ML17341A461
| ML17341A461 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 08/21/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17341A460 | List: |
| References | |
| NUDOCS 8108280069 | |
| Download: ML17341A461 (7) | |
Text
SAFETY EVALUATION REPORT Criteria For Low Pressure Nuclear Turbine Disc Inspection 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 recormends criteria for scheduling disc inspections that provide a very low probability of disc failure prior to inspection.
Me 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-i.
tion of each is described below.
There are several major criteria involved in setting inspection schedules.
Hasically, 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 largg enough to be of concern.
A'nalytic 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 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
'eyway),
material strength, and environment.
They have selected a conservative
'i08280069 8i082i:
PDR ADQCK 05000250
., P PDR
'0 upper bound basis and developed equations that define a conservative crack growth rate for each disc.
Me have reviewed the Mestinghouse methodology and find the g'rowth-rate equations to be acceptable.
B. 'Critical Crack:Size Mestinghouse has used the usual LEFH model to calculate critical crack size, taking into consideration effects of crack shapes expected, in dif-ferent locations (bore or keyway).
The fracture toughhess values used in the calculations are determined from actual charpy
'V tests on each disc, using the common.Rol f-Novak correlation.
Mestinghouse al so 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 irregu'Ear and branched.
Me 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 All'owed Mestinghouse has proposed two methods, for applying 'this factor.
Qne in-II volves a very conservative critical crack size calculation using the Rolf-Novak value of toughness, and then permitting operation until a crack geom to a predicted maximum of 75K 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 realist~c crack
I
~
0
size and then permitting operation until a crack grows to a predicted maximum of 50K 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)
Oiscs 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)
Oiscs 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.
4f These inspection schedules may be varied to co'incide with scheduled outages.
Westinghouse recomendations in this regard should be followed.
Sumar "and Conclusions We agree that the Westinghouse crack growth rate equations for bore and keyway cracks are acceptable.
Me agree with the alternative Westinghouse critical crack size calculational
- method, using a value of fracture toughness increase of 205 'above the Rolf-Novak value.
.C.
4!e 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.
0
'4 I