ML20213E193
| ML20213E193 | |
| Person / Time | |
|---|---|
| Site: | Columbia  |
| Issue date: | 07/13/1982 |
| From: | Johnston W Office of Nuclear Reactor Regulation |
| To: | Tedesco R Office of Nuclear Reactor Regulation |
| References | |
| CON-WNP-0528, CON-WNP-528 NUDOCS 8207220044 | |
| Download: ML20213E193 (21) | |
Text
.
o JUL 131982 Docket No. 50-397 MEMORANDUM FOR:
Robert L. Tedesco, Assistant Director for Licensing Division of Licensing FROM:
William V. Johnston, Assistant Director Materials & Qualifications Engineering Division of Engineering
SUBJECT:
WASHINGTON PUBLIC POWER SUPPLY SYSTEM (WPPSS), WASHINGTON NUCLEAR PROJECT NO. 2, SAFETY EVALUATION REPORT INPUT Plant Name:
Washington Nuclear Project No. 2 Suppliers: Westinghouse; Burns & Roe Licensing Stage: OL Docket Number: 50-397 Responsible Branch & Project Manager:
LB #2; Rajender Auluck Reviewer:
J. O. Schiffgens Description of Task:
Safety Ealuation Report Input Review Status:
Complete The Component Integrity Section of the Materials Engineering Branch, Division of Engneering, has reviewed Section 3.5.1.3 Turbine Missiles in the Final Safety Analysis Report for Washington Nuclear Project No. 2.
Based upon additional information supplied by the applicant in a letter from G.D. Bouchey to A. Schwencer, dated March 2, 1982, we have revised our input to Section 3.5.1.3 of the Safety Evaluation Report (SER) which is included in the Attachment.
t I
!vi William V. Johnston, Assistant Director for Materials & Qualifications Engineering Division of Engineering
Attachment:
As stated cc:
See Page 2
Contact:
J.O. Schiffgens x-28099 4
82d722OO44 8207.13
- ADOCK 05000397 cO
I 1902 Robert L. Tedesco cc:
R. Vollmer D. Eisenhut J. Youngblood S. Pawlicki W. Hazelton R. Klecker D. Sellers R. Auluck J. Gleim E. Sullivan J. Schiffgens DISTRIBUTION:
MTEB Reading File N
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ATTACHMENT 1 WASHINGTON NUCLEAR PROJECT NO. 2 DOCKET NUMBER 50-397 SAFETY EVALUATION REPORT MATERIALS ENGINEERING BRANCH COMPONENT INTEGRITY SECTION 3.5.1.3 Turbine Misstes A.
Review Basis 1.
Introduction During the past 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, particularly those of Westinghouse design.
Within this time period, there has actually been a Westinghouse turbine disk failure at one f acility Yankee Atomic Electric Company.
Furthermore, recent inspections of General Electric turbines l
l have also resulted in the identification of disk bore cracks.
In view of current experience and NRC safety objectives, we are emphasizing the turbine missile generation probability (i.e. turbine system integrity) in our reviews of the turbine 1
missile issue and eliminating the need for elaborate and somewhat ambiguous analyses of strike and damage probabilities given an assumed 1
. turbine failure rate.
Although straightforward in principle, the latter calculations have to be based on detailed facility information and assumptions as to missile shape and size, missile energies, ba rrier penetration potential and ultimately to the likelihood of damaging a facility safety system.
Generally, there are significant differences between Licensees or applicants submittals and the final evaluation by the staff.
Nevertheless, t he ~ staf f 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 analyses or additional calculations for other facilities are unlikely to change this conclusion.
Therefore, expensive and time
\\
I consuming strike probability analyses on the part of applicants / licensees and/or the NRC l
staff are judged to be unwarranted.
In conclusion, the new approach we are using j
improves turbine generator system reliability by the review and regulation of the probabili'ty I
of generating missiles.
This will reduce considerably the analytical burden placed on licensees, conserve NRC resources, and still maintain the high level of protection of public
,e,--
. health and safety.
2.
Review Criteria According to General Design Criterion 4, of Appendix A to 10 CFR Part 50 (Ref. 1), nuclear power plant struct gres, systems and components important to safety shall be appropriately protected against dynamic effects, including the effects of missiles.
Failures that could occur in la rge steam turbines of the main I
turbine generator have the potential for ejecting large high-energy missiles that can damage plant structures, systems and components.
The safety objective of this SER review topic is to assure that ' structures, systems, and components 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:
l 1.
The integrity of the reactor coolant pressure i
- boundary, 2.
The capability to shut down 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 l
l
4-exposures of 10 CFR Part 100, " Reactor Site C ri te ri a" (Ref. 2).
Typical safety-related systems are listed in Regulatory Guide (RG) 1.117 (Ref. 3).
The probability of unacceptable damage due to turbine missiles (F 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 Pb (b) the probability of turbine casing CRAJ,
ejected missiles perforating intervening, barriers and striking gafety related structures, systems, or
(-F%), a n d
( r. ) the probability of st ek components structures, syste.ms, or components failing to P3 perform their safety function (iH5 ).
I According to current NRC guidelines stated in Standard Review Plan (SRP) Section 2.2.3 l
(Ref. 5) and R.
G.
1.115 (Ref. 6), the probability of unacceptable damage from turbine missiles should be less than one f
chance in ten million per yearg or an 10 ' qs individual plant, i.e.,
P < 44-4 per year.
l l
l
r s
5-i 3.
Past Procedure In the past, analyses for construction permit (CP) and operating license (OL) reviews a sumed the P
probability of missile generation
) to be 10 g L app roximately 40-4 pe r tu rbi ne y e a r, based on the 7
historical failure rate (Ref. g).
The strike S
i'f estimated (Ref. 7) based on probability WQ) was
~z postulated missile siges, shapes, and energies, and on availabLe plant specific information such as, turbine placement and orientation, numbe r and type of intervening ba rriers, ta rget geometry, and potential missile trajectories.
The damage 3Pp probability 4 45) was generally assumed to be 1.0.
The overalL probability o unacceptable damage to safety related systems (Pt), which is the sum over all targets of the product of ti.ese probabilities, l
was then evaluated for compliance with the NRC safety goal.
This logic places the regulatory emphasis on the strike probabiliy, i.e.,
having g
h established an individual plant safety goal of 49-f e
-,--------,-e
- - - - -, - - ~ - -, - - - - - - ~ -----i w
6-pe r yea r, or less, for the probability of unacceptable damage to safety related systems due tg, turbine missiles, this procedure requires that I f i o,.) g 445 b e less than or equal to +9-3.
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" turbin.e a
f$
reliability, and the difficulty of calculating 44 in a relatively unambiguous and systematic manner.
4 New Procedure The new approach places the burden for i
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 i
l probabilities.
These methods are to relate disk design, materials properties, and inservice j
volumetric inspection interval to the l
design overspeed missile generation probability, l
l l
l and to related overspeed protection system characteristics, and stop and control valve design and inservice test interval to the destructive overspeed missile generation probability.
Following vendor submittal of such reports to the NRC for review, the vendor wilL provide to ap p li c a nt s and Licensees tables of missile generation probabilities versus time (inservice volumetric disk inspection interval 1
for rated speed or design overspeed failure, and inservice valve testing interval for destructive overspeed failure) for their pa rticula r turbine, which could then be used to e s t ab li s h inspection schedules which meet NRC safety objectives.
It is the staff's view that the NRC safety I
objective with regard to turbine missiles is best expressed in terms of two sets of criteria applied to the missile generation probability.
One set of criteria is t
be applied to favorably oriented 3,o 9t
- turbines, ELip(tctal), and the other is to be 9
T Pu applied to unfavorably oriented turbines, P&eg (total).
These criteria may be summarized as folLows:
(a)
The general requirement f or turbine at reactor start up is that rp li abi li t y P+
10-44, total) be Less t h a n 40 p e r y e a r, and 7V, (PM4 1054 that d (total) be le s s t h a n t3=T p e r y e a r;
4 i F)hp/g (b)
When, during operation, the value of 70-4 (total) increases togreaterthante-K[per 10.3 year, but less than +9-3 per year, or the 4
i ofQ{h/(total) increases to greater value 40' Sap 10.4 t h a n 46-!F p e r y e a r, but less t h a n M-4p e r year, the turbine is permitted to. remain in use until the next scheduled ref ueling outage, at which time the licensee should take action to reduce the missile generation probability to meet criterion (a) before j
i returning the turbine to service.
Exemptions may be granted for valid technical reasons or severe economic hardship.
(c)
When, during operation, thevaluepgff(#/'
>b (total) increases to g r e a t e r t h a n it*5 p e r 10.-
year, but les s t h a n 1022 "p e r y e a r, or the v a lu e,gf Tky/ (t ot al) increases to gretter
%3,
-.g V
than t94 per year, but ists t h a n 49-3 p e r year, the turbine is to be isolated'from the s t e am supply within 90 days, at which time the licensee must take action to reduce the missile generation probability to meet criterion (a) before returnir.g the turbine to service.
(d)
If, at any time during operation, the value t o. g.
of P (total) increases t r g r e a t e r t h.a n ttT:7 per year or the value of
- V (total)
. =_ _ _ _ -
e 10 3 de increases to greater t h a n 44-4 p e r y e a r, the turbine is to be isolated from the steam supply within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, at which time the i
licensee must take action to reduce the i
missile generation probability to meet criterion (a) before returning the turbine to i
service.
a Applicants and Licensees with the turbines from vendors who have not yet performed analyses of i
turbine missile generation, or who have performed t
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:
l (a)
An inservice inspection program for the steam 1
turbine assembly must be developed and 1
implemented to provide assurance that disk i
flaws that might lead to brittle failure of a disk at speeds up to design speec wilL be detected.
The inservice inspection program
)
for the turbine assembly is to include the P
folLowing:
i i
The turbine shouLd be disassembled at f
approximately 3 year intervals, during refueling or mainenance shutdowns coinciding with the inservice inspection l
P 10 -
t schedule as required by ASME Boiler and Pressure Vessel Code,Section XI, and there 1
should be complete inspection of alL
~
normalLy inaccessible parts such ae couplings, coupling bolts, turbine shafts, low pressure turbine blades, low pressure disks, and high pressure rotors.
This inspection should consist of visual, surf ace, and volumetric examinatio'ns, in accordance with the procedures of the turbine
+
c j
manufacturer.
1 (b)
An inservice inspection program for main steam and reheat valves which includes the folLowing provision is to be implemented:
i l
i.
At approximately 3 year intervals, during 4
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 i
l diameters should be checked for proper I
clearance.
j
- 91 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 an'd observing directly the valve motion as it moves smoothly to a fully closed position.
B.
Evaluation 4
1 For Wa:hington Nuclea r Proj ect No. 2, the steam and power conversion system generates steam in a direct cycle BWR and converts it to electric power i
in a turbine generator manuf actured by Westinghouse Electric Corporation.
The placement and 4
orientation of! the turbine generator is unf avorable with respect to the station reactor buildings; that is, there are safety related targets inside the low trajectory missile strike zone.
T h'e t u r bi ne is a tandem-compound type (single s h a.f t ) with one double-flow high pressure t u r b i. n e, three double-flow low pressure turbines, and a rated rotational speed of 1800 rpm.
The major portion of manufacture was performed during 1975.
l l
i A turbine failure resulting in the rupture of the turbine casing is approximtely equivalent to a main steam line failure outside containment.
For l
a BWR such a failure 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 SRP 15.6.4 (Ref. 9) must be satisfied in orde r to meet the criteria of this review area.
1.
Destructive Overspeed Fai. lure Prevention The turbine generator has a turbine control and overspeed protection system which is designed to control turbine action under all normal or abnormat conditions and to ensure that a turbine trip frem full load will not cause the turbine to overspeed beyond acceptable limits so as to minimize the probability of generating turbine missiles, in accordance with the requirements of GDC 4 (Ref. 1).
The turbine control and overspeed protection system is, therefore, essential to the overall safe operation of the plant.
Turbine control is accomplished with a digital electrohydraulic control (EHC) system.
The I
l EHC system consists of an electronic governor using solid state control techniques in combination with a high pressure hydraulic actuating systen.
The system includes electrical control circuits for steam pressure control, speed control, load control, i
and steam control' valve positioning.
L
. There are four methods of turbine overspeed control protection:
the noimal speed governor (EHC), the overspeed protection controller (OPC), thefrechanical'over$oced trio mechanism,
.and the electrical overspeed trip.
The EHC modJLates the turbine control valves to maintain desired speed load characteristics within 2 to 3 rpm of desired speed.
The primary function of the OPC is to avoid excessive turbine
At 103 percent of rated speed, the OPC solenoids open, closing the governor and intercept valves to arrest the overspeed before it reaches the trip setting of 111 percent of rated speed.
After turbine coastdown to synchronous speed, the digital r.ystem takes control and maintains the turbine generator at synchronous speed.
The mechanical overspeed sensor trips the turbine stop, control, and combined intermediate valves by deenergizing the hydraulic fluid systems when 111 percent of rated speed is reached, thereby maintaining turbine speed below 120 percent of rated speed and causing unit coastdown to turning gear operation.
The electrical backup overspeed sensor trips these same valves when 111.2 percent of rated speed is reached by independently deenergizing the hydraulic fluid system.
These overspeed trip systems
. can be tested while the unit is online.
The staff has reviewed these systems and has concluded that the turbine generator overspeed protection system meets the guidelines of SRP 10.2 (Ref. 10) and can perform its design safety function.
The overspeed protection controller, the mechanical overspeed t rip mechanism and electrical overspeed trip are to be inspected and tested periodically during reactor operation.
The manner and frequency of the inspection and testing will take into consideration the manufacturers recommendations in conjunction with the plant generating requirements. AcccordingLy, the applicant's inservice inspection and testing program for the main steam control and stop valves.and reheat intercept and stop. valves includes the fotLowing:
(1 ) At least once per 40 months, at least one main steam control valve, one main steam stop valve, one reheat intercept valve, and one reheat stop valve are to be dismantled and inspected.
(2) At least once a week, the main steam control and stop valves and reheat intercept and stop valves are to be exercised by closing each and observing the valve motion.
. has submitted to the staff a report describing an analysis according to which the probability of generating missiles at destr'ctive u
10,g x44-6{,pe'runit overspeed is 1.7 per year (Ref. 11).
This report is under review to determine the acceptability of the analysis, and the adequacy of the manufacturer's recommended and the~ applicant's implemented overspeed protection inspection and testing, procedures and schedules.
Until this review is completed, the staff accepts the applicants program.
2.
Design Overspeed Failure Prevention Failures of turbine disks at or'below the design overspeed, nominally, 120 percent of normal operating speed, are caused by a non-ductile material f ailure at nominal stresses lower than the yield stress of the material.
Sin ce 1979, the staff has known of the stress corrosion cracking problems in low pressure rotor disks of Westinghouse turbines.
Westinghouse has developed and implemented procedures for inservice volumetric inspection of the bore and keyway areas of low pressure turbine disks.
They have also prepared and submitted reports f o r tJ R C review which
. describe their methods for detrmining turbine missile generation probabilities due to stress corrosion cracking (Refs. 12 a nd 13).
R e su lt s stemming from the methods and procedures described in these reports are accepted by the staff until review of the reports is completed.
Probabilities of a low pressure turbine disk or associa.ted blade ring fragment becoming a missile have been provided by. Westinghouse to the applicant for their particular turbine.
Missile generation probabilities were provided for each disk on each low pressure turbine, as a function of inspection interval (i.e.,
turbine operating time between inspections for racks).
In the analysis which produced these
.pMfvalues, it is assumed that a crack initiates at the beginning of service life or immediately after an inservice inspection during a i
refueling outage.
For a given disk, the l
j probability of rupture due to stress corrosion is the probability that a crack ' exists in the disk bore whose depth is equal to or greater than a calculated critical crack depth.
The critical crack depth is calculated using
(
standard fracture mech.<nics methodology, and is based on actual material properties for the disk, and normal operating temperatures for
. the turbine.
Data from field inspections are used to estimate a) the probability of crack initiation in the various disk types, and b) the crack growth rate
' assuming cracks initiate at the beginning of service life or after an inservice inspection.
Using appropriate probability distributions for crack growth rate and critical crack depth, a numerical analysis technique is used to calculate the probability of disk rupture.
This value is a function of the inspection interval during which a crack may initiate and propogate.
Energy absorption techniques are used to evaluate whether a given disk or f ragment is contained or escapes the turbine casing upor rupture.
The NRC c ri t e ri a for unfavorably oriented turbines apply to Washington Nuclear Projcet No. 2.
According to the Westinghouse analysis, E %
the total missile generation probability, 41, 16.g t h a n ' &-9{p e r y e a r a t will be less r
start up.
10.g[
t h a n 10 p e r y e a r t h e In order to keep P1 Less turbine dould have to be inspected at two year intervals.
With an inspection interval of three years, assuming a refueling outage P1 schedule of about 18 mo n t h s, fi-f a l l s in the
i 18 -
2 0.G
,b.4 range M-6 to 1 M per year in the interval between the first and second refueling outage.
Therefore, the staff concludes that the missile generation probability is sufficiently low, provided all low pressure turbine disks are volumetrically insperted within three years or by the second refueling outage.
If no cracks are found on inspection continued use of a three year inspection interval is considered acceptable.
If cracks are found on inspection the inspection schedule will have to be changed according to the depth of the cracks and an accompanying Westinghouse analysis.
C.
Summary Until staff review of the submitted Westinghouse reports is completed,.
We conclude that the turbine missile risk for Washington Nuclear Project No. 2 is acceptable provided both of the following conditions are met:
1)
Volumetric inspection of all low pressure turbines are conducted in accordance with Westinghouse procedures within three years of startup or during the second refueling outage.
O w
9 2)
The NRC interim c ri t e ri a (inspection and testing, schedules and procedures) described in section A-4,'and discussed in sections B-1 and B-2, of this SER are implemented.
D.
References L.
Volume 10, Code of Federal Regulations, Part 50, Appendix A,
" General Design C riteria f or Nuclear Plants."
2.
Vo lume 10, Code of Federal Regulations, Part 100, " Reactor Site Criteria."
3.
Regulatory Guide 1.117, " Tornado Design Classification,"
J une 1976.
4.
See February 16, 1982 Memorandum for All NRR Prof essional Staff from J.
L.
Funches,
Subject:
Proposed Poli cy Statement on Safety Goals for Nuclear Power Plants.
5'.
Standard Review Plan Section 2.2.3,
" Evaluation of Potential l
Accidents Revision 1.
l 6.
Regulatory Guide 1.115, " Protection Against Low-Trajectory Turbine Missiles," Rev.
1, J u ly 1977.
7.
S.
H.
Bush, " Probability of Damage to Nuclear Components,"
Nuclear Safety, li, 3, (May-J une) 1973, p.
187; and S.
H.
- Bush, "A
Reassessment of Turbine-Generator Failure Probability," Nuclear Safety, 19, 6, (Nov.
Dec.) 1978,
- p. 681.
l 8.
S t anda rd R evi ew P la n S e c t ion 3.5.1.3, " Turbine Missiles,"
l Rev.
1.
l 9.
Standard Review Plan Sections 15.6.4, " Radiological Consequences of Main Steam Line Failure Outside Containment (BWR)," Rev. 2.
. 10.
St anda rd Review Plan Section 10.2, " Turbine Generator."-
Standard Review P lan S e ct ion 10.2.3, " Turbine Disk Integrity.
11.
" Analysis of the Probability of the Generation and Strike of Missiles from a Nuclear Turbine," March, 1974.
12.
" Procedures for Estimating the Probability of Steam Turbine Disc Rupture from Stress Corrosion Cracking,"
WSTG-1-P, May 1981.
13.
" Missile Energy Analysis Methods for Nuclear Steam Turbines," WSTG-2-P, May, 198L.
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