisburg, Pennsylvania 17108 Nuclear Division Shippingport, Pennsylvania 15077 l
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Mr..J. J. Carey Duquesne Light Company cc: Mr. J. s. Sieber itanager Nuclear Safety and Licensing Duquesne Light-Company
!:uclear Division Poe,t Office Box 4 Shippingport, Pennsylvania 15077 Resident Inspector U. S Nuclear Regulatory Commission Post Office Box 298 Shippingport, Pennsylvania 15077 4
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SAFETY EVALUATION REPORT Criteria For Low Pressur'e Nuclear Turbine Disc Inspection Westinghouse has prepared a proprietary report covering their investigation and analysis of turbine disc cracking. This mport includes a statistical analysis of all turbine disc cracks found to date and recomends criteria for.
scheduling disc inspections that proviae a very low probability of disc failum prior to inspection.
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 ?
tion of each is described below.
There are several major criteria involved in setting inspection schedule 5.
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 t!'.is approach are:
A. Crack Growtli Rate B. Critical Crack Size C. Fraction of Critical Crack Size Allowed.
The Westinghouse criterion for establishing each of these facters and our evalua-tion is discussed below.
A.
Crack Growth Rate 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 environment. They have selected a conservative i
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t 2-upper bound basis and developed equations that daffne a conservative crack growth rate for each disc. We have reviewed the Westingf.ouse metnodology and find the growth-rate equations to be acceptable.
B.
Critical Crack Size Westinghouse has used the usual LEFM model to calculato critical crack size, taking into consideration effects of crack shapes expacted 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 teughness related to acttal disc cracks because the
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tracks are irregular and branched. We therefore prefer _ the alternative proposed; i.e., to increase the estimate of fracture toughness derived from the Rolf-Novtk 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 increises.
C.
Fraction of Critical Size Allowed Westinghouse has proposed two methods for applying this factor.
One ' :.-
volves a very conservative cr.itical crack size calculation using ths Rolf-Novak v61ue of toughness, and then permitting operation until a crack grows to a predicted maximum of 75% of the critical site. 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 s
3-size and then pemitting operation until a crack grows to a predicted maximum of 50% of critica2 crack size. We prefer this latter approach.
A growth limit of 50% of critical crack size has been the NRC criterion; consequently, an acceptable inspection schedule criteria is maintained as follows:
ll 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
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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.
41 These inspect.fon schedules may be varied to coincide with scheduled outages. Westinghouse recomment.tions in this regard should be followed.
Summary and Conclusions I
A.
'..'e agree that the Nestinghouse crack growth rate equations for bore and keyway ' cracks are acceptable.
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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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We will retain a criterion of relating allowable running time before inspections to the time to reach 1/2 of the critical crack depth, i
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The NRC staff will na longer monitor eaJ1 turbine inspection except through the normal activities of the Office of Inspection and Enforce-4 ment.
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