ML20010F685
| ML20010F685 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 09/01/1981 |
| From: | Clark R Office of Nuclear Reactor Regulation |
| To: | Mayer L NORTHERN STATES POWER CO. |
| References | |
| NUDOCS 8109110176 | |
| Download: ML20010F685 (7) | |
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j4 SEP ' 01 1981 of y A s-Olh Docket Hos.(5
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Mr. L. O. Hayer, Manager Nuclear Support Services Horthern States Power Company 414 41 collet Mall - 8th Floor j
Minnerpolis, Minnesota 55401 W
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Dear Mr. Mayer:
By letter dated February 25, 1980 weinformedalllicensee/brs
+h house low-pressure turbines that stress corrosion cracks wefiE b fGi$
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in the keyway and bore areas of low-pressure discs. Becaus7Ah you perfom ultrasonic inspections on your low-pressure disc %(reque were considered to increase the probability of disc failure M
I A justify that your plant could continue to operate safely.
$,1'd,1' 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-wough all factors related to cracking have not been positively established, operating experience indicates that crack initiation and growth are related j
to disc temperature and raterial characteristics. Westinghouse is continuing i
to evaluate the effect of other manufacturing and operational variables.
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Until a satisfactory solution can be found we believe that it would be pru-dont for you to continue inspecting your low-pressure turbino discs on a i
schedule designed to ninimize the probability that a crack will fom 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 infomation in Memorandum MSTG-1-P, June 1981 (Pro-priet !6 /) for review by the NRC staf f.
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) ' og Our appraisal of the Westinghouse approach is presented in the enclosed mo Safety Evaluation. We conclude that inspection schedules based on the i
d recommendations in the Westinghouse Memorandun C,1 provide an acceptablj 5
high degree of assurance that discs will be insnected before crach can grow to a size that could cause disc failure r speeds up to design speed.
9 in our Safety Evaluation we list four criterea for an acceptable inspection Om schedule.
I request that you comit to use these criteria for future disc gg inspections. He believe that such a cocnitrent will reduce the probability og for a safety problem to such a degree that the NRC staff wnuld no longer 1
need to renitor your turbine inspections except through the nomal activi-
- b ties of our Office of Inspection and Enforcenent. Your coonitment wculd 4
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also elininate the need for you to report these inspection results to the staff or to transmit the conputerized disc data sheets that are prepared by Westinghouse.
Your response to this _ request should be subraitted within 30 days of re-ceipi. ui' Eis b iici.
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2 I also take this opportunity to advise you that on June 11, 1981 t!esting-house t*ansmittr:d two proprietary reports related to turbine missiles for NRC staf f review and evaluation.
"? have been advised by Westinghouse that the nethodolooy described in tnese reports was used to provide its custoaters with estimates of the probability of disc ruptura from stress corrosion cracking and with analyses of potential missile energies. We shall provide \\!estinghouse with our evaluation of this nethodology as soon as our resources permit.
Sincerely, Original Elgned by Robcrt A. Clark Robert %. Clark, Chief Operatinq Res: tors Branch #3 Divisor. of Licensing
Enclosure:
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y ut, Northern States Power Company CC' Gerald Charnoff, Esquire Bernard M. Cranum Shaw, Pittman, Potts and Trowbridge Bureau of Indian Affairs, DOI l
1800 M Street, N.W.
15 S0uth 5th Street j
Washington, D. C.
20036 Minneapolis, Minnesota 55402 Mr. Louis J. Breimburst Mr. R. L. Tan'nner Executive Director County Auditor Minnesota Pollution Control Agency Red Wing, Minnesota 55066 1935 W. County Road B2 Roseville, Minnesota 55113 U. S. Environmental Protection Agency
.Tederak Activities Branch The Environmental Conservation Library Region V Office Minneapolis Public Library ATTN:
Regional Radiation 300 Nicollet Mall Representative Minneapolis, Minnesota 55401 230 South Dearborn Street Chicago, Illinois 60604 fir. F. P. Tierney, Plant Mar,ager Prairie Island nuclear Generating Plant liorthern States Power Company Route 2 Welch, Minnesota 55039 Jocelyn F. Olson, Esquire Special Assistant Attorney General "innesota Pollution Control Agency 1935 '..'. Ccunty Road E2 Reseville, Minneosta 55113 v.5. 'Juclear Regulatery Commission Resicant Inspectors Office 2.0; e =2, Box 5CCA
.:elct, Minnesota 55059 l
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SAFETY EVALUATION REPORT Criteria For Low Pressure Nuclear Turbine Disc Inspection Westinghouse has prepared a proprietary repcrt covering their investigation and analysis of turbine disc cracking. This report includes a statistical analysis of all turbine disc cracks found to date and recommends 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 s
agreement with either the Westinghouse position or with one of the positions i
in those cases where they suggest alternatives. Thes> criteria 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 this approach are:
A. Crack Growth Rate I
B. Critical Crack Size C. Fraction of Critical Crack Cize Allowed.
The Westinghouse criterion for establishing eacn 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 6
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 Westinohouse has used the usual LEFM model to calculate critical crack size, taking into consideracion 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 Re f-Novak correlation. destinghouse also Ore-sented test resuits, 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 conservative, as all of the test data show even larger increases.
C.
Fracticn of Critical Size Allowed Westinghouse has proposed two esthods for applying this factor.
ne in-e volves a.very conservative critical crack size calculation using the Rolf-Novak value of toughnest, 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 s
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. n si.3 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 siza has been the NRC criterion; consequently, an acceptable inspection schedule criteria is maintained as follows:
l} New discs should bo inspected at the first refueling outage, or be-fore any postulated crack would grow to more than 1/2 the critical depth.
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- 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 j$scs, calculating crack growth from the time of the last inspection.
- 3) Discs operating with known and ceasured cracks should be reinspected before 1/2 the time calculated for any crack to grow to 1/?. the criti-cal crack depth.
- 4) These inspection schedules may be varied to coincide with scheduled octages. Westinchouse recommendations in this regard should be followed.
- manary and Conclusions A.
We agree that the Westinghouse crack growth rate equations for bore and keyway cracks are acceptable.
B.
We agree with the alternative Westinghouse critical crack size calculational method, usina 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 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 activities of the Office of Inspection and Enforce-ment.
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