ML19347D150
| ML19347D150 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 03/09/1981 |
| From: | Hazelton W Office of Nuclear Reactor Regulation |
| To: | NRC ATOMIC SAFETY & LICENSING APPEAL PANEL (ASLAP) |
| Shared Package | |
| ML19347D149 | List: |
| References | |
| ISSUANCES-OL, NUDOCS 8103110263 | |
| Download: ML19347D150 (5) | |
Text
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O 03/09/31 UNITED STATES OF AMERICA NUCLEAR REGULATORY COMMISSION BEFORE THE ATOMIC SAFETY AND LICENSING APPEAL BOARD In the fiatter of
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VIRGINIA ELECTRIC AND POWER COMPANY
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Docket Nos. 50-338 OL
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50-339 OL (North Anna Nuclear Power Station,
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Un.its 1 and 2)
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NRC STAFF RECOMMENDATION RELATING TO REINSDECTION INTERVAL FOR TURBINE DISC AT NORTH ANNA NUCLEAR POWER STATION, UNIT 1 I authored the document entitled " Derivation of Turbinc Disc Crack Growth _ Rates for North Anne Unit 1", dated fiarch 24, 1980 with attachments which was submitted to this Appeal Board under affidavit on March 24, 1980 (copy attached).
I stated therein the Staff's position that "when the postulated crack is less than 50". of'the critical crack size, there is
-sufficient margin to account for any uncertainties in the calculations and therefore no necessity to become-concerned about turbine operations".
(Derivation 'of Turbine Disc Crack Growth Rates for North Anna Unit 1 at 3).
The Staff's position remains the same today.
In Attachment I to the document, I set forth the Staff's approach tc calculating ' turbine disc crack growth rates for operating nuclear power plants.
I-stated therein the Staff's bases for using the following equation-for calculating the critical crack size _for turbine disc bores:
A Q
=
un 1.21 r o
- 810311Q M
. I also provided the assumption used by the Staff for each of the factors in the equation.
Finally, I provided the crack growth rate curve used to derive the growth rate.
(Derivation of Turbine Disc Crack Growth Rates for North Anna Unit 1, Attachment 2, Figure 1).
In a letter to Virginia Electric and Power Company (VEPC0) dated February 9,1981, (Copy provided to the Appeal Board as Attachment 5 to letter from James N. Christman to the Appeal Board members dated February 12, 1981), Westinghouse provided recommended reinspection intervals in months for the existing low pressure 1 turbine rotor at North Anna Unit 1 (LP1 rotor) and the replacement low pressure 2 turbine rotor (LP2 rotor).
Westinghouse stated it had calculated the allowable service life of these rotors based on its interpretation of the NRC computation techniques and found the limiting turbine to be the LP1 rotor requiring reinspection in 21.8 months.
The Staff has independently calculated the estimated crack growth rate for the two turbine discs and confimed that Westinghouse correctly performed the calculations using the Staff's computation techniques set out in my March 24, 1980 document and attachments.
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However, since March 24, 1980, the Staff has received additional infomation regarding actual service experience with Westinghouse turbines f
which have undergone ultrasonic inspection. This includes the information from the recent inspection of the Farley Unit 1 turbine discs.
Based on its-evaluation of all the available additional information received since
- March 1980, the Staff believes that the following modifications should be
4 made in its previously reported criteria to be consistent with this additional information:
1.
The effect of the actual yield strength should be included in a more quantitative way.
The effect of strength level is now considered to be a continuous function instead of the step function shown in the crack growth curves submitted under my March 24, 1980 affidavit.
(Derivation of Turbine Disc Crack Growth Rates for North Anna Unit 1, Attachment 2, Figure 1.)
2.
Bore cracks'are considered to grow faster than keyway cracks in the same disc.
This assumption is based on evalu-ation of data from recent cracking incidents in inspected Westinghouse turbines.
Accordingly, the Staff now believes that separate crack growth rates should be used for estimating potential keyway and bore cracks.
3.
The probable shape of bore tracks should be assumed to be no more than twice as long as they are deep, thus making the calculated critical crack depth greater than the earlier staff model, where the shape of bore cracks was assumed to be four. times 'as long as they were deep.
I would note that this assumed 2 to I ratio was discussed as the most probable case
- in ny March 24, 1980 document.
(Derivation _of Turbine Disc Crack Growth Rates for North Anna, Unit 1, Attachnent I at 5).
With regard to the shape of keyway cracks,-because the keyway itself has an influence on the effective shape of keyway cracks relative to fracture mechanics behavior, the Staff has
. not changed the assumption that keyway cracks should be calculated using a 4 to I ratio (a/2C =.25).
The Staff has recalculated the estimated crack growth rate for the two turbine rotors at North Anna, Unit 1, using computation techniques with the nodifications discussed above. The Staff's recalculation again identified the limiting ' turbine to be the LP1 rotor.
The Staff's current calculations indicate that it will take approximately 25 months for the number 1 disc at the governor end of that rotor to grow to 50% of critical crack size.
While this conclusion is sub.iect to revisior. as additional actual service experience is received and evaluated by the Staff, it is not expected to change significantly.
Accordingly, the Staff recommends that the North Anna Nuclear Power Station, Unit 1 turbine discs t ' reinspected at the next refueling outage.
t a
1
UNITED STATES OF AMERICA NUC1.FAE PEULATORY COMMISSION BEFORE THE ATOMIC SAFEIT A'O LICENSING APPEAE BOAET In the Matter ef
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VIRGINIA ELFITF.IC AND POWER COMPAW
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50-339 OL (Scrth A.na Nuclear Fower Station,
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Units 1 and 2)
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AFFIDAVIT OF WAP.EE; S. HAZELTON STATE OF MAF.YLA O
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COUNTY OF MONTGOMERY ) SS I, Warren S. Hazelton, do depose and state that:
1.
I an a Section Leader, Engineering Branch, Division of Operating Reactors of the United States Nuclear Regulatory Cc=rission.
I at responsible for reviewing =aterials related to aspects of operating nuclear power plants, including the North Anna Unit 1 facility.
A copy of cy professional qualifications was previously subritted to the Appeal board as an attach-cent to the letter fro: Daniel T. Swanson dated February 19, 1950.
2.
I authored the attached document entitled "Derivati_n of Turbine Dis:
Crack Growth Rates for North Anna Unit 1," dated March 24, 1950, with attachments.
3.
I hereby certify that the information contained in this affidavit and attach =ents is true and correct to the best of my knowledge and belief.
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a i Yfl'~n fh.~i. h.!
'Narren S. Hazeltor.-
Subscribed and sworn to before ce this 24th ' day of M._;ch,1980 r
L Lt tr
- 6\\
NotarpPublic My Commission expires:. July-1,1932
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