ML20030D652
| ML20030D652 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 08/28/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20030D650 | List: |
| References | |
| NUDOCS 8109140132 | |
| Download: ML20030D652 (4) | |
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.i SAFETY EVALUATION REPORT L
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-Criteria For Low Pressure Nuclear Turbine Disc Inspection
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' Westinghouse has prepared a proprietary report covering their investiaation and analysis of turb'.ine disc cracking.
This report includes, a statistical analysis of all turbine disc cracks foundi o date and recomends criteria for t
scheduling disc inspections that prov4de a very low probabil.ity of. disc failure l
l 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 y
in those cases where they suggest alternatives. These criteria and our evalua '.
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tion of each'is described b'elow.
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There are several cajor criteria invcived in setting inspect'on scheduies.
Basically, the approach used is to make a conservative prediction'of.how fast a presumed or actual crack vill grow and then schedule an inspection prior to the tire the crack grows large enough to be of concern.
Analytic components of this approach are:
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A. Crack Growth Rate -
i M S. Critical Crack Size C. rrectico of Critical Crt.ck Sire A11cwed.
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The Wettinghouse criterion for establishing each of t, hen factors and our evalua-l-
tion is discussed below _
l A.
Crack Growth Rate
-Westinghouse has performed statistical studies using the field data on crack sizes and shapes as rdated to temperature of operation, location' (bore or keyway), material strer.gth, and environment. They have selected a conservative
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upper bound basis and developed equations that define a conservative l
crack growth rate for each disc.
We have reviewed the Westinghouse r.;athodology and find the gecwth-rate equations to be acceptable.
B.
Critical Crack Size
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.i Westinghouse has used the usual LEFM model to calculate critical crack size, taking into consideration effects of crack shapes expected in dif-
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ferent locaticas (bore or keyway).
The fracture toughness values used
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in the calculations are determined f rom actual charpy V tests on each disc, using the common Rolf-Novak correlation.
Westinghouse also pre-so.ted test results, obtained from both fracture mechanics specimens and a soin test, to show thet this correlation yields over-conservatively luw values of the toughness related to actual disc cracks because the cracks are irregular and branched.
We theref;re prefer the alte native propcsed; i.e., to increase the estimate of fracture touchness derived from the Rolf-Lovak expression by 20S to reflect the effect of the irreg 7
ul a r nature of actual service-induced disc cracks.
This 20S increase is still very conservative, as all of the test data-show even larger increases.
C.
Fractior of_ Criticaijize Allowed
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Westinghouse S.er proposed two methods for applying this f actor.
One in-volves a very conser/ative critical crack size cgiculation 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
size und then permitting operation until a crack grows to a prcdicted raximum 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
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- 1) New discs should be inspected at the first refueling outage, cr be-s fore any postulated crack wculd grow to more than 1/2 the critical depth.
21 Discs previously inspected and found to be free of cracks or that have been repaired to elimiriate 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 neasured crack's 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 coinciee 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
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keyway cracks are acceptable.
3.
We agree with the" alternative Westinghouse critical crack size calculational method', using a value of fracture toughness increase of 20% above the Rolf-
.Novak value.
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He will retain a criterion 0f relating allowable running time before inspections to the time to reach 1/2 of the critical crack depth.
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The NRC staff will no longer nonitor each turbine inspection except through the normal activities of the Office of Inspection and Enforce-i ment.
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