ML20210R487
| ML20210R487 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 04/29/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20210R484 | List: |
| References | |
| TAC-60085, NUDOCS 8605160484 | |
| Download: ML20210R487 (5) | |
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UNITED STATES f
}9, NUCLEAR REGULATORY COMMISSION 5'
E WASHINGTON, D. C. 20065
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4 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION 1
RELATED TO AMENDMENT NO.12 TO FACILITY OPERATING LICENSE NPF-29 GRAND GULF NUCLEAR STATION, UNIT 1 l
MISSISSIPPI POWER & LIGHT COMPANY 1
MIDDLE SOUTH ENERGY, INC.
SOUTH MISSISSIPPI ELECTRIC POWER ASSOCIATION 1
1.0 INTRODUCTION
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By letter dated November 1,1985, as supplemented by letters dated December 10 and 27, 1985, and January 24 and February 26, 1986, Mississippi Power & Light 1
Company (the licensee) requested a change to License Condition 2.C.(26) " Turbine Disc Integrity" to make the inspection interval for the low pressure turbine j
discs 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation instead of one fuel cycle.
The Grand Gulf Nuclear Station (GGNS) Unit 1 main steam turbines consist of one high pressure turbine and 3 low pressure (LP) turbines. The turbines are manufactured by Kraftwerk Union (KWU) of Germany. Because of the massive size j
of the LP rotors, the rotors were not made from a single forging.
- Instead, individually forged discs were shrunk fit to a rotor and keyed in place to prevent discs moving independently of the rotor if the shrink fits were lost due to stress relaxation during operation. Because of the unique design of the LP turbines compared to older turbines, the' licensee believes that turbine inspection at each refueling outage is overly conservative and would impose a j
handship by extending the refueling outage time beyond that otherwise required
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for refueling, surveillance tests, and maintenance.
LicenseCondition2.C.(26) states:
"During each refueling outage MP&L shall ultrasonically inspect the i
bores and keyways of the low pressure turbine discs for indications of cracking. All unacceptable indications and their dispositions shall be reported prior to startup-for the next cycle operation. These inspections shall continue until the potential.for turbine disc cracking has been assessed and an acceptable alternate inspection schedule has been established."
4 The license condition resulted from the NRC staff's review of the GGNS Final Safety Analysis Report (FSAR) Section 10.2.3 " Turbine Disc Integrity" and the generic concern with intergranular stress corrosion cracking (IGSCC) in keyny and bore areas in LP turbine discs.
(Supplement No. I to the GGNS Safety Eval-uation Report (SER), Section 10.2.1, December 1981). At the time of the 1981 review, the staff did not have sufficient information'from the licensee on turbine disc material properties, operating temperatures in the bore area, steam moisture level, methodology of crack calculation, and operating temperature and lubri-cants used during assembly to justify a longer inspection interval. The staff was also concerned about several reported cases of cracking in KWU LP discs'in j
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fossil fuel plants. The staff took a conservative, interim position by assuming the propensity for disc cracking is the same as that of Westinghouse turbines, Hence, the staff required the ultrasonic inspection during the first and every j
refueling outage.
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In the present submittals, the licensee has provided additional information to support its request for a longer inspection interval. The licensee proposes to inspect the high pressure turbine during the first refueling outage, one LP turbine during the second outage, the second LP turbine during the third outage, and the third LP turbine during the fourth outage. At the fifth refueling outage, the inspection cycle repeats itself. This schedule would provide a 50,000 hour0 days <br />0 hours <br />0 weeks <br />0 months <br /> inspection interval for each of the turbines, which is four to five times longer than the inspection interval required by the license condition.
2.0 EVALUATION The staff evaluated the licensee's submittals based on four considerations:
the methodology used for turbine disc analysis and its similarity to the Westinghouse methodology taking into account the unique features of the turbine disc design; the fracture toughness of disc materials; consequences of turbine missiles in the event of disc failure; and, the licensee's commitment to shorten turbine disc inspection intervals if necessary based on inspection results.
2.1 Methodology for turbine disc failure analysis In 1979, Westinghouse observed radial cracks in the bores and keyways of certain LP discs of a nuclear turbine. Since then, Westinghouse has published numerous studies on the methodology of estimating the probability of turbine disc rupture, calculating disc crack growth, and developing criteria for inspection intervals.*
The staff has reviewed Westinghouse's studies and approved their methodology.
As a part of the present review the staff determined whether the licensee used the same methodology as that of the Westinghouse turbine disc failure analysis.
Four aspects of the turbine disc failure analysis are considered significant: critical crack size; crack growth rate; probability of disc crack initiation; and proba-bility of disc failure, given the crack initiation.
The licensee's proposed disc inspection interval was obtained from evaluating these four aspects of the disc j
crack analysis. The staff found that the licensee followed the Westinghouse methodology in calculating critical crack size, crack growth rate, and probability of disc failure. However, insufficient information was provided in the submittals for staff to confirm three areas of the licensee's methodology regarding calculations of the probability of disc crack initiation.
(1) The staff questions the factor of 15 that the licensee used in calcu-lating crack initiation probability. This factor was derived from a KWU test which showed that for tests with oxygen cracks were not initiated for operating times up to 30,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. Tests showed that under adverse environmental
- W.G. Clark, B.B. Seth, D.H. Shafter, " Procedures for Estimating the Probability of Steam Turbine Disc Rupture from Stress Corrosion Cracking", 81-JPGC-Pwr-31, American Society of Mechanical Engineers "M
- conditions, such as flow stagnation and crevices, it took about 2,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> to initiate cracks. The licensee divided 30,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> by 2,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> to obtain the factor of 15. The licensee stated that their keyway design pre-cludes the effects of adverse environmental conditions because of unique design features of the KWU turbine. The KWU LP turbine disc design is some-what different than Westinghouse designs in that the keyway area is not included in the shrink-fit area of the disc-rotor interface. This condition will lower the shrink-fit stresses in the keyway area. Another unique feature in the KWU LP turbine is the radial relief grooves which were premachined into the disc hub area and the rotor, which eliminates the high stress con-centrations at the bottom ends of these keyway bores. This groove is designed to prevent flow stagnation. The different design of the KWU turbines may reduce the probability of disc crack initiation compared to Westinghouse designs but insufficient information was provided for the staff to confirm the factor of 15 used by the licensee. However, the staff believes the omission of this factor would not significantly affect the calculated inspection interval of 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
(2) In KWU's inspection of four LP turbines in the German nuclear plants, no cracks were found after 83,000 service hours.
From these inspection results, the licensee states that they do not expect intergrannular stress corrosion cracking (IGSCC) in their disc design because the disc materials, assembly, and water chemistry in the German power plants are the same as those at GGNS. The licensee did not supply sufficient supporting information to the staff to ascertain if the disc materials and water chemistry of German nuclear plants are similar to those of GGNS.
(3) The licensee st'ated that an inspection of any one turbine at any given number of operating hours will be representative of all three turbines with respect to the initiation and growth of IGSCC. Although the discs in each turbine are fabricated using the same material, manufacturing method, and method of assembly, the possible variations in material properties and operating conditions could cause the disc in one turbine to be more susceptible to IGSCC than the discs in the other turbines.
2.2 Disc materials Criteria for evaluation of turbine disc integrity are contained in Standard Review Plan (SRP) 10.2.3, " Turbine Disc Integrity." During the licensing review in 1981, the staff raised questions about the materials properties of the discs.
Specifically, the staff requested information from the licensee on disc material specifications and strength levels, operating temperatures in the bore area of the discs, lubricant used in the hub area of the discs for assembly, moisture level at the discs, and critical crack size calculation. The licensee's response to the staff questions was documented in Appendix 10A of FSAR and in the November 1, 1985 submittal. The staff considers the licensee's response acceptable.
- 2.3 Consecuences of turbine disc failure Although the licensee stated that the KWll disc design will not crack for operating times up to 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />, the staff nevertheless reviewed the con-sequence of turbine missile generation at GGNS. The probability of unacceptable damage resulting from turbine missiles may be expressed as the product of:
the probability of turbine failure resulting in the ejection of turbine disc (or internal structure) fragments through the turbine casing; the probability of ejected missiles perforating intervening barriers and striking safety-related structures, systems, or components; and, the probability of struck structures, systems, or components failing to perform their safety function. The GGNS turbines have a massive and strong casing which should be able to contain turbine missiles caused by disc failures.
In a previous disc failure of a Westinghouse turbine, turbine disc fragments were contained inside of the turbine casing.
The GGNS Unit 1 turbine generator
.s an unfavorable orientation relative to the reactor building. The orientation places the containment building, auxiliary buildings, diesel generator building, standby service water cooling tower and basin, and control building, within the low and high trajectory missile strike zones of the turbine-generator train. During the operating license review, the licensee e rformed an analysis which indicated that the missile shield around the tur'.ne would stop the postulated worst-case low trajectory missile and the probability of significant damage by the postulated worst-case high trajectory missile 's within acceptable limits as defined in the Standard Review Plan, Section 2.2.3 (NUREG-0800). The staff had reviewed the consequence of turbine missiles and concluded that the total turbine missile risk from low and high trajectory missiles is acceptably low for GGNS Unit 1 so that the plant structures, systems, and components important to public safety are adequately protected against potential turbine' missile damage (GGNS Safety Evaluation Report, Section 3.5.3, September 1981). The staff reviewed its previous evaluation of the con-sequence of turbine missles using the information from this submittal and in the FSAR and re-affirms its previous finding.
2.4 Use of disc inspection results Results from previous inspections of turbines show that cracks may occur randomly among discs. The staff believes that the licensee should not plan their inspection intervals solely from analyses but also should consider operating data and engineering judgment. The licensee has stated that future inspection schedules may be altered depending on results from prior inspections, input from KWU, or unexpected problems with continued operation of any of the GGNS steam conversion units.
If the detection of cracks in one turbine unit is determined to affect the unit's operability, the licensee has committed to inspect the remaining two units for similar conditions during the same outage (December 27, 1985 submittal). The staff accepts this comitment as an indication of the licensee's intent to consider operating data in their inspection program.
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o 2.5 Summary Despite the staff's reservations on the methodology and input data used in cal-culating the probability of disc failure as noted in Section 2.1 above, the staff considers the 50,000 hour0 days <br />0 hours <br />0 weeks <br />0 months <br /> inspection interval and the associated inspection program for three low pressure turbines to be acceptable because:
(1) the overall proba-bility of turbine missile damage to safety related structures systems and components is small and the consequence of turbine failure affecting public safety is accept-ably low, and (2) the licensee has committed to (a) alter the future inspection schedules depending on results from prior (GGNS) inspections, in or unexpected problems arising from turbine operation, and, (b) puts from KWU, inspect the remaining two LP turbines before restarting the unit if cracks are detected in one LP turbine.
Accordingly, the staff concludes that the licensee's requested change in License Condition 2.C.(26) is acceptable. The staff notes that the proposed license condition retains the requirement to report to the NRC any unacceptable indications of cracking identified in low pressure turbine disc inspections..
3.0 ENVIRONMENTAL CONSIDERATION
The amendment involves a change of requirements of facility components located within the restricted area as defined in 10 CFR 20 and a change in surveillance requirements. The Commission made a proposed determination that the amendment involves no significant hazards consideration, and there have been no comments on that proposal. Based on its evaluation, the staff concludes that there is no significant change in types or significant increase in the amounts of any effluents that may be released offsite. There is no significant increase in individual or cumulative occupational radiation exposure because the changes do not affect personnel exposure. Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR Section 51.22(c)(9). Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment' need be prepared in connection with the issuance of the amendment.
4.0 CONCLUSION
The Commission made a proposed determination that the amendment involves no significant hazards consideration which was published in the Federal Register (51 FR 6826) on February 26, 1986, and consulted with the state of Mississippi.
No public comments were received, and the state of Mississippi did not have any coments.
We have concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Comission's regulations, and issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
J. Tsao, Engineering Branch, DBL L. Kintner, BWR Project Directorate No. 4, DBL Dated:
April 29, 1986