ML20032A256
| ML20032A256 | |
| Person / Time | |
|---|---|
| Issue date: | 08/20/1981 |
| From: | Ross B NRC |
| To: | Hazelton W, Pawlicki S NRC |
| Shared Package | |
| ML20032A255 | List: |
| References | |
| NUDOCS 8110280671 | |
| Download: ML20032A256 (8) | |
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UNITED STATES g
g NUCLEAR REGULATORY COMMISSION g
- p WASHINGTON, D. C. 20555
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August 20, 1981 0 ' kA"$
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NOTE T0: Steve Pawlicki Warren Hazelton gy FROM:
Bill Ross On August 19, 1981, I read, to Frank Vaccaro of Westinghouse, the following clarification of the first criterion for turbine disc inspection, i.e.,
"New discs should be inspected at the first refueling outage or before any postulated crack would grow to more than 1/2 the critical depth."
"The initial inspection of a new rotor or disc should be performed before any postulated (calculated) crack would grow to more than 1/2 the critical crack depth. On the basis of our pres we do not expect this interval will be,ent knowledge of Westinghouse discs much less than one fuel cycle or longer than two cycles unless and until these discs are fabricated and installed under conditions more favorable than in the past.
If the calcu-lated interval is less than the scheduled first fuel cycle, the licensee should seek Westinghouse's guidance on delajing the inspection until the refueling outage.
If the calculated period is longer than the first fuel cycle, the licensee should also seek Westinghouse's guidance for scheduling the first inspection at a later refueling outage. Crit ~erion #4 allows the inspection to be sooner or later (within limits acceptable to Westinghouse) than the calculated date so as to coincide with a scheduled outage."
During this conversation Mr. Vaccaro informed me that it was Westinghouse's intent to eventually replace discs that could potentially fonn cracks within the relative short period postulated by the staff with new designs that would have crack-free operation for much longer periods of time. The staff will be kept informed of these developments.
I agreed to transmit a copy of this clarification to Mr. Vaccaro.
4 Bill Ross
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Docket No.
08 SN MIEI3
{ ADDRESS OF LICENSEE)
Y6!/4') M88 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 bcre cracks.
Indica-tion of'one or both types of these cracks has been found at 20 plants. Al-thpugh all factors related to cracking nave n6t been positively established, operating experience indicates that crack initiation and growth are related to dis:: temperature and material characteristics. Westinghouse is continuing to evaluate the effect of other manufacturing and operational variables.
Until a satisfactory solution can be found we believe that it would be pru-dent for you to continue 4specting 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 freavencies and has submitted this information in Memorandum MSTG-1-P, June 1981 (Pro-prietary) for review by the NRC staff.
l 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 schedule.
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 activie ties of our Office of Inspection and Enforcement. Your commitment would alsc eliminate the need for you to report these inspection results to'the staff or to transmit the computerized disc data sheets that are prepared by Westinghouse.
Your response to this request should be submitted within 30 days of re-ceipt of this letter.
L
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 revieu 'nd evaluation. We have been advised by Westinghouse a
that the methodology described in thtse reports was used to provide 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,
, Chief Operating Reactors Branch #
Divison of Licensing
Enclosure:
Safety Evaluation cc w/ enclosure See next page
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SAFETY EVALUATION REPORT Criteria For Low Pressure Nuclear Turbine Disc Inspection Westinghouse has pmpared a proprietary report covering their investigation 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 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 critc~.ia and our eva'ua 1 tion of each is described below.
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 i
the time the crack grows large enough to be of concern. Analytic components of this approach are:
i 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.
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 N
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f 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 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 taug!}hess values used in the calculations are determined from actual charpy V tests on each disc, using the comon-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 toughness 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 conservuive, 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 ca.lculation using the Rolf-Novak value of toughness, and then pemitting 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 E
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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 1ndications 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 tine 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 recommendations in this ~ regard should be followed.
o l
Summary and Conclusions A.
We agree that the Westinghouse crack growth rate equations for bore and j
keyway cracks are acceptable.
l l
g.
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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Q. We will retain a criterion of relating allowable ror.ning time 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,ivities of the Office of Inspection and Enforce-ment.
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i' Docket No.
- 8. Ist('r/4'6'd //Hr"S (ADDRESS OF LICENSEE)
Dear Mr.
l 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 i.
at 15 nuclear plants using GE turbines. No clear indication of stress corro'sion cracking has been discovered, although indications of water cutting and/or erosion have been observed in several. turbines.
'In view of thes2 findings, we have decided to discontinue the NRR staff monitoring of these inspections. Normal activities of the Office of i
Inspection and Enforcement will continue of course.
I k'e 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 degradation, such as stress corrosion cracking, we would expect you to promptly report it l
in the normal manner.
Sincerely,
(
, Chief l
Operating Reactors Branch e Division of Liscensing j
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