ML20138B873

Rev 1 to Engineering Evaluation & Analysis of IGSCC in Low Pressure Discs at Grand Gulf Nuclear Station Unit 1

ML20138B873
Person / Time
Site:	Grand Gulf
Issue date:	09/11/1985
From:	MISSISSIPPI POWER & LIGHT CO.
To:
Shared Package
ML20138B837	List: AECM-85-0333, Forwards Application for Amend to License NPF-29 Revising Tech Specs to Allow Insp of Low Pressure Turbine Discs at Insp Interval Not to Exceed 50,000 H of Operation ML20138B831 ML20138B863 ML20138B870 ML20138B873 ML20138B898 ML20138B920
References
85-M-006, 85-M-006-R01, 85-M-6, 85-M-6-R1, TAC-60085, NUDOCS 8512120403
Download: ML20138B873 (25)

Text

.

ATTAChrkf4T Z ENGINEERING EVALUATION AND ANALYSIS OF INTERGRANULAR STRESS CORROSION CRACKING IN LOW PRESSURE TURBINE DISCS AT GGNS UNIT 1 MECHANICAL SECTION NUCLEAR PLANT ENGINEERING REPORT NO. 85-M-006 Revision 1 Prepared By: f/%r /MD.JK

/ Enginee(16g Supervisor '

Reviewed By: Ch) T//0/99 Inde ent Reviewer 4f .

/

Reviewed By: ~

A , /

Principal Mechanical Engineer Approved By: M 'I lI! Sf Man'ager, Nuclear Plant Engineering fhh D k P

A TTAC H rntN 7* I. 1 Table of Contents -

I. Introduction Page 1 II. Background Page 1 III. Location of L.P. Turbine Disc Cracks Page 2 IV. Apparent Crack Growth Rate Page 5 V. Analysis of GGNS-1 L.P. Turbine Discs Page 7 VI. End of First Fuel Cycle Projected Data Page 9 VII. Other Factors Influencing Disc Failures Page 10

{

I VIII. Data Application Page 12 II. Conclusions Page 13 I. References Page 15 i

i List of Figures Fig. 1 Westinghouse L.P. Disc-Rotor Interface Page 3 Fig. 2 General Electric L.P. Disc-Rotor Interface Page 4

Fig. 3 KWU L.P. Disc-Rotor Interface Page 6 4

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PAGE 1 of 15 j

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y l j Engineering Evaluation and Analysis of Intergranular Stress Corrosion Cracking in Low Pressure Turbine Discs at Grand Gulf Nuclear Station i -

! I. Introduction The occurrence of Intergranular Stress Corrosion Cracking (IGSCC) in '

low pressure turbine discs has been well documented in the available litera-ture (References 2, 3, 4 and 5) for several years. The U.S. Nuclear Regulatory Commission (NRC) has imposed a requirement in the Operating License for GCNS that the low pressure turbine discs be ultrasonically a

inspected for IGSCC in the bore and keyway areas during each refueling I

outage. This Engineering Evaluation and Analysis will show that this i requirement is overly conservative and should be relaxed.

II. Background l This history of turbine discs ICSCC is summarized in the Electric Power f

Research Institute (EPRI) Report NP-2429 LD (Reference 2), " Steam Turbine i

l Disc Cracking Experience, Volume 2: Data Summaries and Discussion." The  !

r j final report was issued by EPRI in June, 1982 and covers the time period up l

to June / July 1980. This report identified the foreign and domestic occur- ,

l rences of disc cracking but did not include the occurrence of disc cracking i

l that was discovered in 1982 in the low pressure turbines of the General i

Electric Co. (GE) design. The Report NP-2429 did not address any lov '

pressure turbines manufactured by Kraf twerk Union (KW), who supplied the low pressure (L.P.) turbines for CCNS-1. The final report for EPRI Research Project RP-2408-1 " Stress Corrosion Cracking in Steam Turbine Dises

(

l' Analysis of Field and Laboratory Data" was prepared in December 1984 by

! Failure Analysis Associates and is published by EPRI as EPRI NP-4056 Final Report (Reference 7). The NP-4056 Final Report includes data up to mid-1984 l

and represents the latest available data and analysis of the L.P. turbine i

I i

ATTACHmser.I PAGE 2 of 15 4

disc IGSCC problem. The report also includes data on the GE L.P. disc cracking failures and some projections for KWU L.P. turbine dises.

When analyzing L.P. turbine dise IGSCC, there are several attributes which need to be addressed. The most important of these attributes are:

(1) the location of the cracking, and (2) the apparent crack growth rate (s/t). These two attributes will be discussed in detail, as they apply to the KWU L.P. turbine discs at GGNS-1.

III. Location and Incidence of L.P. Turbine Disc Cracks IGSCC may occur, and has been documented, in the following areas of a L.P. turbine disc: Bore, Keyway, Rim, and Face of the disc. The EPRI NP-4056 Final Report shows that the statistical rate of incidence for Bore, Rim, and Face cracking is sufficiently similar so that these three areas may be lumped together when performing statistical analyses. The incidence rate of Keyway cracking for all analyzed discs has been shown to be statistically higher than the incidence of Bore cracking (including Rim and Face cracking). In all of the discs analyzed to date for Keyway cracking, it should be noted that the Keyway area is included in the shrink-fit area of the disc to rotor shaft attachment. This is shown in Figures 1 and 2 for Westinghouse and GE disc-rotor shaft attachment areas. This condition is also typical of the AEG and NEI Parsons low pressure turbines, however no sketches of these disc-rotor attachment areas are available. The KWU L.P.

turbines are somewhat different in that the Keyway area is not included in the shrink-fit area of the disc-rotor shaft attachment. This condition will lower the shrink-fit stresses in the Keyway area, which exist in the other manufacturer's designs. After the discs for KWU turbines have been shrunk on the rotor shaft, the Keyways for the locking pins are drilled into the rotor shaft and discs. Another unique design feature in the KWU L.P.

• PAGE 3 of 15 /ITW//Ndr x

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/IT7M CHiritM X turbines is the radial relief grooves which were pre-machined into the disc hub area and the rotor shaft which eliminates the high stress concentrations at the bottom ends of these keyway bores. This condition is shown in Detail A of Fi dure 3.

IV. Apparent Crack Crowth Rate Due to the limited data available on cracked L.P. turbine discs where the actual crack growth rate has been determine by repetitive inspections at significantly greater operating times, the industry has developed an

" Apparent Crack Crowth Rate" term. The apparent crack growth rate is the depth of the crack divided by the total operating hours of the turbine.

This an artificial term in that it is assumed that the crack initiates immediately upon the unit's first start-up and continues at a linear rate for the life of the unit. The conservatism'of this term has been validated by the accuculated data from turbine disc inspections performed to date, and by the minimum number of catastrophic failures due to IGSCC. In the EPRI NP-4056 Final Report an apparent crack growth rate model was developed which is an expansion of the apparent crack growth rate model originally developed by Westinghouse. The Westinghouse crack growth rate model was presented at the 1981 Joint Power Generation Conference as " Procedures for Estimating the Probability of Steam Turbine Disc Pupture for Stress Corrosion Cracking" (paper 81-JPGC-Pvr-31), and is a two variable model. The EPRI model that was developed is a three variable model which was statistically validated using a data base of 228 data points. The model for apparent crack growth rate is: ,

in(a/t)= -4.74 - 9270/T + 0.0337 (Yield Strength) + 4.53 (vt.X Mn)

Where T is equal to the disc operating temperature in *R.

l 9

, PAGE 6 of 15 A TTACHmral Z Circumferential Stress Relief Groove __

Radius: - '

R10 = 0.39 in. +B - -

R22.5 = 0.89 in. }

Diameter: - - - - - - - - ' -

208 = 0.79 in. }:j! !\

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Disk #5 Section B-3 FIGURE 3 Kraftwerk Union Disc - Rotor Shaft Attachment Area I

PAGE 7 cf 15 V. Analysis of GCNS-1 L.P. Turbine Discs

. For the analysis of the GGNS L.P. Turbine Disc the following known data is used.

Disc Steel Properties (Reference 1, A-C PSI Report ER504)

Disc 1,2,4,5 Chemistry: 3.5% Ni, 1.50%Cr, 0.26%C 0.30%Mn 0.15% V max Physical: Tensile 156 KSI max Yield 128-135 KSI Disc 3 Chemistry: 2.9% Ni, 1.50%Cr, 0.26:C, 0.30%Mn 0.15% V max Physical: Tensile 149 KSI max Yield 114-121 KSI Disc Operating Metal Temperatures (Steady State) at Disc-Shaft Shrink Fit l i (Reference 6)

Disc 1 Admission / Exhaust 333*F/257'F Disc 2 " "

257'F/210*F Disc 3 " "

210*F/176*F Disc 4 " "

176*F/145'F Disc 5 " "

145'F/156*F Critical Crack Size ("c) at Keyway (for minimum values of Fracture Toughness) at Rated Speed /120% Overspeed (Reference 6) 1 Disc 1 1.34"/1.10" Disc 4 1.42"/1.10" Disc 2 1.30"/1.02" Disc 5 1.73"/1.34" l Disc 3 2.01"/1.61" I

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/}17//wnrar 1 PAGE 8 of 15 1

Critical Crack Size ("c) at Dise Hub (for minimum values of Fracture Toughness) at Rated Speed /120% overspeed (Reference 6) -

Disc 1 1.93"/1.65" Disc 4 1.81"/1.50" Disc 2 1.77"/1.46" Disc 5 1.97"/1.57" Disc 3 2.56"/2.13" The following analysis will identify two conditions of unit availability, i.e. 100% unit availability and 80% unit availability. For the intent of 4

this analysis the 100% unit availability shall be considered as the worst ca s t. .

Operating hours /18 month fuel cycle = 24 hr/ day X 365 day /yr X 1.5 yr/ cycle X 2 availability Operating hours / Cycle 0 100% availability = 1.314 E4 hours

. Operating hours / Cycle 0 80% availability = 1.051 E4 hours Using the apparent crack growth rate model discussed previously, the apparent crack growth rates for the GGNS-1 L.P. turbine discs (in inches / hour) will be calculated using the exhaust side steady state metal temperature for Keyway cracks since the Keyways are only on the exhaust side of the discs and an arithmetical average of the admission and exhaust temperatures for Bore cracks. The apparent crack growth rates (in inches /

hour) then are:

Dise Keyway (a/t) Bore (a/t) 1 7.057E-6 1.353E-5 2 2.849E-6 4.553E-6 3 9.075E-7 ' 1.326E-6 4 6.444E-7 9.449E-7 5 8.472E-7 7.400E-7 i

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df17/cHNf47.L PAGE 9 of 15 l

VI. End of First Cycle Projected Data To determine the maximum crack size at the end of each cycle, the following ,

formulae are used. l Crack Size = (a/t) X (operations hours 8 % availability)

% of Critical Crack Size = (Crack Size) / (Critical Crack Size ("c)) I i (100)

The following data shows the % of Critical Crack Size of the end of the first cycle 100% Unit Availability At Rated Speed:

Disc  % "c Keyway  % "c Bore 1 6.9 9.2 2 2.9 3.4 3 0.6 0.7 4 0.6 0.7 5 0.6 0.5 At 120% Overspeed:

Disc  % "c Keyway  % "c Bore 1 8.4 10.7 '

2 3.7 4.0 3 0.7 0.8 4 0.7 0.8 5 0.8 0.6 L

Arincuswari PAGE 10 of 15 80% Unit Availability

}

at Rated Speed: -

Disc  % "c Keyway  % "c Bore 1 5.5 7.4 2.3

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2 2.7 3 0.5 0.6 l

4 0.5 0.5 5 0.5 0.4 At 120% overspeed:

Disc  % "c Keyway  % "c Bore 1 6.7 8.6 2 2.9 3.2 3 0.6 0.6 4 0.6 0.6 5 0.6 0.5 Using the worst case of 100% unit availability and considering that per the A-CPSI analysis (Reference 1. Report ER504) shows that the probability of reaching a 120% overspeed condition is 1.6 E-7 per unit year for a 6-flow turbine, the maximum crack size which could develop is less than 11% of the critical crack size for Disc No. 1. The next largest crack is even smaller, at less than 5% of the critical crack size for Disc No. 2.

VII. Other Factors Influencing Dise Failures Another factor which needs to be considered in analyzing the signifi-

cance of the preceding data is the probability of ICSCC initiation. Discs which operate above the Wilson line have not shown any indication of stress

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PAGE 11 of 15

d. , . , +

-c -- corrosion. The Wilson line in a turbine is the point where the steam f flowing through the turbine may initially start condensing into vater.

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Upstream of the Wilson line there cannot be any condensate, only steam; and downstream of the Wilson line there may be a mixture of steam and water.

e Wilson line for the L.P. turbines at GGNS-1 is located near the admis-sion. side- of the No. 2 Disc, therefore Disc No. 1 is upstream and Dise Nos.

2-5 fre downstream of the Wilson line. The CGNS L.P. turbines are reheat f-turbines and will operate a minimum number of hours / year without the reheaters on-line. Based upon these facts, the data presented in the EPRI Report NP-2429LD (Reference 2), and data obtained from the EPRI Seminar of December 1-2, 1983 (General Electric Nuclear SCC experience), Utility Power Corporation (UPC) (an affiliate of Kraf twerk Union and Allis-Chalmers) has developed in Engineering Report ER-8503 (Reference 8), Keyway IGSCC initia-tion probabilities for each disc set on the L.P. turbines at Grand Gulf Unit

1. The crack initiation probabilities which are derived statistically from the U.S., turbine cracking experiences (Reference 2) and KWU experimental

. data (Reference 8) are:

-p Dine No. Keyway Crack Initiation Probability 1 .

6.7E-5

,' 2 1.3E-2 3 ,

1.3E-2

, 4 2.0E-4 5 .

2.0E-4

> The pr'oL' ability of keyway cracking is used throughout the rest of the

. j

' analysis for both bore and keyway cracking since this has been historically

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demonstrated to be the most conservative (highest probability of occurance) value, p

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gyg&Hinedr Z PAGE 12 of 15 l

l VIII. Data Application To apply the data presented previously in order to conserv'atively bound the potential crack size in either the keyway or disc hub a

regions for subsequent fuel cycles, the End of First Cycle Projected 1

Data is multiplied by the number of the cycle for which the end of cycle data is to be determined. Disc 1 would appear to be most limiting, based on the previously developed apparent crack growth

! rates. However, as also previously stated, discs that operate above the Wilson line (such as Disc No. 1) have not shown susceptibility to i

ICSCC. It is at this point in the analysis that the crack initiation 1

4 probabilities must be actively considered. The probability of l

j initiating an intergranular stress corrosion crack in No. I disc is 6.7E-5.

When this extremely low probability is considered in conjunction with l

l the low apparent crack growth rate even if crack is assumed, it is clear that Disc 1 is not limiting. Since Dise No.'s 2 and 3 have an i

IGSCC initiation probability of 1.3 E-2 this probability becomes the i

governing limit. Based upon this higher probability of IGSCC initiation and Dise No. 2's apparent crack growth rate being the second highest of all the discs Disc No. 2 becomes the critical disc.

Assuming that a crack initiates in the disc bore area immediately upon [

initial startup and propagates at the apparent crack growth rate previously identified, Disc No. 2 will suffer catastrophic failure in t 4

approximately 27.6 cycles, or approximately 40.5 service years after unit  ;

i 3

initial startup.

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It is well understood that the " apparent crack growth rate" used in I

p this analysis is an " average" value, determined and validate,d by statistics.

MP&L is not presenting the foregoing analysis with the intent of not per-i forming any turbine disc inspections, only to demonstrate that during the 3

early life of the turbine the probability of experiencing disc cracking is ex,tremely low. .

i ,

In actual practice, the data generated by using the apparent crack

, growth rate model and the probabilities of crack initiation would be supple-t l

mented and modified by ae'tual disc inspections to verify: 1) actual crack s

init'dation and 2) actual crack growth rates. Once l these values are deter-mined plant specific crack initiation and growth rates could be developed.

l.

From that point, the run/ retire decisions would be determined by conven-tional fracture mechanics analysis techniques.

d IX. Concluslons Based upon the preceding analysis, the NRC requirement to inspect the L.P. turbine discs at each refueking outage is extremely conservative and

could impose undue hardship in the forms of, excessive outage lengths,

, increased outage direct costs and increased power replacement costs upon the utility.A The conservatism of the " apparent crack growth rate" used in the

preceding analysis is that the crack is assumed to initiate immediately

! upon the initial start-up of the turbine. In actual fact, the initiation of

{ ICSCC requires a finite time for the susceptible material to be exposed to K

j . the working environment before any cracking can occur. This finite time generally is in the order of years, therefore the actual crack rate may i '

l significantly exceed the apparent crack growth rate without invalidating l the analysis based on the apparent crack growth rate.

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A T M cso m oor I PAGE 14 of 15 Since all three of the GCNS-1 low pressure turbines are fabricated using the same grade of material for the respective discs in each turbine (i.e.: the material for disc no. 1 is the same in all 3 L.P.'s, the material for disc no. 2 is the same, etc.) and since all three L.P.

turbines operate under nearly identical inlet conditions (steam temperature, pressure, enthalpy and steam chemistry), any one of the turbines may realistically be considered as representative of all of the L.P. turbines. MP&L therefore believes that a " sampling" inspection of cne turbine at any given number of operating hours will be representative of all three turbines with respect to the initiation and growth of IGSCC. The use of a " sampling" type inspection would thereby mitigate some of the hardships imposed on the utility.

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References:

1. GGNS FSAR Vol. 16, Chapter 10, Appendix 10A (includes the following Allis-Chalmers Power Systems Inc. Engineering Reports:

ES-43 (1/76); Engineering Report ER-503 " Turbine Missile Analysis",

Amendment 1 (12/1/75); Engineering Report ER-504 " Probability of Turbine Missiles" (10/75))

2. Electric Power Research Institute; Final Report NP-2429LD " Steam Turbine Disc Cracking Experience" Vol.1-7; June 1982

3. Electric Power Research Institute; Final Report NP-2970 " Residual Stresses in LP Turbine Discs"; March 1983

4. Electric Power Research Institute; Final Report NP-3691 " Stress Corrosion Cracking in Steam Turbine Disks: Survey of Data Collection, Reduction, and Modeling Activities", September 1984

5. Electric Power Research Institute; Final Report NP-3634 Vol. 1

" Properties of Turbine Disk Materials'; July 1984

6. Allis-Chalmers Power Systems Inc.; Engineering Report ER-8105 " Critical Crack Sizes of Grand Gulf LP Turbine Disks"; November 1981

7. Electric Power Research Institute; Final Report NP-4056 " Stress Corrosion Cracking in Steam Turbine Disks: Analysis of Field and Laboratory Data"; May, 1985

8. Utility Power Corporation; Engineering Report ER-8503 " Probability of Disk Cracking Due to Stress Corrosion - Grand Gulf Unit 1"; March 1985 t

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• Attachm:nt II w

Excerpt

• From Engineering Report No. ER-503

• Complete Report may be found in GGNS FSAR Appendix 10A l

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Similar data and calculations were used to determine the characteristics of missiles from the LP disks #1 through #4. The results are given in Section 1.4 for the case of burst at 184% of rated speed for disks I, 2 and 3, and 181% for disk 4 which, due to its design would fall at this slightly lower speed.

For an assumed burst at 120% of rated speed disks #1 through #4 would be contained by the turbine casings. In this regard, analysis indicates that the following minimum burst saeeds woJid be required to provide sufficient i

initial energy to produce a disk missile which is just barely capable of emerging from the outer casing:

Missile Threshold Speed Disk r/ min t of rated i 2900 161 2 3220 179 3 2900 161 4 2960 164 5 3042 169 The missile threshold speeds of greater than 160% of rated provide a substantial probability that no significant turbine disk missiles would be produced even in the event of a serious overspeed incident. The crash ring built into the inner casing over disk #5 provides a missile threshold speed and missile characteristics which are of similar magnitude to those for disks il through #4. If this ring were not provided, the missile hazard from disk #5 would be much greater than f rom disks #1 through #4.

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ER-503 7/75

a. ....._-

, A77Acamsar f ALLIS-CHALMEERS POWaiR SYSTEiMO,IIMC.

r 5 impact between the missile and casing, it is estimated that the axial exit angle will be within a narrow range of possibilities. For disk 5, the most likely axial exit angle is estimated to be +8.4* toward the exhaust end of the LP casing and the range of angles which could occur with lower probability is from 0* (radial) to 16.8*. For each of the other disks 1 through 4, the most likely axial exit angle is +6*, and the range is from -7* to +19' with lower probability.

2. HYPOTHETICAL OVERSPEED FAILURE 2.1 Overspeed incident The hypothetical production of disk missiles requires that the turbine actually

, reach more than 180% of rated speed. In order to reach this high speed, the

! turbine controls and/or valves must fall such as in the following hypothetical incident followino a load rejecti6n:

a) Immediately after load rejection, the electro-hydraulle control (EHC) system would normally close the turbine control valves as a function of lost load. In the case of a full load rejection, the EHC would limit the transient overspeed to less than 10%. Assume the EHC falls j to close the turbine control valves, b) In response to the negative load gradient, a load-rejection relay (LRR) .

would act directly on the hydraulic control system to rapidly close

! all control valves, and the mechanical-hydraulic control (MHC) system would close all control valves as a function of speed independent of the EHC. It is assumed that the NHC and LRR also fall and speed rises above 110% of rated.

ER-503 7/75

. . - - .. ......._.____.e, AITACHP1MT 2 ,

ALLIS-CHALMERS PC)WER SYS' REMS, cm I c) At approximately 111% of rated speed the overspeed trip system would 3 trip all turbine stop, control and extraction valves closed and limit the transient overspeed excursion to less than 120% of rated speed.

Assume the overspeed trip system also falls.

d) As speed increases above 120% of rated, the shrunk-on disks of the LP turbine rotors would begin to loosen at about 130%, and at 150%

of rated speed the bores of the last stage disks would theoretically

. be eccentric to the shaf t by more than 7 mils creating a large unbalance of the rotors. The unbalance forces magni fled by cri tical speed characteristics would produce extremely large shaking forces resulting in bearing failures and severe interference between rotating and stationary parts which would probably brake the uni t to a halt.

The thrust bearing trip device and the low bearing oil pressure trip would probably act to trip the unit.

I e) The generator rotor teeth would begin to yield at about 150% and the retaining rings would burst at about 155% of rated speed. The ring fragments would either be retained by the generator stator casing or possibly emerge as relatively unhazardous missiles. The retaining ring failure would tend to increase overall braking of the unit to a l

l stop; however, it is assumed the speed continues to increase.

I f) At 170 to 175% of rated speed the generator rotor teeth would fall, and slot wedges and coils would fly outward producing enormous additional unbalar,ce forces which would probably fracture the generator rotor. In the worst case the rotor would separcte into two partsi

-Il- ER-503 7H5

, AT74cHmfar x ALue-CHALMERS POWER SYSTEMS. NMC.

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even in this case the rotor fragments would probably not penetrate the surrounding stator core vthich is a very massive structure.

g) At 180 to 185% of rated speed one or more disks on the LP rotors would fall, producing turbine missiles. The calculated failure speed of disk #4 is about 181%; and for disk #5 it is about 184%. At this speed, it is assumed that also disks #1, 2 and 3 would burst.

2.2 Disk Stress and Failure Further details of the hypothetical overspeed incident are shown in Figure MA4.16 in which the average tangential stress in an LP last stage disk is plotted as a function of speed. in the range from rated speed to 120% of rated speed, the maximum disk stress at the shrunk fit is less than one-half of the burst strength of the rotor material, which provides a large safety factor for normal operation.

The disk stress and energy both increase as a function of speed up to about 180% of rated speed at which it is likely that the 44-inch last stage blade vanes would detach and fly off.

)

End blade vane detachment is assumed to occur when the centrifugal stress (bending neglected) in the blade equals the ultimate tensile strength of the blade material. This is calculated using the dimensions, density and ultimate strength of the blade material. $1nce the blades are of the free-standing type, there are no Interconnections between blades to complicate the calcula-tion. The configuration of the 44" 1800 rpm end blade is shown in Figure MA 4.06.

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ER-503 7/75 f ,

l Attichm*nt III i

1 Excerpt

• From Engineering Report No. ER-504

• Complete Report may be found in GGNS FSAR Appendix 10A J16ATTC85091301 - 8

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• ER-504 10/75 C 1. INTRODUCTION 3

. This report provides information on the probability of occurrence of turbine misslies from burst-type failure of low pressure (LP) blade disks of 1800 r/ min turbines designed for nuclear power plarit applications. Two distinct cases are covered: 1) LP disk failure at or near design speed (<120% of rated speed), and

2) LP disk burst due to an excessive overspeed incident.

For the first case, information is provided on the turbine disk integrity including details of design, manufacture and quality assurance which provide very high reliability against disk failure in the design speed range. In addition, it should be noted that even if a disk burst occurs in the design speed range, the disk fragments would be contained by the turbine casings. The details of

/ this analysis are given in A-CPSI Engineering Report #ER-503 (1) which shows that burst speeds in excess of about 160% of rated would be required to yield external turbine disk missiles. Since an LP turbine disk missile can only penetrate the turbine casings at a turbine generator speed in excess of about 160%,

the probability of significant turbine disk missiles within the design speed range of 120% is considered zero.

For the second case, LP turbine disk missiles with the highest energy level are defined for a 130 to 185% speed range, where the average tangential disk stress is equal to 85% of the disk material ultimate strength. The probability of turbine misslies for this case is assumed to be equal to the prchability of an overspeed incident of greater than 120% of rated speed. This is determined by means of a reliability analysis of the turbine valves, speed control and overspeed trip systems. The results of reliability analysis t

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AT7did*14 r1 ma_ue-cma_msras powsa evernme.nwo. O ER-504 10/75 are values of 1.6 x 10 ~7 per unit year for a 6-flow turbine generator and 2.1 x 10 ~2 per unit year for a 4-flow turbine-generator. These values are Intended for use as "P;" in the equation P = Py xP 2xP where 3 P = overall probability of turbine missile damage, P = striking probability, and 2

P = damage pr bability. Information needed to calculate P 2and P is 3 3 given in A-CPSI Engineering Report #ER-503 h

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