ML20040F841
| ML20040F841 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 02/08/1982 |
| From: | Crutchfield D Office of Nuclear Reactor Regulation |
| To: | Kay J YANKEE ATOMIC ELECTRIC CO. |
| References | |
| TASK-03-04.B, TASK-3-4.B, TASK-RR LSO5-82-02-045, LSO5-82-2-45, NUDOCS 8202100364 | |
| Download: ML20040F841 (5) | |
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L February 08, 1982 G
Docket No.50-029
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{S8 72 Mr. James A. Kay
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'p Senior Engineer - Licensing 9
Yankee Atomic Electric Company D
1671 Worcester Road Framingham, Massachusetts 01701
Dear Mr. Kay:
SUBJECT:
SYSTEMATIC EVALUATION PROGRAM TOPIC III-4.B. TURBINE MISSILES - YANKEE R0WE Enclosed is a copy of our draft evaluation of Systematic Evaluation Program Topic III-4.8.
You are requested to examine the facts upon which the staff has based its draft evaluation and respond either by confirming that the facts are correct, or by identifying errors and supplying the corrected information.
We encourage you to supply any other material that might affect the staff's evaluation of this topic or be significant in the integrated assessment of your facility.
Your response is requested within 30 days of receipt of thir. letter.
If no response is received within that time, we will assume that you have no comments or corrections and will consider this topic complete.
Sincerely, 55 r
I we b8) osu Dennis H. Crutchfield, Chief hgg2ggo!
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9 Operating Reactors Branch flo. 5 g p
PDR Division ofLLicensing j
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Enclosure:
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Mr. James A. Kay cc Mr. James E. Tribble, President Yankee Atomic Electric Company 25 Research Drive Westborough, Massachusetts 01581 t
Greenfield Community College 1 College Drive Greenfield, Massachusetts 01301 Chairman Board of Selectmen Town of Rowe Rowe, Massachusetts 01367 Energy Facilities Siting Council 14th Floor One Ashburton Place
~ Boston, Massachusetts 02108 U. S. Environmental Protection Agency Region 1 Office ATTN: Regional Radiation Representative JFK Federal Building Boston, Massachusetts 02203 Resident Inspector Yankee Rowe Nuclear Power Station c/o U.S. NRO Post Office Box 28 Monroe Bridge, Massachusetts 01350 e
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SYSTEMATIC EVALUATION PROGRAM TOPIC 111-4.8 YANKEE R0WE TOPIC III-4.B. TURBINE MISSILES I.
INTRODUCTION The purpose of this topic is to assure that, with respect to potential turbine missiles, all structures, systems and components important to safety either have adequate protection by means of structural barriers or have an acceptably low probability of damage.
II.
REVIEW CRITERIA 10 CFR 50, Appendix A, GDC 4 III. RELATED SAFETY TOPICS AND INTERFACES III-4.C, Internally Generated Missiles IV.
REVIEW GUIDELINES Standard Review Plan (SRP) Section 3.5.1.3, Regulatory Guides (R.G.)
1.115 and 1.117.
V.
EVALUATION During November 1979, the NRC staff became aware of low pressure turbine disc cracking in Westinghouse turbines at several operating plants.
Additional inspections at other plants possessing Westinghouse turbines also indicated cracking,thus implying a generic problem applicable to plants with Westinghouse turbines.
Consequently, on February 25, 1980, the NRC issued 50.54 (f) letters to utilities with Westinghouse low pressure turbines requesting information related to this problem.
Both Westinghouse and the NRC staff have been following this problem closely and have developed independent crack growth models.
The findings of a multi-plant action, B-46, " Turbine Missiles," concluded that an inservice inspection schedule based on an approach developed by Westinghouse for their turbines provides an acceptably high degree of assurance that discs will be inspected and cracks detected before any crack could grow to one-half of the size that could cause disc failure at speeds up to design speed.
. The low pressure turbine in use at Yankee Rowe at this time was manufactured by Westinghouse. Also, it was of a design unique to nuclear applications in that the discs were inaccessible to the specifically developed ultrasonic inspection technique that has been successfully used at the other affected plants.
Shortly after the generic disc cracking problem was identified, the low pressure turbine at Yankee Rowe failed catastrophically.
However, it should be noted that all of the disc fragments were retained in the casing. The cause of the failure was identified as stress corrosion cracking in the bore and keyway region at the steam inlet of the turbine.
The Yankee Atomic Electric Company (YAEC) was provided with a safety evaluation report for this inservice inspection approach and was re-quested to commit to use the four criteria listed on page 3 of the safety evaluation report (Reference 1). YAEC responded to this request (Reference 2) and committed to the use of the four criteria except for the first three discs of the low pressure turbine. The first three discs of this new turbine will be integrally forged with the rotor, thus, there will be no bore or keyways for these discs. This design precludes the possibility of stress corrosion cracking of the type that this topic addresses and, therefore, the inspection criteria of the safety evaluation report need not be applied to the integrally forged discs. As a result of this comitment, an acceptably high degree of assurance is provided, which assures that disc failures at design speed will not occur.
In addition, as a result of the turbine cracking problem. Westinghouse has revised its probability analysis of damage to safety related struc-tures, systems and components of damage.
The analysis includes utilizing the following individual probabilities:
(1) the probability of turbine failure leading to the ejection of turbine missiles due to design speed failures and destructive over-speed failures (P1), (2) the probability of r.trike (P2) and (3) the probability of damage (P3). Since the evalua-tion of the latest Westinghouse probability analyses has not been completed by the staff, criteria, considering turbine cracking and the implemented inservice inspection program, has not been established for determining Pl. The staff, upon completion of the Westinghouse review, will determine what actions, if any, are required.
The staff determined that the licensee perfoms a testing program for the overspeed protection system.
Specifically,'there are three different tests performed on a routine basis as follows.
At every turbine overhaul and at each refueling oucage the following two tests are performed:
1.
Overspeed protective test - actually overspeed turbine to trip set point to close stop and governing valves.
2.
As turbine is brought up to speed, stop and governing valves are tested to verify system operability.
~l l Every month, while the plant is operating, power is reduced and each 90vernor and stop valve combination is se:;tentially exercised.
VI.
CONCLUSION The inspection progra:n comitted to by YAEC (References 1 and 2) provides an acceptably high degree of assorance that turbine discs will not fail at speeds up to design speed. The testing program of the overspeed protection system, including the stops and control valves at Yankee Rowe, provides reasonable assurance that the overspeed pro-tection system will remain operable and, thereby, limit the likelihood that overspeed past the design conditions would occur.
The staff concludes, for an interim period until a decision is reached regarding the need for updated probabilistic analysis of the turbine missile hazard, the probability of damage from turbine missiles is acceptably low.
Should further reviews of operating plants and/or additional requirements be deemed necessary, the Yankee Rowe plant will be included with that operating plant action.
REFERENCES 1.
Letter from D. M. Crutchfield (NRC) to J. A. Kay (YAEC), dated August 28, 1981.
2.
Letter from J. A. Kay (YAEC) to D. M. Crutchfield (NRC), dated October 9,1981.
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