ML20198E715

Notifies & Provides NRC W/Supporting Documentation Re Util Plans to Replace HP & LP Turbine Rotors on Main Turbines at Limerick Station.Turbine Rotor Mod Work Scheduled to Be Completed During Next Refueling Outage.Encl Withheld

ML20198E715
Person / Time
Site:	Grand Gulf, Limerick
Issue date:	07/25/1997
From:	Hunger G PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML19317C568	List: ML20198E715
References
NUDOCS 9708080066
Download: ML20198E715 (12)

Text

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Station Support Department N

PECO NUCLEAR nco rm c_

A Unit of PECO Energy NNN$f July 25,1997 Docket Nos. 50 352 50-353 tJcense Nos. NPF-39 NPF-85 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555

Subject:

Limerick Generating Station, Units 1 and 2 Replacement of Main Turbine Rotors Gentlemen:

The purpose of this letter is to notify the NRC, and provide supporting documentation, regarding PECO Energy's plans to replace the high pressure (HP) and low pressure (LP) turbine rotors on l'w Main Turbines at Limerick Generating Station, Units 1 and 2. The new turbine rotor assemblies aie being designed and manufactured by Siemens Power Corporation (SPC). These new advanced turbine rotors will replace the existing three (3) LP turbine rotors and one (1) HP turbine rotor on each of the Main Turbines. The existing et'rbine rotors were manufactured and installed by General Electric (GE). The turbine rotor modification work is scheduled to be completed during the next refueling outage for each unit at LGS (i.e., Unit 1 - April 1998 and Unit 2 - April 1999).

The replacement of the HP and LP turbine rotors on the Main Turbines is intended to accomplish the following:

. Eliminate the possibility of Stress Corrosion Cracking (SCC) in the area of the turbine disk /

key-ways,

. Reduce the degree of erosion of the turbine rotating and stationary parts,

. Ennance net turbine output capability by taking advantage of more efficient steam path Oj/j designs, and

. Extend the interval between inspections required for the turbine maintenance program by utilizing improved manufacturing techniques and selection of material.

The proposed turbine retrofit modification work will minimize the impact on the existing Main Turbine system equipment, components, and support systems by making use of existing material and equipment. The interfacing equipment, components, and system functions are being maintained within the current plant design basis, with only minor changes being required.

SPC has provided an updated missile probability analysis methodology in support of this proposed turbine modification work to account for the retrofitted turbine design in conjunction with interface of the existing GE turbine overspeed controls which are [ Lot being tr.odified. A p, revised turbine maintenance program was also developed to maintain / provide the bases for the Wf,4'i', T probability analysis and assumptions. With a few exceptions, which are considered unique for

& the LGS tprbine retrofit modificatio , the basic principles of the methodology used by SPC are (W(1(dK Lir Mid If OMf \lll@l[\l[lll3l((l((l[Ill,I 9708080066 970725 '

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July 25,1997 Page 2 the same as those used by SPC in previous studies which have been reviewed and accepted by

- the NRC (e.g., Grand Gulf, Comanche Peak, and Connecticut Yankee). The exceptions which are considered unique for LGS are as fellows:

1. The probabilities will support an initial rotor inspection interval of 100,000 operating hours while implementing a Quarterly / Quarterly / Quarterly (Q/Q/Q) turbine valve (i.e.,

stop valves, controls valves, and combined-intermediate valves) test program.

2. A numerical Monte Carlo simulation technique has been developed and utilized for probabilistic computations. It is a simpler and more versatile technique for probability analyses and allows the use of more simulation cycles for determining ti.e probability of a missile due to burst .

3. Existing GE turbine overspeed controls and protection failure probabilities, as related to valve test frequency, have been integrated into SPC's missile probability analysis methodology, The application of the SPC methodology for determining missile probability in support of turbine retrofit modification work at LGS, concluded that the probability of generating a turbine missile is maintained w! thin the NRC accepted limit of 1 x 104 per year for a 10 year inspection interval (87,600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br />) and a O/Q/Q valve testing frequency. The NRC acceptance limit of 1 x 104 is based on the value documented in Sections 3.5.1.3 and 10.2 of NUREG 0991," Safety Evaluation Report Related to the Operation of Limerick Generating Station, Units 1 and 2."

The following attachments are provided which support design and implementation of this plant modification.

. Attachment 1 - Modification Summary

. Attachment 2 Missile Probability Analysis Overview

. Attachment 3 - Siemens Engineering Report ER-9605,' Missile Probability Analysis Methodology for Limerick Generating Station, Units 1 and 2, With Siemens Retrofit Turbines

• Revision 2, June 18,1997

. Attachment 4 - Engineering Report ER-8503," Probability of Disk Cracking Due to Stress Corrosion,' Grand Gulf Unit 1, March 1985

. Attachment 5 - Engineering Report ER-8402," Probability of Disk Cracking Due to Stress Corrosion," Comanche Peak Unit 1, August 1984

. Attachment 6 UPC Engineering Report ER-8611," Turbine Missile Analysis for 1800 rpm Nuclear LP-Turbines with 44-inch Last Stage Blades *

. Attachment 7 UPC Engineering Report ER 8605a," Probability of Disk Cracking Due to Stress Corrosion," Connecticut Yankee, Replacement LP Rotors, July 1986 (Revision 1, June 1987)

. Attachment 8 - Siemens Report, " Theoretical Bases and Calculations for Extending the Intetval Between Testing of the Turbine Valves

• Attachments 2-8 contain information of a proprietary nature to SPC. Therefore,in accordance with the requirements of 10 CFR 2.790(a)(1)(1)(4) we are requesting that the contents of Attachments 2-8 be withheld from public disclosure since they contain information pertinent to trade secrets and commercial or financialinformation considered to be privileged or confidential.

In accordance with the requirements of 10CFR2.700(b)(1) the required affidavit supporting this request is enclosed.

. ,, e . . p. e -

July 25,1997 Page 3 -

i The retrofitted turbines do not require changes to the Limiting Conditions of Operation (LCO) or Surveillance Requirements (SRs) as specified in the Technical Specifications (TS). However, the commitments associated with the methods for turbine maintenance, determination of the probabilities of turbine missile generation, and turbine stop valve and turbine control valve test frequencies, as currently stipulated in Section 3.5.1.3 of NUREG 0991,

• Safety Evaluation Report Related to the Operation of Limerick Generating Station, Units 1 and 2,* and Supplement 8 to NUREG-0991 will be modified in support of this plant modification. The attached supporting documentation describes the methodology, maintenance program, and the intended valve testing frequency which is being proposed. Therefore, PECO Energy would appreciate the NRC's review and concurrence of the attached information by February 28,1998,in support of our efforts to replace the turbine rotors at LGS.

If you have any questions or require additionalinformation, please do not hesitate to contact us.

Very truly yours, G. A. Hunger, Jr.

Director Licensing Attachments Enclosure cc: H. J. Miller, Administrator, Region I, USNRC (w/o attachments / enclosure)

N. S. Perry, USNRC Senior Resident inspector, LGS (w/o attachments / enclosure)

ENCLOSURE "

SIEMENS POWER CORPORATION AFFIDAVIT LIMERICK GENERATING STATION UNITS 1 AND 2 MAIN TURBINE ROTOR REPLACMENT

I l

l AFFIDAVIT STATE OF WISCONSIN COUNTY OF MILWAUKEE i

I, Lamont J. Jenkins being duly sworn, hereby say and depose:

1. I am Vice President, Turbine Uervices, for Siemens Power Corporation, (SPC), and as such I am authorized to execute this Affidavli.

2. I am familiar with SPC's detailed document control system and policies which govem the protection and control of information.

3. I am familiar with Engineering Report ER-9605 " Missile Probability Analysis Methodology for Limerick Generating Stations, Units 1 & 2 with Siemens Retrofit Turbines" Revision No. 2, dated June 18,1997 including the referenced documents: Engineering Report ER-85-03, " Probability of Disk Cracking Due to Stress Corrosion," Grand Gulf Unit 1, dated March 1985, Engineering Report ER-8402, " Probability of Disk Cracking Due to Stress Corrosion," Comanche Peak Unit 1, dated August 1984, UPC Engineering Report ER-8611,

" Turbine Missile Analysis for 1800 rpm Nuclear LP-Turbines with 44-inch Last Stage Blades" ,

Engineering Report ER-8605, " Probability of Disk Cracking Due to Stress Corrosion," Connecticut Yankee, Replacement LP Rotors, dated July 1986 (Revision 1 dated June 1987), and Siemens Report, " Theoretical Bases and Calculations for Extending the interval Between Testing of the Turbine Valves", referred to as " Documents." Information containeo in these Documents have been classified by EPC bs proprietary in accordance with the control system and policies established by SPC for the control and protection of information, i1

]

4. Then Documents contain information of a proprietary and confidential nature and is of the type customarily held in confidence by SPC and not made available to the public. Based on my experience, I am aware that other companies regard information of the kind contained in the Documents as proprietary and confidential.

5. The Documents have been made available to the U.S. Nuclear Regulatory Commission in confidence, with the request that the information contained in the Documents will not be disclosed or divulged.

M 6. The Documents contain information which is vital to a competitive advantage of SPC and would be helpful to competitors of SPC when competing with SPC.

7. The information contained in the Documents is considered to be proprietar; by SPC because it reveals certain distinguishing aspects of SPC mechanical design methodology which secure competitive advantage to SPC for disc design optimization and marketability, and includes information utilized by SPC in its business which affords SPC an '

opportunity to obtain a competitive advantage over its competitors who do not or may not know or use the information contained in the Documents.

8. The disclosure of the proprietary information contained in the Documents to a competitor would permit the competitor to reduce its expenditure of money and manpower and to improve its competitive position by giving it valuable insights into SPC mechanical design methodology and would result in substantial harm to the competitive position of SPC.

9. The Documents contain proprietary information which is held in confidence by SPC and is not available in oubib sources, j

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_ , , , , _ , _ _ , _ , , , , _. . ._ _ _ _ _ _ _ _ . _ . . J

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l 10.- In accordance with SPC policies governing the protection and control of information, proprietary information contained in the Documento have been made available, on a limited basis, to others outside of SPC only as required and under suitable agreement providing for nondisclosure and limited use of the information.

11. SPC policy requires that proprietary information be kept in a secured file or area and distributed on a need-to-know basis.

12. Information in these documents prevides insight into SPC mechanical design methodology developed by SPC. SP0 has invested significant resources in developing the methodology as well as the strategy for this application. Assuming a competitor had available the same background data and incentives as SPC, the competitor might, at a minimum, develop the information for the same expenditure and money as SPC.

THAT the statements made hereinabove are, to the best' of my knowledge,' -

information, and belief, truthful and complete.

FURTHER AFFIANT SAYETH NOT.

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i SUSCRIBED before me this I%

day'of ukU .1997

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ATTACHMENT 1 MODIFICATION

SUMMARY

LIMERICK GENERATING STATION UNITS 1 AND 2 MAIN TURBINE ROTOR REPLACEMENT l

Attachment 1 ,

Limerick Generating Station Units 1 and 2 Main Turbine Rotor Replacement Modification Summary DJSCUSSION Each unit at the Limerick Generating Station (LGS) was originally provided with one (1) tandem compound Main Turbine Generator fumished by the General Electric Company (GE). Each GE Main Turbine-Generator consists of one double flow, high pressure (HP) turbine section and three (3) double flow, low pressure (LP) sections. Each is a nonreheat turbine,1800 RPM, with 38 inch last siegs buckets, exhausting to a multipressure condenser. The turbine was furnished with six extraction points; one HP section extraction, one cross-around piping extraction, and four (4) LP section extractions. The cross-around piping also delivers steam to six (6) vertical moisture separator vessels of the nonreheat type for retum of steam to the LP turbines, and steam to three (3) Reactor Feed Pump Turbines.

This plant modification is designed to support the replacement of the high pressure (HP) and low pressure (LP) turbine rotor assemblies on the Main Turbines at LGS, Units 1 and 2. The new replacement turbine rotors are being designed and manufactured by Siemens Power Corporation (SPC). The new turbine rotors will replace the three (3) LP rotors and one (1) HP rotor which were manufactured and installed by General Electric (GE) on each unit. The replacement LP rotors are a " shrunk-on-disk" design. The HP rotor is a "monoblock" design. SPC has provided a new methodology for analyzing missile generation probability which considers the design of the replacement rotors in combination with the existing GE turbine control system.

This proposed plant modification is scheduled to be installed on LGS Unit i during 1RO7 (scheduled for April 1998) and LGS Unit 2 during 2RO5 (scheduled for April 1999). Implementation of this plant modification will help to minimize the overall impact on the LGS Operations staff, in that, the extension of the interval between inspections required by the maintenance program will result in fewer load drops, and a resulting decrease in potential for operator error and reactivity management concems. In addition, the Siemens turbine rotors are a proven design in Europe and the United States with many years of successful operating experience.

NRC General Design Criterion 4,

• Environmental and Missile Design Bases," of Appendix A to 10CFR50 requires, in part, that structures, systems, and components important to safety be appropriately protected against the effects of missiles that might result from failures, such as high energy missiles resu! ting from a steam turbine failure. The NRC evaluation of the effects of turb;ne failure on the public health and safety has generically followed the guidance stipulated in Regulatory Guide (RG) 1.115," Protection Against Low.

Trajectory Turbine Missiles," and the NRC Standard Review Plan (SRP), i.e., NUREG-0800, Sections 10.2, 10.2 3, and 3.5.1.3. According to NRC guidelines stated in RG 1.115, the orobability of unneceotable damaae from turbine missiles (P4) should be less than or equal to 1 x 10# per year for an individual plant. In order to maintain P4 less than 1 x 10 , the NRC requires licensees to ensure that the probability of ceneratinc a turbine minile (P1) satisfies turbine reliability requirements criteria. For an " unfavorably oriented turbine," such as the LGS, Units 1 and 2, Main Turbines, the probability of generating a turbine missile (P1) is required to be less that 1 x 104per year as documented in NUREG-0991," Safety Evaluation Report Related to the Operation of Limerick Generating Station, Units 1 and 2."

In order to assure that the turbine missile generation probability (P1) follows the turbine reliability criteria, the NRC requires applicable licensees to maintain a Turbine System Maintenance Program (TSM Program),

including Low Pressure (LP) Turbine inspection intervals based on manufacturer's calculation of missile generation probabilities. Both NUREG-0991 (Section 3.5.1.3) and the Unit 1 Operating License (Section 2.C.9) required that such a TSM Program, which meets the guidance stipulated in the NRC SRP and RG i.115, be submitted to the NRC for review and approval. The current LGS TSM Program was approved by I

Attachment 1 Main Turbine Rotor Replacement l Modification Summary Page 2 of 4 the NRC, as documented in NUREG-0991, Supplement 8, and letters from the NRC dated November 3, 1987, and May 9,1989, and was based on the GE Turbine Missile Generation Probability Analysis Methodology and criteria c5cumented in NUREG 1048," Safety Evaluation Report for the Operation of Hope Creek Generating Station," Supplement 6.

The industry experience of LP turbine component failures due to stress corrosion cracking has led PECO Energy to take a proactive approach to assure that significant stress corrosion cracking (SCC) problems at LGS are not encountered. This plant modification will replace both LP turbine rotors (three each) on the LGS, Units 1 and 2. Main Turbines in order to correct and/or prevent distress. In addition, this modification will replace HP turbine rotors on the Main Turbines based on the associated cost benefit.

Since the LP turbines are a

• shrunk-on-disk" design, Limerick Generating Station will be required to maintain a TSM Program. The replacement LP turbines will meet or exceed the original equipment design, manufacturing, and inspection criteria evaluated and accepted by the NRC as documented in NUREG-0991 and NUREG4991, Supplement 8. Therefore, the replacement turbines will meet the acceptance criteria delineated in the NRC's SRP.

SPC has provided a methodology (Attachment 3) for analyzing missile generation probability that addresses the design of the replacement LP turbines in combination with the ex! sting GE Main Turbine Control System.

Attachme-t 2 " Missile Probability Analysis Overview" is provided to highlight the SPC design features ard overall conservatisms which were incorporated into the SPC methodology. Use of the SPC methodology in support of the LGS TSM Program ensures continued compliance with the guidance of RG 1.115 and ultimately, GDC 4. The basic principles of the SPC methodology (Attachment 3) are the same as those used in previous SPC studies (including Grand Gulf, Comanche Peak, and Connecticut Yankee) that have been reviewed and acceptcJ by the NRC. The methodology addresses the same three (3) major components that were considered by the GE methodology and accepted by the NRC as documented in NUREG-1048. These three (3) components are:

1. probability of turbine overspeed,

2. wheel burst probability, and

3. probability of casing penetration.

The LGS TSM Pregram utilizes a "probabilistic approach" to determine required intervals for inspection and maintenance /replacemen' of the LP turbines. The original GE missile generation analysis limited inspection intervals to 6 years. The current TSM Program reflects the 6-year inspection interval, except for a one-time exception on Unit 1's "B " and "C" LP turbines rotors as documented in an NRC letter dated Januarv 2,1996.

The SPC methodology (Attachment 3) evaluates inspection intervals up to 100,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> (approximately 12 years). The proposed change to the LGS TSM Program limits the maximum inspection interval to 10 years, which satisfies the criteria delineated in SRP Section 10.2.3.

The TSM Program also includes required intervals for performing in-service testing of the Main Turbine Stop Valves, Control Valves, and Combined-intermediate Valves as documented in NUREG-0991 and NUREG-0991, Supplement 3. The test intervals are also listed as surveillance requirements in Section 3/4.3.8 of the LGS, Units t and 2, Technical Requirements Manuals (TRMs). Currently, the Main Turbine Stop and Combined-Intermediate Valves are tested once per week. The Main Turbine Control Valves are tested once per month. The test intervals are TSM Program and TRM requirements since they provide inputs to the current GE missile generation probability analysis regarding the reliability of the Main Turbine overspeed protection system. Performance of the valve testing at the* stated irtervals supports the current LP turbine inspection intervals, and therefore, the probability of a turbine missile is less than 1 x 104 per year.

Attachment 1 Main Turbine Rotor Raplacement Modification Summary Page 3 of 4 The SPC methodology (Attachment 3) ovaluates a maximum test interval of 3 months for Main Turbine Stop Valves, Control Valves, and Combined-intermediate Valves. Based on the assumptions in the SPC methodology, the valve test intervals support LP turbine inspection intervals of 100,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> and maintains the probability of turbine missiles less than 1 x 104 per year. Extension of valve test intervals based on missile generation probability has previously been accepted by the NRC as indicated in NUREG-0991, Supplement 3, Section 10.2.

Since the proposed chan;;es to the TSM Program deviate from previously approved commitments associated I

with the implementation of the TSM Program and its basis (i.e., the missile probability analysis methodology),

the SPC Missile Probability Analysis Methodology and the proposed changes to the present TSM Program are being submitted to the NRC for review and acceptance.

SAFETY DISCUSSION The proposed changes do not alter LGS's conformance to RG 1.115 and GDC 4. Additionally, the proposed changes do not impact any Technical Specifications (TS) Limiting Condition for Operation (LCO) or Surveillance Requirement (SR).

Replacement of the turbine rotors on the Main Turbines requires use of the Siemens' Missile Probability Analysis Methodology (Attachment 3) to support the Turbine System Maintenance Program (TSM Program) at LGS. The current TSM Pro 0 ram was based on the GE Turbine Missile Generation Probability Analysis Methodology and inform # ion contained in NUREG-1048. The LGS turbine system design will continue to satisfy the guence M RG 1.115 and requirements of GDC 4, with regards to turbine missile generation.

The maximum interval betwe an LP turbine inspections will be limited to 10 years of operation, which is consistent with the criteria dolineated in the NRC SRP. The maximum intervals for turbine valve testing will be revised to once every 3 nonths.

The replacement of the Main Tubine rotor %semblies will not alter the assumed function, failure modes, or reliability of the retained GE Oversp# Protection Systems. As a result, performance of the periodic operational verifications that are inputs to the GE Missile Generation Probability Methodology will be maintained for the application of the Siemens Missile Probability Analysis Methodology. In addition, the method of testing as specified in NUREG-0991, Supplement 3, Section 10.2, and periodic inspection of the turbine stop valves, control valves, combined-intermediate valves, and extraction check valves as delineated in NUREG-0991 and NRC letters dated November 3,1987 and May 9,1989, are unchanged by the proposed revisions to the present Turbine System Maintenance Program.

The Siemens' Missile Probability Analysis Methodology (Attachment 3) supports the installation of replacement turbines that are being designed, manufactured, and inspected to meet or exceed the quality of the originalinstalled equipment.

Currently, there are no LGS specific accidents evaluated in the Safety Analysis Report (SAR) for the effects of turbine generated missiles. Implementation of the plant modification will continue to ensure that the probability of generating a turbine missile is maintained less than the current NRC accepted limit of 1 x 104 per year. Meeting this requirement assures compliance with GDC 4 , and precludes the necessity o.'

evaluating any specific accident as a result of turbine generated missiles as referenced in NUREG-0991, i

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Attachment 1 Main Turbine Rotor Replacement Modification Summary Page 4 of 4 The Main Turbines and their overspeed protection systems are not safety-related and perform no safety function. Replacement of the turbine rotors will not alter, degrade, or prevent the response of active or

. passive components of this system from functioning as originally designed.

The issue of turbine rotor integrity has been addressed generically by the NRC's establishment of a limit on turbine missile generation such that the probability of unacceptable damage from turbine missiles should be 3

less than or equal to 1 x 10 per year, as specified in RG 1,115, in order to meet the requirements of GDC 4.

To accomplish this, the probability of generating a turbine missile must be maintained less than 1 x 104 per year as indicated in NUREG-0991. The replacement of the turbine rotors will continue to maintain the turbine missile genet ation probability less that the limit specified by the NRC as documented in NUREG-0991 and NUREG-0991, Supplement 8 Replacement of the turbine rotors will not alter, degrade, or prevent the response of Equipment important to Safety. NUREG-0991, Section 10.2 , does describe the

• turbine control system" as "important to the overall safe operation of the plant." The funcit00 of the control system related to overall plant safety is to piovide overspeed protection to minimize the probability of generation of turbine misslies, in accordance with the requirements of GDC 4. Revising the test intervals for the valves does result in an increase, above that for the current valve test intervals, in the probability of a turbine overspeed event assumed in the Siemens Missile Probability Analysis Methodology (Attachment 3). Although this individual probability value has increased, the overall probability of a turbine generated missile is maintained less than the limit of 1 x 104 per year (refer to Attachment 3) as required to meet GDC 4.

Replacement of the turbine rotors and extension of the valve testing frequency will not adversely affect the function or failure modes of any Equipment important to Safety . Therefore, the consequences of a malfunction of any Equipment important to Safety remain unchanged. The Siemens' Missile Probability Analysis Methodology (Attachment 3) has demonstrated that the probability of a turbine generated missile is 4

less than 1 x 10 per year which is the limit specified in NUREG-0991. As a result, the probability of

" unacceptable damage from turbine missiles" is maintained less than or equal to 1 x 10# per year for an individual plant, as stipulated in NUREG 0991 and RG 1.115. This plant modification will continue to ensure compiiance with the requirements of GDC 4 with regards to the possibility of"intemal missiles" generated by a catastrophic turbine-generator failure.

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