ML20033B195
| ML20033B195 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 08/21/1981 |
| From: | Clark R Office of Nuclear Reactor Regulation |
| To: | Burstein S WISCONSIN ELECTRIC POWER CO. |
| References | |
| NUDOCS 8112010013 | |
| Download: ML20033B195 (3) | |
Text
Oh(xh C5 M 211E b/[h Docket Mos. 0-266 and 50 ' 1 e
p *MOg8 '[I Mr. Sol Burstein M
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Executive Vice President Wisconsin Electric Power Cocoany
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Dear Mr. Burstein,
v By letter dated February 25, 1930 we inferred all licensee / users of Vesting-house icw-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 perforn 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 imre 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, operatinq experience indicates that crack initiation and grwth are related to disc te-'perature and r,aterial characteristics. Mestinghouse 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 insrecting your low-pressure turbine discs on a schedule designed to rininize the probability that a crack will form and grow to a depth that would cause a disc to rupture. Mestinghouse has deve-leped a rethod to detemine safe inspection and re-inspection frequencies and has subnitted this informatio. in Menorandum MSTG-1-P, June 1981 (Pro-prietary) for review by the NRC staff.
Our anpraisal of the Westinchouse approach is presented in the enclosed Safety Evaluation. We conclude that inspection schedules based on the recomendations in the Westinpbouse Memorandun 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 uur Safety Evaluation we list four criteria for an acceptable inspection schedul e.
I request that you comit to use these criteria for future disc insocctions.
"e beliave that such a ccmitment will reduce the probability for a safety problea, to such a degree that the liRC staff would no longer need to ronitor your turbin? inspections except through the nor-al activi-ties of our Office of Inspection and Enforcer.cnt. Your comitment would also elir,inate the need for you to report these inspection results to the staff or to transmit the co puterized disc data sheets that are prepared by Westinghouse.
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,.a 2-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 review and evaluation. We have been advised by Westinghouse that the methodology described in these 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 nethodology as soon as our resources pemit.
Sincerely, original elened byi i
ELC {et Robert A. Clark, Chief Operating Reactors Branch #3 Divison of 1.icensing
Enclosure:
Safety Evaluation cc w/ enclosure See next page DISTRIBUTION:
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Mr. Bruce Churchill, Esquire Mr. William Guidemond Shaw, Pittman, Potts and Trowbridge USNRC Resident Inspectors Office 1800 M Street, N. W.
6612 Nuclear Road Washington, D. C.
20036 Two Rivers, Wisconsin 54241 Joseph Mann Library 1516 Sixteenth Street Two Rivers, Wiscensin 54241 Mr. Glenn A. Reed, Manager Nuclear Operations Wisconsin Electric Power Company Point Beach Nuclear Plant 6610 Nuclear Road Two Rivers, Wisconsin 54241 Mr. Gordon Blaha Town Chairman Town of Two Creeks Route 3 Two Rivers, Wisconsin 54241 Hs. Kathleen M. Falk General Counsel Wisconsin's Environmental Decade 114 N. Carroll Street Madison, Wisconsin 53703 U. 5. Environmental Protection Agency Federal Activities Branch Regien V Office ATT!.':
Regional Radiaticn Representative 230 5. Dearborn Street Chicago, Illinois 60604 Chairman Public Service Commission of Wisconsi'n Hills Farms State Office Building Madison, Wisconsin 53702 e.
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SAFETY EVALUATION REPORT Criteria For Lce Pressure Nuclear Turtine Disc Inspection Introduction Westinghouse has prepared 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 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 ~
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 hcw 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 this approach are:
A. Crack Growth Rate B. Critical Crack Size
. C. Fraction of Critical Crack Size Allowed.
Discussion and Evaluation The Westingnouse 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 g9hh
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. a 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 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 froc. 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 conservative, as all of the test data show even larger increases.
C. ' Fraction'of Critical Size Allowed Westinghouse has proposed two methods fer app 7ying 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 conservative) augmented toughness value (discussed in B above) that gives a larger and more realistic crack a
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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 criterionf 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 l
using the same criterion as for new discs, calculating crack growth i
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 outages. Westinghouse recommendations in this regard should be followed.
Sumary and Conclusions A.
We agree that the 14estinghouse crack growth rate equations for bore and s
j keyway cracks are acceptable.
I g.
We agree with the alternattve Westinghouse critical crack size calculational i
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.
f 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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