ML20030E153
| ML20030E153 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 09/04/1981 |
| From: | Clark R Office of Nuclear Reactor Regulation |
| To: | Groce R Maine Yankee |
| References | |
| NUDOCS 8109180056 | |
| Download: ML20030E153 (7) | |
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4 Mr. Robert H. Groce Senior Engineer - Licensing Q
, %[cg Maine Yankee Atenic Power Cor Jany f.!
q 1671 Worcester Road p
FraW ilham, Massachusetts 01701 7
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Dear Mr. Groce:
J, By letter dated February 25, 1080 we informed all licensee / users of W house low-pressure turbines that stress corrosion cracks ucre 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.
All Westinghouse low-pressure turbines at operating nuclear power plants
'x have now been inspected, at least once, for keyway and bore cracks.
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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, cperating experience, indicates that crack initiation and growth are related to disc tewerature and material characteristics. Westinghouse is continuing to evaluate the effect of other manufacturing and operational variables.
U Until a satisfactory solution can be found we believe that it would be pru-dent for pu to continue inspecting your low-pressure turbine discs on a schedule designed to minipize the pn hability that a crack will form and '
grow to a depth that would cause a disc to rupture. Westinghouse has deve-loped a method to deternine safe inspection and re-inspection frequencies and has subnitted this information in Menorandun !!STG-1-P, June 1981 (Pro-prietary) for review by the NRC stsff.
Our appraisal of the Westinghouse approach is presented in the enclosed Safety Evaluation. He conclude that inspection schedules based on the reconmendations in the Westinghouse Memorandun will provide an acceptably high degree of assurance that discs will be inspected beforo cracks can 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 schedule.
I request that you commit to use these criteria for future disc inspections. We believe that such a commitnent will reduce the probability for a safety problem to such a degree 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 Enforcenent. Your connitsent would also elininate the need for you to report these inspection results to the staff or to transmit the coryuterized disc data sheets that are prepared by Westinghouse.
Your response to this request should be suboitted within 30 days of re-ceipt oT rn u n; m.
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2 I also take this opportunity to advise you that on June 11. 1981 Westing-nouse transmitted two proprietary reports related to turbine nissiles for l
HRC staff review and evaluation. We have been advised by Westinghouse that the methodology described in these reports was used to provide its custorers with estimates of the probability of disc rupture from stress corrosion cracking and with analyses of potential missile energies. We shall provide Westinghouse witr. our evaluation of this nethodology as soon as our resources pemit.
Sincerely, Original signed by Robert A. Clark Robert A. Clark, Chief Operating Reactors Branch #3 Divison of Licensing
Enclosure:
Safety Evaluation 8
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eunp OFFICIAL RECORD COPY uso m ini.- m.,,o 5 NRC FORM 318 (10-80) NRCM G240
Maine Yankee Atomic Power Company cc:
'E. W. Thurlow, Pref.taent Mrs."L. Patricia Doyle, President Maine Yankee Atomic Power Company SAFE POWER FOR MAINE Edison Drire Post Office Box 774 Augusta, Maine 04336 Camden, Maine 04843 Mr. Donald E. Vandenburgh First Selectman of Wiscasset Vice President - Engineering Municipal Building Yankee Atomic Electric Company U. S. Route 1 20 Turnpike Road Wiscasset, Maine 04578 Westboro, Massachusetts 01581 Stanley R. Tupper, Esq.
John A. Ritsher, Esquire Tupper and Bradby Ropes & Gray 102 Townsend Avenue 225 Franklin Street Boothbay Harbor, Mair,c 04538 Boston, Massachusetts 02110 David Santee Miller, Esq.
Mr. Rufus E. Brown 213 Morgan Street; N. W..
Deputy Attorney General Washington, D. C.
20001 State of Maine Augusta, Maine 04330 Mr. Paul Swetlanii Resident Ir.spector/ Maine Yankee Mr. Nicholas Barth c/o U.S.N.R.C.
Executive Director P. O. Box E Sheepscot Valley Conservation Wiscasset, Maine 04578 Association, Inc.
P. O. Box 125 Mr. Charles B. Brinkman Alan, Maine 04535 Manager - Washington Nuclear Operations Combustion Engineering Inc.
Wiscassett Public Libcary Association 4853 Cordell Avenue, Suite A-1
.High Street Bethesda, Maryland 20014 Wiscasset, Maine 04578 Mr. John H. Garrity, Director Mr. Torbet H. Macdonald, Jr.
Nuclear Engineering & Licensing l
Office of Energy Resources Maine Yankee Atomic Power Company State House Station #53 Edison Drive Augusta, Maine 04333 Augusta, Maine 04336 Robert M. Lazo, Esq., Chairman Atomic Safety and Licensing Board U.S. Environmental Protection Agency U.S. Nuclear Regulatory Comm'ssion Region I Office Washington, D. C.
20555 ATTN: EIS COORDINATOR JFK Federal Building Dr. Cadet H. Hand, Jr., Director Boston,Massachgsetts 02203 Bodega Marine Laboratory University of California Bodega Bay, California 94923 O
Mr. Gustave A. Linenberger Atomic Safety and Licensing Board State Planning Officer U.S. Nuclear Regulatory Commission Executive Department Washington, D. C.
20555 189 State Street Augusta, Maine 04330 s
SAFETY EVALUATION REPORT Criteria For Low Pressure Nuclear Turbine Disc Inspection Westinghouse has prepared 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 recommends crieeria for scheduling disc inspections that prtvide a very low probability of disc failure 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 cri'teria and our evalua-i tion of each is described below.
There are several najor 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 1
this approach are:
l A. Crack Growth Rate L
B. Critical Crack Size C. Fraction of Critical Crack Size Allowed.
l The Westinghouse criterion for establishing each of these factors and our evalua-tion is discussed below.
l A.
Crack Growth Rate l
Westinghouse has performed statistical studies using the field data on crack sizes and shapes as related to temperature of operation,. location (bore or l
keyway), material strength, and environment. They have selected a conservative
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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 equations to be acceptable.
B.
Critical Crack'S.ize 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 toughhess values used in the calculations are determined from actual charpy V tests on each i
disc, using the common' Rolf-Novak correlation. Westinghouse a' Iso pre-sented test results, obtained from both fracture mecha~nics 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 estimat of fracture toughness derived from the Rolf-Novak exp-assion by 20% to reflect the effect of the irregy 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 Westinghousa has proposed two methods for applying this factor.
One in-volves a very conservative critical crack size calculation using the Rolf-Novak value of toughness, and then permitting operation until a crack grows to a predicted maximum of 75% of the critical size. An altersitive approach is to use the more realistic (but still conservative) augmented toughness value (discussed in.3 above) that gives a larger and more realistic crack d
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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 growth limit of 50% of critical crack size has been the NRC criterion; consequently, an acceptable inspection schedule criteria is maintained as
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follows:
1} 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 previtasly inspected and found to be free of cracks 6r that have been repaired to eliminate all indications should be reinspected using the same criterion as for new discs, calculatir.g crack growth from the time of the last inspection.
- 3) Discs operating with known and measured cracks should be reinspected before 1/2 the tine 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 outages. Westinghouse recommendations in this regard should be followed.
Summary and Conclusions A.
We agree that the Westinghouse crack growth rate equations for bore and keyway cracks are acceptable.
- s. W'e 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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Q. We will retain a criterion of relating allowable running time befort inspections ta the time to reach 1/2 of the critical cracht depth.
D.
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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