ML20010H213
| ML20010H213 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 08/26/1981 |
| From: | Varga S Office of Nuclear Reactor Regulation |
| To: | Stiede W COMMONWEALTH EDISON CO. |
| References | |
| NUDOCS 8109240168 | |
| Download: ML20010H213 (8) | |
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NRC PDR L PDR TERA NSIC Ai]G gg1981 ORB #1 Rdg DEisenhut WRoss Mr. Wayne Stiede CParrish Commonwealth Edison Company OELD Post Office Box 767 IE-3 Chicago, Illinois 60690 ACRS-10 Gray File
Dear Mr. Stiede:
I am enclosing, for your information, copies of letters tnat are being mailed to licensee / users of Westinghouse and General Electric low-pressure turbines.
These letters define the present position of the Office of Nuclear Reactors Regulation related to the subject of keyway and bore cracks in low-pressure turbine discs. The cooperation of the Westinbhouse Disc Integrity Task Force during our joint efforts to control the impact' of this type of cracking has been greatly appreciated. As you will observe from these letters we are recomending that liccnsee/ users continue to work closely with both vendors in the future.
If significant changes occur as the result of future disc inspections and/or design the staff will review the positions described in the Safety Evaluation and will reopen connunication with the licensees and vendors if considered advisable.
Sincerely, gysginal sicned WA S. A. VW Steven A. Varga, Chief Operating Reactors Branch #1 Division of Licensing
Enclosures:
Letter to Westinghouse turbine
- users Letter to General Electric turbine users C,,
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Docket No.
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~O (ADDRESS OF LICENSEE)
Y6!/4j Dear Mr.
By letter dated February 25, 1980 we informed all licensee / 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.
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 typt:3 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 temperature and material characteristics. Westinghouse is continuing to evaluate the effect of otner manufacturing and operational variables.
!Jntil a satisfactory solution can be found we believe that it woulu ' u pru-dent for you to continue inspecting your low-pressure turbine discs on a schedule designed to minimize the probability that a crack will form and grow to a depth that would cause a disc to rupture. Westinghouse has deve-loped a method to determine safe inspection and re-inspection frequencies and has submitted this information in Memorandum MSTG-1-P, June 1981 (Pro-prietary) for review by, the NRC staff.
Our appraisal of the Westinghouse approach is presented in the enclosed Safety Evaluation. We conclude that inspection schedules based on the recommendations in the Westinghouse Memorandum will provide an acceptably high degree 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 four criteria for an acceptable inspection schedul e.
I request that you commit to use these criteria for future disc inspections. We believe that such a commitment 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 normal activi-ties of our Office of Inspection and Enforcement. Your commitment 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 i
by Westinghouse.
Your resper,se to this request should Se submitted within 30 days of re-ceipt of this letter.
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s-I also take this opportunity to advise you that on June 11, 1981 Westing-house transmitted two proprietary reports related to turbine mist,iles for NRC staff review and evaluation. We have been advised by Westinghouse that the methodology described in these reports was used to prov'de its customers with estimates of the probability of disc rupture from 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,
, Chiet Operating Reactors Branch #
Divison of Licensing
Enclosure:
Safety Evaluation cc w/ enclosure See next page 4
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(ADDRESS OF LICEN.SEE)
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Dear Mr.
-In our letter of May 1980 we recommended that you initiate a program to inspect the keyway and bore areas of your low preisure 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 clear indication 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 monitoring of these inspections. Normal activities of the Office of Inspection and Enforcement will continue of course.
We recommend that you continue to perform disc inspections, as appropriate, and continue to work with General-Electric Company to establish safe inspection schedules. Should you find any significant degradat. ion, such as stress corrosion cracking, we would expect you to promptly report it in the normal manner.
Sincerely,
, Chief Operating Reactors Branch #
Division of Liscensing G
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1.
SAFETY EVALUATION REPORT l
Criteria For Low Pressure Nuclear Turbine Disc Inspection Westinghouse hac 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 (nd recommends criteria for scheduling disc inspections that provide a very low probability of c1sc failure
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prior to inspection.
We have evaluated each of the criteria presented in the report and ar; 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 each is described below.
l 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.
A'alytic components of n
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 m.scussed below.
l A.
Crack Growth Rate L
Westinghouse has performed statistical studies using the field data on crack
' sizes and shapes as related to tehperature of operation, location (bore or i
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 Size W stinghouse has used the usual LEFM model to calculate critical crack e
size, taking into consideration effects cf crack shapes expected in dif-ferent locations (bore or keyway). The fracture taughhess 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 :.;schanics 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 toughness derived from the Rolf-Novak expression by 20% to reflect the effect of the irregd 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-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 concervative) augmented toughness value (discussed ir. 8 above) that gives a larger and more realistic crack m7 T
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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 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 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
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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.
B.
Re agree with the alternattve Westinghouse critical crack size calculational method, using a value of fracturc toughness increase of 20% ~above the Rolf-Novak value.
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.C.
We will retain a criterion of relating allowable running tim before
. 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 act,1vities of the Office of Inspection and Enforce-ment.
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