ML20010F906
| ML20010F906 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 08/31/1981 |
| From: | Clark R Office of Nuclear Reactor Regulation |
| To: | Leasburg R VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| Shared Package | |
| ML20010F902 | List: |
| References | |
| NUDOCS 8109150096 | |
| Download: ML20010F906 (7) | |
Text
_ _ _ _ _ _ _ _ _
~
4
[
UNITED STATES
[..
-k NUCLEAR REGULATORY COMMISSION W &)f f j,
,i.
WASHINGTON, D. C. 20555 AUG 311981 l
%..V 7
Docket Nos. 50-338 and 50-339 Mr. R. H. Leasburg Vice President - Nuclear Operations Virginia Electric and Power Company Post Office Box 26666 Richmond, Virginia 23261
Dear Mr. Leasburg:
25, 1980 we informed all licensee / users of Westing-By letter dated February house low-pressure turbines that stress corrosion cracks were being found Because these cracks in the keyway and bore areas of low-pressure discs.
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 Indica-have now been inspected, at least once, for keyway and bore cracks.
Al-tion of one or both types of these cracks has been found at 20 plants.
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 other manufacturing and operational variables.
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 minimize the probability that a crack will form and Westinghouse has deve-grow to a depth that would cause a disc to rupture.
loped a method to determine safe inspection and re-inspection frecuencies 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 We conclude that inspection schedules based on the Safety Evaluation.
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 We request that you commit to use these criteria for future disc schedul e.
We believe that such a commitment w111 reduce tne probability inspections.
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 by Westinghouse.
Your response to this request should be submitted within 30 days of re-ceipt of this letter.
8109150096 810904 PDR ADOCK 05000339 0
ppg
2 We 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 methodology as soon as our resources permit.
Sincerely, k
kL-k-
obert A. Clark, Chief Operating Reactors Branch #3 Divison of Licensing
Enclosure:
Safety Evaluation cc w/ enclosure See next page l
l l
l l
l
_,,,_.,,.,_....r.....
- m.. -. _. _ _ - _, _. ~.
Virginia Electric and Power Company cc:
Richard M. Foster, Esquire Mr. James Torson Musick, Williamson, Schwartz, 501 Leroy Leavenworth & Cope, P.C.
Socorro, New Mexico 87891 P. O. Box 4579 Boulder, Colorado 80306 Mrs. Margaret Dietrich Route 2, Box 568 5
Michael W. Maupin, Esquire Gordonsville, Virginia 22042 Hunton, Williams, Gay and Gibson P. O. Box 1535 Mr. James C. Dunstance Richmond, Virginia 23212 State Corporation Commission Commonwealth of Virginia Alderman Library Blandon Building Manuscripts Department Richmond, Virginia 23209 University of Virginia Charlottesville, Virginia 22901 Mrs. June Allen North Anna Environnental Coalition Mr. Edward Kube 8720 Lockmoor Circle Board of Supervisors Wichita, Kansas 67207 Louisa County Courthouse P. O. Box 27 U.S. Environmental Protection Agency Louisa, Virginia 23093 Region III Office ATTN: EIS COORDINATOR Ellyn R. Weiss, Esquire Curtis Building Sheldon, Harman, Roisman and Weiss Oth and Walnut Streets 1725 I Street, N.W. Suite 506 Philadelphia, Pennsylvania 19106 Washington, D. C.
20006 Mr. Paul W. Purdom Mr. W. R. Cartwright, Station Manager Environmental Studjes Institute P. O. Box 402 Drexel University liineral, Virginia 23117 32nd and Chestnut Streets Philadelphia, Pennsylvania 19104 Mr. Anthony Gambardella Office of the Attorney General Atomic Safety and Licensing 11 South 12th Street - Room 308 Appeal Board Panel Richmond, Virginia 23219 U.S. Nuclear Regulatory Commission Washington, D. C.
20555 Mr. Edward Webster Resident Inspector / North Anna c/o U.S.N.R.C.
Route 2, Box 78A Mineral, Virginia 23117 Mr. J. H. Ferguson Executive Vice President - Power Virginia Electric and Power Company Post Office Box 26666 Richmond, Virginia 23261
SAFETY EVALUATION REPORT Criteria For Low Pressail Nuclear Turbine Disc Inspection Westinghedse has prepared a proprietary report covering titir investigattor.
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 agreement with either the Westinghouse position or with one of the positions in those cases where they suggest alternatives. These criteria and our evalus '
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 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 dis :ussed 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, loc: tion (bore or keyway), material strength, and environment. They have selected a conservative
[
l
\\
upper bound basis and developed equations that define a conservative crack' growth rate for each disc. We inave reviewad the Westinghouse methodology and find the growtherate 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 coman Rolf-Novak correlation. Westinghouse also pre-sented test results, obtained from both fracturt mechanics specimens and a spin test, to shew 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; 1.c., 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 cracts. This 20% increase is still very conservative, as all of the test data show even larger increases.
C.
Fraction of Cr.tical 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 conservative) augmented toughness value (discussed in B above) that gives a larger and more realistic crack l
l.
m
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 operati'.g with known and measured cracks should be reinspected before 1/2 t e tire calculated for any crack to grew 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.
Summary and Conclusions A.
We agree that the Westinghouse crack growth rate equations for bore and keyway cracks are acceptable.
We agree with the alternattve Westinghouse critical crack size calculational g.
method, using a value of fracture toughness increase of 20% above the Rolf-Novak value.
_ __ _ _. ~ _.
. : ~
4-C. He will retain a criterion of relating allewable 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 af Inspection ar.d Enforce-ment.
1 l
l h
h I
r L
i i
._--vy._,.
m_,.._,.,
...,......,--_,_,,..-_m-_,_._,,,__
._.m o_,,m-x.-_-.../,.,-., _,,...,..,,