ML20054K139
| ML20054K139 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 06/29/1982 |
| From: | James Shea Office of Nuclear Reactor Regulation |
| To: | Counsil W NORTHEAST NUCLEAR ENERGY CO. |
| References | |
| TASK-03-04.B, TASK-3-4.B, TASK-RR LSO5-82-06-117, LSO5-82-6-117, NUDOCS 8207010225 | |
| Download: ML20054K139 (16) | |
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June 29, 1982 f a lN y
Docket No.50-245 LS05-82 117 Mr. W. G. Counsil. Vice President Nuclear Engineering and Operations Northeast Nuclear Energy Company Post Office Box 270 Hartford, Connecticut 06101
Dear Mr. Couns11:
SUDJECT: SEP TOPIC III-4.B. TURBINE MISSILES HILLSTONE NUCLEAR POWER STATION, UNIT 1 Enclosed is our final evaluation of SEP Topic III-4.B.
It is based on a Safety Analysis Report which you supplied on August 31, 1981, and other information supplied by you and on Docket No. 50-245 The evaluation concludes acceptability of the turbine missile risk at your facility providing certain inspections described in the SER are followed.
The evaluation will be a basic input to the integrated safety assess-ment of your facility.
It may be changed in the future if your facility design is changed or if NRC criteria are modified before completion of the integrated assessment.
Sincerely.
[ggf h 7dl M,s)
>ta Ct James Shea, Project Manager Operating Reactors Branch No. 5 Division of Licensing
Enclosure:
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l Mr. W. G. Counsil cc William H. Cuddy, Esquire State of Connecticut Day, Berry & Howard Office of Policy & Management Counselors at Law ATTN:
Under Secretary Energy One Constitution Plaza Division Hartford, Connecticut 06103 80 Washington Street Hartford, Connecticut 06115 Ronal d~ C. Haynes, Regional Administrator Nuclear Regulatory Commission Region I Office f
631 Park Avenue King of Prussia, Pennsylvania.19406
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Northeast Nuclear Energy dompany ATTN:
Superintendent Millstone Plant P. O. Box 128 Waterford, Connecticut 06385 Mr. Richard T. Laudenat Manager, Generation Facilities Licensing Northeast Utilities Service Company P. O. Box 270 Hartford, Connecticut 06101 Resident Inspector c/o U. S. NRC P. O. Box Drawer KK Niantic, Connecticut 06357 First Selectman of the Town of Waterford Hall of Records
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200 Boston Post Road Waterford, Connecticut 06385 John F. Opeka Systems Superintendent Northeast Utilities Service Company P. O. Box 270 Hartford, Connecticut 06101 U. S. Environmental Protection Agency
' Region I Office ATTN:
Regional Radiation Representative JFK' Federal Building Boston, Massachusetts 02203
SEP REVIEW 0F TURBINE MISSILES TOPIC III - 4.8 FOR THE MILLSTONE NUCLEAR POWER STATION, UNIT NO. 1 O
l MILLST0flE NUCLEAR POWER STATION, UNIT NO. 1 TABLE OF CONTENTS I.
INTRODUCTION 1
II.
REVIEW CRITERIA III.
RELATED SAFETY TOPICS AND INTERFACES IV.
REVIEW GUIDELINES i
1.
Safety Goal 2.
Past Review Procedures 3.
New Procedures j
V.
EVALUATION VI.
SUMMARY
VII.
REFERENCES 1
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I.
INTRODUCTION The safety objective of TOPIC III, 4.8 is to assure that structures, systems, and components important to safety are adequately pro-tected from potential turbine missiles.
Of those systems important to safety, this SER Topic is primarily concerned with safety-related systems; i.e., those structures, systems, or components necessary to perform required safety functions and to ensure:
The integrity of the reactor coolant pressure boundary, a.
b.
The capability to shut down the reactor and maintain it it in a safe shutdown condition, or The capability to prevent accidents that could result c.
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 ccmponents important to safety shall be apprcpriately protected agair.st dynamic effects, including the effects of missiles.
Failures that cculd occur in large steam turbines of the main turbine generatcr have the potential for ejecting large high-enercy missiles that can damage plant structures, systems and components.
Typical safety-related systems are listed in Regulatory Guide (EG) 1.127 (Ref. 3).
During the past few years the results of turbine inspecticas at operating nuclear facilities show that cracking to varicus degrees has occurred at the inner radius of turbine wheels, also referred to as disks.
(Ref s. 4 and 5) The staff has been folloveir; this development closely and, together with respective turbine manufacturers, is in the process of develcping criteria a-d procedures for establishing turbine wheel inspection frecuencies,
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and guidance from turbine overspeed system maintenance and testing
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to preclude wheel failures.
In view of current experience and NRC safety objectives (Ref. 6), we are emphasizing the turbine missile generation probability (i.e., turbine system integrity) in our reviews of the turbine missile issue and eliminating the need for elaborate and somewhat ambiguous analyses of strike and damage probabilities given an assumed turbine failure rate, as described in Regulatory Guide 1.115 (Ref. 7) and Standard Review Plan (SRP) Section 3.5.1.3 (Ref. 8).
The staff concludes, based on our reviews of many facilities, that the probability of a turbine missile striking and damaging a safety system is in a relatively narrow range depending on turbine orientation.
More refined analys'es or additional calculations for other facilities are unlikely to, change this conclusion.
Therefore, expensive and time consuming strike probability analyses on the part of applicants / licensees and/or the NRC staff are judged to be unwarranted.
The new approach, discussed in Section IX of this SER Topic, improves turbine generator system reliability by review and regulation of the probability of generating missiles.
III. RELATED SAFETY TOPICS AND INTERFACES The scope of review for this SER Topic was limited to avoid duplication of effort since some aspects of the review were performed under related topics.
The related topics and their subject matter are identified in Table 1.
Each of the related topic reports contains the acceptance criteria and review guidance for its subject matter.
Topic XV-18 is particularly significant since a turbine failure resulting in the rupture of the turbine casing is apprcximately equivalent to. a main steam line failure outside containment, which for a BWR releases primary coolant steam and radioactivity to the environment.
- Hence,
regardless of the probablity 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 cf this SER Topic.
IV. REVIEW GUIDELINES '
1.
Safety Goal In accordance with its previously announced " Plan for Developing a Safety Goal" (45 FR 71023, October 27, 1980) (Ref. 6), the Nuclear Regulatory Ccmmission has published for public c:=ent a proposed policy statement on safety goals for nuclear power Because of the substantial uncertainties inherent plants.
in prcbabilistic risk assessments of potential reactor accidents, especially in evaluation of accident consequences, the C:m-mission proposes the following limitations on the probability-of a core melt as a provis,ional guideline for NRC staff use in the course of reviewing and evaluating probabilistic risk assessments of nuclear power plants:
"The likelihood of a nuclear reactor accident that results in a large-scale core melt should norcally be less than one in 10,000 per year of reactor operation."
For anticipated plant populations of not less than 100 and nor more than 1,000, during the service life of plants now operating or being designed and constructed, this safety objecti.~e requires that the probability of an accident that results in a :arge-scale core melt not exceed the range "one chance in te-rillion to one chance in one million" per year for an individtil plant.
t.
TABLE 1 RELATED SAFETY TOPICS AND INTERFACES TOPIC #
TITLE VII-3 Systems Required for Safe Shutdown IX-3 Station Service and Cooling Water Systems IX-5 Ventilation Systems XV-3 Loss of External Load, Turbine, Trip, Loss of Condenser Vacuum, Closure of Main Steam Isolation Valve (BWR), and Steam Pressure Regulation Failure (Closed)
XV-7 Reactor Coolant Pump Rotor Seizure and Reactor Ccolant Pump Shaft Break XV-18 Radiological Consequences of Main Steam Line Failure Outside Containment (BWR)
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:
The integrity' of the reactor coolant pressure boundary.
a.
b.
The capability to shut down the reactor and maintain it in a cold shutdown condition, and c.
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."
This definition goes beyond the Commissions stated " class" 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 withlotential consequences greater than the Part 100 guidelines, the probability of unacceptable damage from turbine missiles should be less than one change in ten million per year for an individual plant, i.e.,
-7 P4 < 10 per year.
It shot.ld be noted that this is consistent with current NRC guidelines stated in SRP Section 2.2.3 (Ref. 9).
i 2.
Past Review Procedure The probability of unacceptable damage due to turbine missiles (P ) is generally expressed as the product of (a) the proba-4 bility of turbine failure resulting in the ejection of
turbine disk (or internal structure) fragments through the turbine casing (P ), (b) the probability of ejected missiles 1
perforating intervening barriers and striking safety related structures, systems, or components (P ), and (c) the proba-2 bility of stuck structures, systems, or components failing to perform their s'afety function (P ).
3 In the past, analyses for construction permit (CP) and operating license (OL) reviews assumed the probability of missile genera-
~4 tion (P ) to be approximately 10 per turbine year, based on y
the historical failure rate. (Ref. 10) The strike proba-bility (P ) was estimated based on postulated missile sizes, 2
shapes, and energies, and on available plant specific information such as, turbine placerent and orientation, number and type of intervening barriers, target geometry, and potential missile trajectories.
The damage The probability (P ) was generally assumed to be 1.0.
3 overall probability of. unacceptable da:Ege to safety-related systems (P ), which is the sum over all targets 4
of the product of these probabilities,*.cas then evaluated for ccmpliance with the NRC safety goal.
This logic places the regulatory emphasis on the strike probability, i.e_.,
-7 having established an individual plant safety goal of 10 per year or less, for the probability of unacceptable damage to safety-related systems due to tu'rbine missiles, this-3 be less than or equal to 10 procedure requires that P2 This approach requires a great deal of effort on the part of applicants / licensees and the staff due to its explicit disregard for the " actual" turbine reliability, and the in a relatively un mbiguous difficulty of calculating P2 and systematic manner.
P
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 probabilities to the review of missile generation probabili-ties.
The new approach, which emphasizes turbine reliability, improves regulation of turbine generator systems reliability, reduces considerably the analytical burden placed on licensees, conserves NRC resources, and at the same time maintains the high level of protection 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 volumetric (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 desi;n, caterials prcperties, and inservice volumetric inspection interval to the design overspeed missile generation probability, and to relate overspeed protection system character-
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istics, and step and centrol valve design and inservice iest interval to the destructive 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 interval for rated speed or design overspeed failure, and inservice
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valve testing interval for destructive overspeed failure) for their particular turbine, which could then be used to establish inspection schedules which meet the NRC safety objectives.
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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 turbine orientation:
(a) An inservice inspection program for the steam turbine assembly must be developed and implemented to provide assurance that disk flaws that 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 main 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 rotcrs.
This inspection should consist of vise,al, surface, and volumetric examinations, in accordance with the procedures of the turbine manufacturer.
(b) An inservice inspection program for main steam and reheat valves which includes the following provision is to be implemented:
At approximately 3 year intervals, during refueling 1.
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
.g.
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 The Millstone Nuclear Power Station, Unit No.1, turbine generator Since the was manufactured by the Ger.eral Electric Company.
initiation of operation in December i970, the following ultra-sonic rot'or and disc inspections have been performed:
Results Year Inspection 1974 Discs on both LP rotors No significant defects 1978 Discs on LP A rotor No significant defects No significant defects 1980 HP rotor Indications on LP discs 1980 Discs on LP B rotor These are in addition to the normal rotor surface examinaticns (i.e., visual, magnetic particle, and liquid penetrant) con-ducted at more frequent intervals.
The ultrasonic exacinaticn by General Electric in November 1980 gave one indication on each These indications suggest crack depths of.14 of two wheels.
inches and.21 inches.
These were considered shallow and not i
expected to affect the structural integrity of the discs.
General Electric recommends that, in general, their turbines should be reinspected within 6 year intervals.
A depth of defense against an.overspeed event is provided by two series valves on the HP turbine inlet, two series valves on the LP turbine inlet, and four independent overspeed control logic circuits for valves.
Only seven out of 87 turbine trip events that have occurred at Millstone Unit No. 1 involved turbine' over-speed, and none of these.resulted in a peak rpm greater than 110% of synchronous.
The most recent overspeed incident (1976) was a total electrical load rejection event due to switch,. ard flashover during a hurricane.
The mechanicai emergency trip logic, set to limit the overspeed peak to 120% synchronous, has never been required during operation.
However, the ability to satisfactorily send the proper closure signal to the stop valves is periodically verified on-line.
In addition, during the r,efueling outage the mechanical emergency trip system (set to activate at 110% of synchronous speed) and the backup mechanical emergency trip system (set to activate at 112%
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of syncnronous speed) are tested during an sctual overspeed test.
The ability of the stcp and control valves o successfully respond to these signals is checked by peri:dic stroke testing.
These tests have never detected any valve sticking or other abnormal valve stroking condition.
Additicnally, there is a 5'0% valve inspection during each refueling outage.
These inspections have determined the need for expected refurbishing and parts replacement, but no significant abnormal conditions which might affect operability have been detected with either the turbine stop valves or control valves.
The NRC has requested the Northeast l'uclear Energy Company to supply us with a description of the t2st precedures and.
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schedules for the Millstone Nuclear Power Station, Unit No. 1, turbine (a) overspeed protection system, and (b) main steam stop and control valves and reheat stop and intercept valves.
The staff has not yet received all the requested information.
General Electric has not'yet completed an analysis of turbine missile 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 SER Topic, apply to the Millstone Nuclear Power Station, Unit No.1 turbine.
VI.
SUMMARY
Until General Electric reports are submitted to the staff and reveiwed, we conclude that the turbina missile risk for lii11 stone Nuclear Power Station, Unit No.1, is acceptable provided both of the following conditions are met:
(a) Volumetric inspection of icw pressure turbine A is conducted in accordance with General Electric precedures during the next refueling outage.
(b) The NRC interim criteria (inspection and testing, s'chedules and procedures) described in Section IV-3 of this SER Topic are implemented.
VII. REFERENCES 1.
Volume 10, Code of Federal Regulations, Part 100, " Reactor Site Criteria."
2.
Volume 10, Code of Federal Regulations, Part 50, Appendix A,
" General Design Criteria for Nuclear Power Plants."
3.
Regulatory Guide 1.117, " Tornado Design Classification,"
June 1976.
4.
See NUREG/CR-1884, " Observations and Comments on the Turbine Failure at Yankee Atomic Electric Company, Rowe, Massachusetts" March 1981.
5.
See Preliminary Notification of Event or Unusual Occurrence --
PN0 - III 104 - " Circle in the hub of the eleventh stage wheel in the main turbine" at Monticello Nuclear Power Station, Nov. 24, 1981.
6.
See February 16,1982 Memorandum ~for All NRR Professional Staff from J. L. Funches,
Subject:
Proposed Policy Statement on Safety Goals for Nuclear Power Plants.
7.
Regulatory Guide 1.115, " Protection Against Low-Trajectory Turbine Missiles," Rev. 1, July 1977.
8.
Standard Review Plant Section 3.5.1.3, " Turbine Missiles,"
Rev. 1.
9.
Standard Review Plan Section 2.2.3, " Evaluation of' Potent.31 Accidents," Rev. 1.
10.
S. H. Bush, " Probability of Damage to Nuclear Corponents,'
Nuclear Safety, 14, 3, (May-June) 1973, p. 187; and S. H. Bush, "A Reassessment of Turbine-Generator Failure Probability," Nuclear Safety, 19, 6, (Nov. - Dec.) 1978,
- p. 681.
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