ML17194B304

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Forwards Final Evaluation of SEP Topic III-4.B, Turbine Missiles. Plant Does Not Comply W/Proposed Schedule for Turbine Disk Inservice Insp.Need for Implementation to Be Addressed in Plant Integrated Assessment
ML17194B304
Person / Time
Site: Dresden 
Issue date: 09/16/1982
From: Oconnor P
Office of Nuclear Reactor Regulation
To: Delgeorge L
COMMONWEALTH EDISON CO.
References
TASK-03-04.B, TASK-3-4.B, TASK-RR LSO5-82-09-059, LSO5-82-9-59, NUDOCS 8209220178
Download: ML17194B304 (10)
v • d • e


Text

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September 16, 1982 Docket No. 50-237 LS05 09-059 Mr. L. DelGeorge Director of Nuclear Licensing Commonwealth Edison Company Post Office Box 767 Chicago, Illinois 60690

Dear Mr. DelGeorge:

SUBJECT:

SEP TOPIC III-4.B, TURBINE MISSILES DRESDEN NUCLEAR POWER STATION., UNIT 2 Enclosed is the staff's final evaluation for SEP Topic III-4.B, Turbine Missiles for the Dresden Nuclear Power Station, Unit 2. This evaluation incorporates those comments provided by your letter dated August 30, 1982.

The staff's draft safety evaluation dated May 26, 1982, contained a probabil i~tic study of turbine missil~ risk based on a geometric strike probability for the missHe t~rgets and a separate a~alysis of the per-,

forabil ity of 1nterven,rig barriers. 'That study was ~ased on historical failure rates of turbfoe whe.~l materials. Currently, the staff has

  • shifted the review procedure for turbine missile generation to emphasize turbine rel?iability. The NRC staff has developed interim criteria ~~ard ing turbine inspection programs and schedules. A detailed discussion* of,

the current NRC approabh* is given in Section IV of this report. This final *safety evaluation has been revised to reflect the staff's emphasis on* turbine re13abf11ty.

The review of the Dresden 2 facility with regard to current interim criteria St;;c>l/

has concluded that Dresden 2 does not comply with the proposed schedule for

( )

turbine ldlisk inservice fnspe~tfon *. The need fori)'iiJfplementation of th,is OS"- '-"G Ii:,

inspection schedule will be addressed in the Dresden 2 Plant Integrated Asse*ssment.

AoO **

Sincerely, c;. S~o. / G f

  • O~igin*al signed by:

Paul ID;>'Connor, Project Manager Operating Reactors Branch No. 5 Division of Licensing

Enclosure:

As stated c

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OFFICIAL RECORD COPY USGPO: 1961-335*960

    • ,--*J-
  • Mr. L. DelGeorge cc Robert G. Fitzgibbons Jr.

Isham, Lincoln & Beale Counselors at Law Three First National Plaza Suite 5200 Chicago, Illinois 60602

. Mr. Doug Scott Plant Superintendent Dresden Nuclear Power Station Rural Route #1 Morrh, 11 l i noi s 60450

  • The Honorable Tom Corcoran United States House of Representatives Washington, D. c.

20515

  • U *. S. Nuclear Regulatory Commission Resident Inspectors Office Dresden Station RR 11 Morris, Illinois 60450 Mary Jo Murray Assistant Attorney General Environmental Control Division 188 W. Randolph Street Suite 2315 Chicago, Illinois 60601 Chainnan Board of Supervisors of

&Grundy County Grunay*tounty Courthouse Morris, Illinois 60450 John F. Wolf, Esquire 3409 Shepherd Street Chevy Chase, Maryland 20015 Dr. Linda W. Little 500 Hermitage Drive Raleigh, North Carol'L1a 27612.

Judge Forrest J. Remick

. The Carriage House - Apartment 205 2201 L Street, N. w.

Washington, D. c.

20037 Dresden 2*

Docket No. 50-237 Revised 5/19/82 Illinois* Department of Nuclear Safety 1035 Outer Park Drive, 5th Flooi Springfield, Illinois 62704 U. S. Environmental Protection Agency Federal Activities Branch.

Region V Office ATTN:

Regional Radiation Representative 230 South Dearborn Street Chicago, Illinois 60604 James G. Keppler, Regional Administrator Nuclear Regulatory Commission, Region III 799 Roosevelt Street Glen Ellyn, Illinois 60137

~..*

SYSTEMATIC EVALUATION PROGRAM TOPIC III-4.B DRESDEN NUCLEAR POWER STATION, UNIT 2 TOPIC:

111-4.B, Turbine Missiles I.

INTRODUCTION The safety objective of Topic III-4.B is to assure that structures, systems and conponents important to safety are adequately protected from potential turbine missiles.

Of those systems important to safety, this topic is primarily concerned with safety-related systems; i.e.,

those structures, systems or components necessary to perform required safety functions and to ensure:

1.

The integrity of the reactor coo 1 ant pressure boundary,

2.

The capability to shutdown the reactor and maintain it in a safe shutdown condition, or

3.
  • The capability to prevent accidents that could result in potential offsite exposures that are a significant fraction of the guideline exposures of 10 CFR Part 100,,"Reactor Site Criteria" (Ref. 1 ).

II.

REVIEW CRITERIA According to General Design Criterion 4, of Appendix A to 10 CFR Part 50 (Ref. 2), nuclear power plant structures, systems and components import-ant to safety shall be appropriately protected against dynamic effects of missiles.

Failures that could occur jn large steam turbines of the main turbine-generator have the potential for ejecting large high-energy*

missiles that can damage plant structures, systems and components.

Typical safety-related systems are listed in *Regulatory Guide (RG~: 1.117 (Ref. 3).

RG 1. 115 (Ref. 4) describes methods for protecting safety-related systems against low trajectory missiles (LTMs) resulting from turbine failure, and outlines methods for evaluating an~ calcuJating the probability of unacceptable damage to these systems.

Turbine missile safety evaluations are prepared with the aid of the above Regulatory Guides and Standard Review.

Plan (SRP) Sectton 3~5.1.3 (Ref. 5) and 2.2.3 (Ref: 6).

Ill.

RELATED SAFETY TOPICS AND INTERFACES The scope of review for this topic was limited to avoid duplication of effort since some aspects of the review were perform~d under related topics.

The related topics and their subject matter are identified in Table I.

Each of the related topic reports contain the acceptance criteria and review guidance for its subject matter.

The review for this topic make specific

L

  • and direct use of information provided in reviews of Topics VII-3 and XV-18.

Topic XV-18 is particularly significant since turbine failure resulting in,the rupture of the turbine casing is approximately equival-ent to a mai,.n steam line failure outside containment. which cfor a BWR releases primary coolant steam and radioactivity to the environment.

Hence, regardless of the probability of turbine missiles striking safety-related structures, systems or components, the criteria of Topic XV-18 must be satisfied in order to meet the criteria of this topic.

IV.

REVIEW GUIDELINES

1. Safety Goal In accordance with its previously announc~d 11Plan for Developing a Safety Goal 11 (45 FR 71023, October 27, 1980) (Ref. 6), the Nuclear Regulatory Commission has' published for public comment a proposed policy statement on safety goals for nuclear power plants.

Because of the substantial uncertainties inherent in probabilistic risk assessments of potential reactor accidents especially in evaluation of accident consequences, the Commission proposes the following limitations on the probability of a core melt as a provisional guideline for NRC staff use in the course of reviewing and evaluating probabilistic risk assessments of nuclear power plants: -

11The likelihood of a nuclear reactor accident that results in a large-scale core melt should nonnally be less than one in 10,000 per year of reactor operation."

For anticipated plant populations of'not less than 100 and no more than 1,000, during the service life of the plants now operating or being designed and constructed, this safety objective requires that the probability of an accident that results in a large-scale core melt not exceed the range 11one chance in ten million to one chance in one million 11 per year for an individual plant.

~*

In satisfying the requirements of General Design Criteria 4, systems important to safety, for turbine missile evaluations, are defined to be the safety-related systems, i.e., those structures, systems or*.

components necessary to perform required safety functions and to ensure:

a.

The integrity of the reactor coolant pressure boundary.

b.

The capability to shutdown the reactor and maintain it in a cold shutdown condition, and

c.

Th.e capability to prevent accidents that could result in potential offsite exposures that are a significant fraction of the guideline exposures of 10 CFR Part 100, 11 Reactor Site Criteria,"

  • This definition goes beyond the Corrmissions stated 11class 11 of accidents covered by the numerical guidelines on safety goals, i.e., it includes unacceptable damage to all systems required to satisfactorily limit offsite exposure, not just those required to prevent a large-scale core melt. Therefore, based on the Commissions safety goal, allowing for initiating events other than missile generating turbine failure, and keeping in mind that for our purposes unacceptable damage is not just a large-scale core melt but any release of radioactivity with potential consequences greater than the Part 100 guidelines, the probability of unacceptable damage from turbine missiles should be less than one chance in ten million per year for an individual plant, i.e., P4 < l0-7 per year.

It should be noted that this is consistent with current NRC guidelines stated in SRP Section 2.2.3 (Ref. 9). '

2.

Past Review Procedure The probability of unacceptable damage due to turbine missiles (P4) is generally expressed as the product of (a) the probability of turbine failure resulting in the ejection of turbine disk (or internal structure) fragments through the turbine casing (P1), (b) the probability of ejected missiles perforating intervening barriers and striking safety-related structures, systems or components (P2), and (c} *the probability of stuck structures, systems or components failing to perform their safety function (P3).

In the past, analyses for construction permit (CP) and operating license (OL) reviews assumed the probability of missile generation (P1) to be approximately 10-4 per turbine year,".based on the historical failure rate (Ref. 10).

THe strike probability. (P2) was estimated based on postulated missile sizes, shapes, and energies, and on available plant specific information such as, turbine placement and orientation, number and type of intervening barriers, target geometry, and potential missile trajectories. The damage probability (P3) was generally assumed to be 1.0.

The overall probability of unac~ept~ble damage to safety-related systems (P4), which is the sum over all targets of the product of these probabilities, was then evaluated for compliance with the NRC safety goal.

This logic places the regulatory emphasis on the strike Qrobability, i.e., having established an individual plant safety goal of lo-7 per year or less, for the probability of unacceptable damage to safety-related systems due to turbine missiles, this procedure requires that P2 be less than or equal to lo-3. lhis approach requires a great deal of effort on the part of applicants/licensees and the staff due to its explicit disregard for the 11actual 11 turbine reliability, and the difficulty of calculating P2 in a relatively unambiguous and systematic manner.

  • 3.

New Procedure The staff has made a complete shift of emphasis in the review of turbine missile evaluations, away from the review of strike prob-abilities to the review of missile generation probabilities.

The new approach, which,emphasizes turbine reliability, improves regulation of turbine generator systems reliability, reduces con-siderably the analytical burden placed on licensees, conserves NRC resources, and at the same time maintains the high level of pro-tection of public health and safety.

The new approach places the burden for demonstrating turbine reliability on the turbine vendor.

This shift of emphasis requires nuclear steam turbine manufacturers to develop and implement volu-metric (ultrasonic) examination techniques suitable for inservice inspection of turbine disks and shaft, and to prepare reports for NRC review which describe their methods for determining turbine missile generation.probabilities. These methods are to relate disk design, materials properties, and inservice volumetric inspection interval to the design overspeed missile generation probability, and to relate overspeed protection system characteristics, ~nd stop and control valve design and inservice test interval to the destruc-tive overspeed missile generation probability.

Following vendor submittal of such reports to the NRC for review, the vendor will provide to applicants and licensees tables of missile generation probabilities versus time (inservice volumetric disk inspection in-terval for rated speed or design overspeed failure, and inservice valve testing interval for destructive overspeed failure) for their particular turbine, which could the~be used to establish inspection schedules which meet the NRC safety obj~ctives.

Applicants and licensees with the turbines from vendors who have not yet performed analyses of turbine missile generation, or who have performed analyses but have not yet submitted formal reports to the NRC for review, are expected to* meet the ;following interim criteria, regardless of turbtne orientation.

a.

An inservice inspection program for the steam turbine assembly must be developed and implemented to provide assurance that disk flaws might lead to brittle failure of a disk at speeds up to design speed will be detected.

The inservice inspection program for the turbine assembly is to include the following:

The turbine should be disassembled at approximately 3-year intervals, during refueling or maintenance shutdowns coinciding with the inservice.

inspection scheduled as required by ASME Boiler and Pressure Vessel Code,Section XI, and there should be complete inspection of all normally inaccessible parts, such as couplings, coupling bolts, turbine shafts, low-pressure turbine blades, low-pressure disks, and high-pressure rotors.

This inspection should consist of visual, surface, and volumetrfc examinations, in accordance with the procedures of the turbine manufacturer.

,>.J

  • b.

An inservice inspection program fro main steam and reheat valves which includes the following provision is to be implemented:

i. At approximately 3-year intervals, during refueling or maintenance shutdowns coinciding with the inservice inspection schedule required by Section XI of the ASME Code for reactor components, all main steam stop and control valves, and reheat stop and intercept valves should be dismantled and visual and surface examinations conducted of valve seats, disks, and stems.

Valve bushings should be inspected and cleaned, and bore diameters should be checked for proper clearance.

ii. Main steam stop and control valves and reheat stop and intercept valves should be exercised at least once a week by closing each valve and observing directly the valve motion as it moves smoothly to a fully closed position.

V.

EVALUATION We have reviewed factors that have a bearing on the probability of missile generation at both design and destructive overspeed.

ProteGtion against destructive overspeed failure is accomplished by three independent systems; i.e., a normal speed governor, and mechanical and electrical backup overspeed control systems.

Assuming 100% steam flow and 100% on the load selector, the control valves will throttle to try to limit overspeed to a setpoint of 105%

of. turbine speed.

Due to large quantities of steam contained in the turbine and separators, turbine speed may increase even after control valves have closed.

Upon which the intercept valves~will throttle closed at 105% to a setpoint of 107% of turbine speed, If turbine speed increased to 110%,

a mechanical turbine trip would occur, closing the main stop valves as well as the control and intercept valves.

As backup protection for the 110%

mechanical trip, there is an electrical overspeed trip of 112%.

Every.

refueling outage the turbine is tested for a mechanical overspeed trip of ll0% turbine speed and an electrical over~peed trip of ll2% turbine speed (Ref, 11 )

  • The following Dresden Station procedures (Ref. 12) are used to assure reli-ability of the turbine overspeed protection system: a)

Each of the.mai:n. stop valves, the extraction valves for the D, C, and ~ extractions, and each of the six intercept stop valves and intercept valv~s are exercixed daily; b) the control valves and 24-volt Master Trip Solenoid valves are exerci~ed

  • weekly; and c) the turbine auxiliary system is tested weekly.

Both visual and surface examinations and inspections are conducted on the valve seats, disks and stems unless radiation exposure is considered to be prohibative, in which case the component is replaced.

Valve bushings are inspected for c~ndition and bore diameter.

The licensee retains the services of the turbine manufacturer (General Electric) during the inspection to assure that adequate examinations are performed, unacceptable defects are corrected and reassembly of components is properly performed.

General Electric also is responsible for making reco~mendations for necessary repairs includ-ing consideration for the need to inspect additional valves when unac-ceptable flaws are encount~red. The staff concludes that the Dresden Station procedures constitute adequate assurance of turbine overspeed control system reliability and that the interim criteria requirements for inservice inspection of main steam and reheat valves are met.

During the oast several years the results of turbine inspections at operating nuclear facilities indicate that cracking to various degrees has occurred at the inner radius of turbine disks or wheels.

Some of the turbines in which wheel bore cracks have been identified are of General Electric design.

The staff has been following this *development closely and, together with the respective turbine manufacturers, is in the process of developing new criteria and procedures for establishing turbine wheel inspection frequencies, as well as guidance for turbine overspeed control system maintenance and testing to preclude wheel failures.

As a result of cracks found recently, General Electric has provided specific inspection recommendations to utility owners for each LP turbine not yet inspected, which will have been in service for six years or more as of the end of 1982.

Included are both machines operating in BWR and PWR plants.

These recommendations involve inspecting each machine within a period of one year or less.

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With regard to the Dresden Station Unit No. 2 L.P. turbines, a January 1981 wheel bore ultrasonic examination by General Electric detected indications on the bore surfaces of numerous wheels.

All of these indications were

.shallow and not expected to affect the structural.integrity of the wheels.

Keyway indications were also detected.

These indications varied from less than.03 inches to a maximum of.29 inches.

General Electric has not yet completed an analysis of turbine missil~

generation probabilities.

When their reports are completed and submitted to the NRC, they will be reviewed and evaluated by the staff. Until then, the NRC interim criteria, described in Section IV-3 of this topic evaluation apply to the Dresden Nuclear Power Station, Unit No. 2 turbine.

The licensee does not comply with this interim criteria.

)

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  • VI.

CONCLUSION We have concluded that the licensee meets current licensing criteria with the exception of interim turbine disk inservice inspection schedule.

The need for implementation of the interim turbine disk inspection program will be evaluated during the plant Integrated Assessment.

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~ l I I

l.

TOPIC #

VII-3 IX-3 IX-5 XV-3 XV-7 XV-18 TABLE I RELATED SAFETY TOPICS AND INTERFACES TITLE Systems Required for Safe Shutdown Station Service and Cooling Water Systems Ventilation Systems Loss of External Load, Turbine Trip, Loss of Condenser Vacuum, Closure of Main Steam Isolation Valve (BWR),

and Steam Pressure Regulation Failure (Closed)

Reactor Coolant Pump Rotor Seizure and Reactor Coolant Pump Shaft Break Radiological Consequences of Main Steam Line Failure Outside Containment (BWR) 4

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