ML20030E008
| ML20030E008 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 08/28/1981 |
| From: | Clark R Office of Nuclear Reactor Regulation |
| To: | Lundvall A BALTIMORE GAS & ELECTRIC CO. |
| References | |
| NUDOCS 8109170280 | |
| Download: ML20030E008 (7) | |
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AUG 2 81981 gISTofRtlTION:
Docket F11D JHeltemes NRC PDR Gray File
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DEisenhut q-1 4 Mr. A. E. Lundvall, Jr.
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l Vice President - Supply sEP 0098F ACRS-10 Baltinore Gas & Electric Company
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b P. O. Box 1475 t
Baltimore, tiaryland 21203 P
Dear Mr.-1.undva11:
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a In our letter of May 1980 we recomended that you initiate a progran to inspect the keyway and bore areas of your Calvert Cliffs Unit 1 pressure discs for stress corrosion cracking. This action was prompted by the discovery of such cracks in Westinghouse turbines.
So far, the inspection program has covered one or more low pressure rotors at 15 nuclear plants using GE turbines. No cicar indicetion of stress corrosion cracking has been discovered, although indications of water cutting and/or erosion have been observed in several turbines.
In view of these findings, we have decided to discontinue the NRR staff nodtoring of those inspections. Nomal activities of the Office of l
Inspect'on and Enforcement will continue of course.
We recorrend that you continue to perfom disc inspections, as appespriate, and continue to work with General Electric Joapany to establist afe inspection schedules. Should you find a9y significant 4 gradation, such as stress corrosion cracking, we would expect you to promptly report it in the nomal manner.
With regard to Calvert Cliffs Unit 2, by letter dated February 25,1980 we ir. formed all licensee / users of Westinghouse 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 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 material characteristics. Westinghouse is continuing ta avaluate the effect of other manufacturing and operational-vuiables.
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. 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 schedule designed to nininize the probability that a crack will fora and grow to a depth that would cause a disc to rupture. Westinghouse has deve-loped a method to detemine safe inspection and re-inspection frequencies and has submitted this infornation in Memorandun PSTG-1-P, June 1981 (Pro-prietary) for review by the NRC staff.
Our appratsal of the Westinghouse approach is presented in'the enclosed Safety Evaluation. We conclude that inspection schedules based on the reconnendations in the Westinghouse Memorandum will provide an acceptably high degree of assurance that discs will be inspected before cracks can qrow 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 comit to t.se these criteria for future disc inspections. Us believe that such a conmitrent will reduce the probability for a safety problem to such a degree that the NRC staff would no longer need to monitor your turbine inspections except through the nornal activi-ties of our Office of Inspection and Enforcerent. Your comitment would also eliminate the need for you to report these inspection results to the staff or to transnit the computerized disc data sheets that are prepared by Westinghouse.
Your response to this request should be subnitted within 30 days of re-criot of this letter.
I also take this opportunity to advise you that on June 11, 1981 Westing-house transmitted two proprietary reports reltted to turbine nissiler. for imC staff review and evaluation. We have been ady! sed by Westinghouse that the nothodology described in these reports was used to provide its custre.ers with estirotes of the probability of disc rupture froat stress corrosion crack 1no and witb analyses of potential missile energies. We shall provide Westinghouse with our evaluation of this nethodology as soon as our resources permit.
Sincerely, d W cunAl e'ene Q, d,o, O dI \\ W h' Robert A. Clark, Chief Operating Reactors Drarch #3 Divison of Licensing
Enclosure:
Safety Evaluation cc w/ enclosure See next page P@bft)zer
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m Baltimore Gas and Electric Company cc:
James A.. Biddison, Jr.
"Hs. Mary Harrison, President General Counsel Calvert County Board of County Commissioners Baltimore Gas and Electric Company Prince Frederick, MD 20768 P. O. Box 1475 Baltimore, MD 21203 U. S. Environmental Protection Agency Region III Office George F. Trowbridge, Esquire Attn:
EIS Coordinator Shaw, Pittman, Potts and Trowbridge Curtis Building (Sixth Floor) 1800 M Street, N. W.
Sixth and Walnut Streets Washington, D. C.
20036 Philadelphia, PA 19106 Mr. R. C. L. Olson, Principal Engineer Mr. Ralph E. Architzel Nuclear Licensing Analysis Unit Resident Reactor Inspector Baltimore Gas and Electric Company NRC : 7spection and Enfch ament Room 922 - G&E Butiding P. O. Bos 437 P. O. Box 1475 Lusby, MD 20657 Baltimore, MD 21203 Mr. Charles P. Brinkman Mr. Leon B. Russell Manager - Washington Nuclear Opereilons Plant Superintendent Combustion Engineering, Inc.
Calvert Cliffs Nuclear Power Plant 4853 Cordell Avenue, Suite A-1 Maryland Routes 2 & 4 Bethesda, MD 20014 Lusby, MD 20657 Mr. J. A. Tieman, Manager Bechtel Power Corporation Nuclear Power Department Attn: Mr. J. C. Judd Calvert Cliffs Nuclear Power Plant Chief Nuclear Engineer Maryland Routes 2 & 4 15740 Shady Grove Road Lusby, MD 20657 Gaithersburg, MD 20760 Mr. W. J. Lippold, Supervisor Combustion Engineering, Inc.
Nuclear Fuel Management Attn: Mr. P. W. Kruse, Manager Baltimore Gas and Electric Company Engineering Services Calvert Cliffs Nuclear Pow;-> Plant P. O. Box 500 P. O. Box 1475 Windsor, CT 06095 Baltimore, Maryland 21203 Public Document Room Mr. R. E. Denton, General Supervisor Calvert County Library Training & Technical Services Prince Frederick, MD 20678 Calvert Cliffs Nuclear Power Plant Maryland Routes 2 & 4 Director, Department of State Planning Lusby, MD 20657 301 West Preston Street Baltimore, MD 21201 Mr. R. M. Douglass, Manager Quality Assurance Department Administrator, Power Plant Siting Program Fort Smallwood Road Complex Energy and Coastal Zone Administration P. O. Box 1475 Department of Natural Resources Baltimore, MD 21203 Tawes State Office Building Mr. T. L. Syndor, General Supervisor Operations Quality Assurance Calvert Cliffs Nuclear Power Plant Maryland Routes 2 & 4 Lusby, m 20657
7 w-SArETY EVALUAT.yN REPORT Criteria For Low Pressure Nuclear Turbine Disc inspection Westinghouse has trepared.a proprietary report covering their investigation and analysis of turbine disc cracking. This report incluaes a statistical analysis of all turbine disc cracxs found te date and reconnends criteria for scheduling disc inspections that provide 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 criteria and our evalua-1 tion of each is described below.
There.re several major criteria involved in setting inspection schedules.
Basically, the approach used is to make a conservative prediction of how fast a presured 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 establishing each of these factors and our evalua-tion is discussed below.
A.
frackGrowthRate Wastinghouse 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
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.c upper bound basis and developed equations that' define a conservative--
crack growth rate for each disc. We have reviewed the Westinghouse methocology and find the growth-rate equations 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 values used its the calculations are determined from actual charpy V tests on each disc, using the comon Rolf-Novak correlation. Weninghouse also pre-f sented test results, obtained from both fracture mechanics spdcimens 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 toughness derived i
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 sizi calculation using the Rolf-Novak value of toughness, and then pennitting operation until a crack grows to a predicted maximum of 75% cf the critical size. An alternative a'pproach is to use the more realistic (but still conservative) augmented toughness value (discussed in B above) that gives a larger and more n:alistic crack 9
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' size and then pennitting 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 schedul'e criteria is maintained as 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 previously inspected and found to be fr'ee 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 cf 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 de 6h.
- 4) These inspection schedules may be varied to coincide with scheduled outages. Westinghouse recomendations in this regard should be followed.
Summary and Conclusions A,
We agree that the Westinghcuse crack growth rate equations for bore and keyway cracks are acceptable.
3.
We agree with the alternative 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. We will retain a criterion of relating allcwable running time-befort-inspections to the time to reach 1/2 of the crit' cal crack depth.
D.
The NRC staff will no longer monitor cach turbine inspection except through the normal activities of the Office of Inspection and Enforce-ment.
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