ML20138B863
| ML20138B863 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 11/01/1985 |
| From: | MISSISSIPPI POWER & LIGHT CO. |
| To: | |
| Shared Package | |
| ML20138B837 | List: |
| References | |
| TAC-60085, NUDOCS 8512120399 | |
| Download: ML20138B863 (6) | |
Text
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BEFORE THE UNITED STATES NUCLEAR REGULATORY COMMISSION LICENSE NO. NPF-29 DOCKET No. 50-416 IN THE MATTER OF MISSISSIPPI POWER & LIGHT COMPANY and MIDDLE SOUTH ENERGY, INC.
and SOUTH MISSISSIPPI ELECTRIC POWER ASSOCIATION AFFIRMATION I, O. D. Kingsley, Jr., being duly sworn, stated that I am Vice President, Nuclear Operations of Mississippi Power & Light Company; that on behalf of Mississippi Power & Light Company, Middle South Energy, Inc., and South Mississippi Electric Power Association I am authorized by Mississippi Power & Light Company to sign and file with the Nuclear Regulatory Commission, this application for amendment of the Operating License of the Grand Gulf Nuclear Station; that I signed this application as Vice President, Nuclear Operations of Mississippi Power & Light Company; and that the statements made and the matters set forth therein are true and correct to th best of my knowledge, information and belief.
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D. N Kin sie STATE OF MISSISSIPPI COUNTY OF HINDS SUBSCRIBED AND SWORN TO before me, a Notary Public, in and for the County and State above named, this f S1 day of 4/tt/C4n her 1985.
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i My commission expires:
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Attachm2nt to AECM-85/0333 Page 1 of 6 (0LCR-NLS-85/05)
SUBJECT:
Facility Operating License No. NPF-29, Page 9, License Condition 2.C.(26)
DISCUSSION:
The present license condition requires all the low pressure (LP) turbine discs to be ultrasonically inspected for IGSCC in the bores and keyways during each refueling outage.
Mississippi Power & Light (MP&L) proposes that this license condition be amended to allow for the inspection of the LP turbine discs on an inspection interval not to exceed 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation.
It is respectfully requested that the NRC provide MP&L with a decision on this proposed license amendment by January 6, 1986.
The inspection of the LP turbines requires the development of inspection procedures and the procurement of inspection equipment, both of which are long lead time items. A decision by January 6, 1986 will allow adequate time for final planning and scheduling of the first refueling outage.
JUSTIFICATION: License Condition 2.C. (26) resulted because of a history of corrosion crack initiation associated with LP turbine discs manufactured by certain domestic turbine / generator vendors.
The turbine / generator system at Grand Gulf Nuclear Station (GGNS) was manufactured by Kraftwerk Union (KWU). The design of this turbine is significantly different than the designs which have shown a history of corrosion crack initiation.
Because of its unique keyway design, the KWU LP turbine disc is less susceptible to stress corrosion cracking than other designs. The KWU design minimizes stress within the keyway area which would contribute to corrosion crack initiation.
This is accomplished in the KWU LP turbine by not locating the keyway in the shrink-fit area of the disc-rotor shaft attachment. This design results in lower shrink-fit stresses in the keyway than in other designs. High stress concentrations at the bottom end of the keyway bores have been eliminated by a machined radial relief groove in the disc hub area and the rotor shaft. This groove is open to the outside over the entire circumference by a 1 mm gap between the disc and the shaft. This gap allows for breathing and prevents trapping of corrosion products in the keyway. To eliminate possible condensation within the keyways, the KWU LP turbine keyways are located where the metal temperature is higher than the temperature of the surrounding saturated steam. These unique design features make the KWU LP turbines less susceptible to stress corrosion cracking than other designs.
For KWU LP turbines that have undergone ultrasonic inspections to date, no crack initiation has been found. This includes turbines that have accumulated up to 80,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of service. More details on the KWU keyway design are provided in the attached MP&L Nuclear Plant Engineering (NPE) Report No. 85-M-006 (Attachment I).
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Attachment to AECM-85/0333 Page 2 of 6
)
In addition to the design of the KWU turbines which lessen the unit's susceptibility to stress corrosion cracking, MP&L maintains high reactor water quality to ensure the protection of the reactor and other power conversion components. This is accomplished by incorporation of reactor vendor water quality criteria, fuel warranty requirements and other industry recognized recommendations. This ensures that during normal operating conditions that steam quality either meets or exceeds the. quality requirements as recommended by KWU.
Using the available data on cracked LP turbine discs where the actual crack growth rate has been determined by repetitive inspections, the Electric Power Research Institute (EPRI) has developed an apparent crack growth rate model.
In Attachment I the apparent crack growth rate model developed by EPRI was used to predict potential corrosion crack initiation and growth rates in the KWU LP turbine discs. The results from this model can be used in projecting at what point in the life cycle of the turbine one could reasonably expect to encounter problems due to corrosion cracking. The results are not used in making a decision on whether to continue to run or retire a unit. The run/ retire decision for a system would continue to be made by conventional fracture mechanics analysis techniques following the verification of crack initiation and the actual crack sizes by inspection of the system.
In applying the apparent crack growth rate model for GGNS, worst case conditions for cycle I were assumed:
(1) 100% plant availability (13,140 hours0.00162 days <br />0.0389 hours <br />2.314815e-4 weeks <br />5.327e-5 months <br /> of operation)
(2) One 120% overspeed event The projected end of cycle 1 crack sizes were calculated and then compared to the calculated critical crack sizes for the LP turbines. The critical crack sizes for GCNS's LP turbines were documented in Allis Chalmers Power Systems, Inc. (ACPSI)
Engineering Report ER-8105 submitted to the NRC by AECM-82/103,
. March 23, 1982 (A copy of this report is provided as Attachment V).
At the end of cycle 1 the apparent crack growth rate model predicts a maximum crack size of less than 11% of the calculated critical crack size for the GGNS LP turbine discs.
This demonstrates that even under the worst cycle.I conditions, no stress corrosion cracking problems are predicted by the apparent crack growth rate model and an inspection of the LP turbines discs so early in operation is unwarranted.
l
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Attachmsnt to AECM-85/0333 Page 3 of 6 This is especially true when one considers the conservatism of the apparent crack growth rate model.
For example, the model assumes a crack is initiated immediately upon the initial start-up of the unit and then continues to grow throughout the life of the unit. When in actual fact, the initiation of IGSCC requires a finite time for the susceptible material to be exposed to the working environment before any cracking can occur. This finite time has been shown to be generally in the order of years.
Because KWU turbine / generator systems are designed and tested for safe operation of speeds of up to 120% of rated speed, rotor failure at or below this speed could only occur due to defective material, inadequate quality assurance or design error. Due to the high reliability provided by the design, manufacture and quality assurance of the LP discs and rotor, the probability of such a failure is extremely low.
If cracks or other defects were present and resulted in a disc failure, the KWU LP turbine is designed to contain internally all missiles of less than 160% of rated speed as documented in ACPSI Engineering Report ER-503 (Attachment II).
For KWU LP turbines in which no cracks or other deficiencies exist, Engineering Report ER-503 documents that speeds of up to 180%
of rated speed are required before disc failure is calculated to occur. As documented in ACPSI Engineering Report ER-504 (Attachment III), the probability of experiencing an overspeed event of greater than 120% of rated speed for a six flow KWU turbine is calculated to be 1.6E-7 per unit year.
It is also conservatively assumed that this is also the probability of producing an external turbine missile from a KWU LP turbine.
It is the intent of MP&L to implement as a planning basis a turbine / generator maintenance program similar to that developed and recommended to MP&L by KWU. Adherence to the recommendations of this program will ensure the inspection of the HP turbine, generator / exciter and LP turbines over an interval of four fuel cycles. In accordance with this program the first refueling outage should consist of the disassembly and inspection of both the HP turbine and generator / exciter. This is to be followed by an inspection of the three LP turbines by completion of the fourth refueling outage.
Following the inspections of the LP turbines, the inspection sequence begins over with the HP turbine, generator / exciter and the three LP turbines being reinspected by the completion of each fourth refueling outage.
KWU recommends an inspection of the HP turbine following the i
first fuel cycle due to a potential problem with the turbine blades. KWU units have in the past experienced a tendency for the blades to loosen slightly resulting in a slight blade pitch change. This condition, if left unattended, may cause the unit to experience vibration problems. KWU also recommends that J16ATTC85091301 - 3
Attachmsnt to AECM-85/0333 Page 4 of 6 the generator / exciter unit be inspected early in the service life of the unit for loose components especially within the end winding area. Early inspections of the HP turbine and generator / exciter are considered prudent maintenance practice in order to eliminate potential problems with the operation of these units.
As noted, it is the intent of MP&L to implement a turbine / generator inspection interval as outlined in the KWU maintenance program.
It is therefore anticipated that the LP turbine inspections will be performed during the second, third and fourth refueling outctes.
It is also the intention of MP&L that all future refueling outage critical paths be controlled by refueling outage activities only. Therefore, in order to ensure that the inspection of the LP turbines does not impact future outages from either a critical path or a complexity standpoint, it is requested that License Condition 2.C.(26) be amended to require the LP turbines be inspected prior to reaching 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation. The inspection of all three LP turbines by each fourth refueling outage would ensure : hat all LP turbines were inspected prior to reaching 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation.
The 50,00 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> between inspections is arrived at by calculati and plotting the probability of having a disc rupture versus service Fours between inspections. The point where the probability : disc rupture equals the NRC reliability criterir. if IE-5 is at approximately 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
This is covered in detail in the attached Utility Power Corporation (UPC) Engineering Report ER-8503 (Attachment IV).
The GGNS first refueling outage is estimated to extend for approximately 13 weeks and involves approximately 57 work items, several of which are very large complex design changes.
MP&L is continuing to aggressively review the work schedule for possible ways to optimize and thus reduce the estimated 13 weeks. The first refueling outage work scope now consists of the disassembly and inspection of the HP turbine and the generator / exciter. This work is estimated to require approximately 10 weeks and 35,000 manhours and, as such, represents a significant portion of the total work scope to be performed during the first refueling outage. The addition of an ultrasonic inspection of the LP turbines will prolong the refueling outage by as much as 11 weeks resulting in a refueling outage of up to 24 weeks.
It has also been estimated that any future refueling outage which included the inspection of all 3 LP turbines in its work scope would be substantially prolonged.
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Attachmtnt to AECM-85/0333 Page 5 of 6 The length of time required to perform inspections of the HP turbine, generator / exciter and the LP turbines is affected by the available laydown area within the turbine building and the restricted access to cutside laydown area. A wall constructed in the turbine building to separate the Unit II construction site from Unit I prevents the simultaneous inspection of the HP turbine, generator / exciter and all LP turbines.
This is due to the fact that with the wall in place, large pieces of equipment cannot be removed to the outside for laydown and the laydown area within the turbine building is very limited. As a result, the addition of an ultrasonic inspection for all three LP turbines along with the already planned HP turbine and generator / exciter inspection would lengthen the first refueling outage by as much as 11 weeks and require an additional 32,000 manhours. In estimating the length of time required to perform the LP inspections, it was assumed that the wall restricting access to outside laydown area would remain in place. Although the constraint of restricted laydown area can be overcome by the disassembly and the reassembly of the turbine building wall, this is in itself a very large task and may not result in a substantial schedule reduction over the estimated 24 weeks.
The requirement to inspect the LP turbines during the first refueling outage would impose undue hardship on MP&L in the form of prolonged outage time, increased outage direct cost and increased power replacement costs. The inspection of the LP turbines during the first refueling outage is contrary to the long range maintenance program as recommended by KWU.
Additionally, MP&L believes that this and past submittals presents adequate technical justification to demonstrate that the requirement to inspect all LP turbines at each refueling outage is unnecessarily conservative and onerous.
It is therefore requested that License Condition 2.C.(26) be amended to require that the LP turbine inspection interval not exceed 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation.
SIGNIFICANT HAZARDS CONSIDERATION:
The proposed change to the operating license does not involve:
1)
A significant increase in the probability or consequences of a previously evaluated accident because the unique design features of the KWU turbine reduce the likelihood of corrosion crack initiation and failure of a disc resulting in an external missile so that inspection at the proposed longer interval does not significantly increase the probability or consequences of a previously evaluated accident.
J16ATTC85091301 - 5
Attachment to AECM-85/0333 Page 6 of 6 2)
Creating the possibility of a new or different kind of accident from any previously evaluated accident because the possibility of a turbine disc failure was addressed in the FSAR.
3)
A significant reduction in a margin of safety because the proposed license condition has no effect on any analysis as previously presented in the FSAR or on any governing technical specifications.
Therefore, the proposed change does not involve a significant hazards consideration.
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