ML19340C341

Turbine Missile Damage Probability Analysis for North Anna Units 1 & 2, Summary Rept Prepared for Util

ML19340C341
Person / Time
Site:	North Anna
Issue date:	11/06/1980
From:	STONE & WEBSTER ENGINEERING CORP.
To:
Shared Package
ML19340C331	List: ML19340C330 ML19340C341
References
ISSUANCES-OL, NUDOCS 8011140545
Download: ML19340C341 (18)

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Sl?SIARY REPORT ON THE

_R'RBINE MISSILE DAMAGE PR03ABILITi ANALYSIS FOR NORTH ANNA UNITS 1 AND 2 FOR VIRGINIA ELECTRIC AND POs'ER COMPANY BY STONE & WEBSTER ENGINEERING CORPORATION (R FS . C ALC . 1307 5. 01-NM ( B ) -3 S S-DKA) 8 011140 $4/5

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INTRODUCTION

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This report provides an overview of a pr6bability study on turbine missile damage at the North Anna site. It includes a summary of the findings, a brief discussion of the method by which they were 6btained, and an interpretive discussion of their meaning. The analytical basis for the results and conclusions presented in this report can be found in Stone & Webster calculation 1307 5. 01-NM (B) -33 8-DKA. The sole purpose of this report is to clearly present the findings of this calculation independent cf the detailed technical justification upon which they are based. Due to the lack of P1 data for the HP Rotor, this summary is based on damage from the low pressure turbine hoods only.

Based on the characteristics of the HP Rotor and its postulated missiles, it is expected that the resulting increase due to the inclusion of the HP Rotor will be insignificant.

OBJECTIVE The narpose of the calculation is to compute the total pr6bability of incurring unacceptable damage to essential systems at the North Anna site as a result of a tu.bine failure. This is done to satisfy the requirements of Regulatory Guide 1.115 (Ref. 1) and Standard Review

f. 2). A conservative analysis and the acceptance Plan 3.5.1.3 criteria (Rg/ annum, as allowed by Standard Review Plan 2. 2.3 (Ref. 9),

of 10 is used to arrive at an acceptable inspection frequency for the Westinghouse turbines.

Four distinct situations will be considered to arrive at the limiting case for minimum inspection frequency:

1. Damage to Unit 1 dua to Low Trajectory Missiles

2. Damage to Unit 2 due to Law Trajectory Missiles

3. Damage to Unit I due to High Trajectory Missiles

4. Damage to Unit 2 due to High Trajectory Missiles For each case above, the probability of damage will be calculated based on the failure probability of both turbines at the North Anna 1 and 2

. The maximum value calculated for any case will be compared to the sitg/

10~ annum criteria to establish the governing inspection frequency.

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The calculation is limited in scope to the tao existing units at the dorth Anna cite. However, the ef fect of turbine failures in future units is dibcus sed on a qualitative basis. ,

1 ASSUMPTIONS

1. The failure of a disc f rom the law pressure hood is assumed to create three or four equal segments with the properties defined in Ref. 4. The pr6bability of generating either 90 deg or 120 deg segments is assumed equal.

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2. The only targets of high trajectory missiles are the roof s of structures and the only targets of low trajectory missiles are the walls of structures. (Since the containment dome has a projected area in both plan and elevation views, it is a ta rge t for both types of missile.)

3. Interior discs (discs 1-4) may have trajectories up to 5 deg off the plane of rotation. End discs (disc 5) may have tra-jectories from 5 deg to 25 deg off the plane of rotation, but only in the direction of the hood end.

4. For the purpose of calculating strike probabilities, the mis-sile is assumed to be a point obj ec t .

5. The origin of internal disc missiles is located at the center of the hood and the origins of the outer disc missiles are assumed to be 6 feet to either side of the hood center.

6. If a missile penetrates a barrier, it is assumed to follow the same trajectory it had before striking the barrier, but 'the kinetic energy is reduced (i.e. , the barrier is not a scattering source). If the missile does not penetrate a barrier, ricochets and secondary missiles are not considered.

7. In a barrier penetration analysis, only the velocity normal to the barrier surf ace is relevant.

S. No earthquake or pipe rupture occurs concurrently with the postulated failure of the tu rb ine.

9. The column 9 line is the divider between Units 1 and 2.

10. For the destructive overspeed case, it is assumed t.. the I f a ilc re rate for any one of the 20 los pressure turbine discs is 1/20 of the total.

11. The failure of the high pressure rotor is assumed to create l eight equal segments with the properties defined in Ref. 4. j I

All missiles created are assumed to originate at the center of the high pressure hood and have trajectories up to 5 deg off the plane of rotation.

12. It is assumed that, for every fragment created by a low pressure disc failure or a high pressure rotor failure, there will be i

corresponding cylinder and blade ring f ragment s as de tailed in l Re f. 4.

13. When using the mcdified NDRC formula for calculating missile pe r f ora tion , it is adequate to use the average missile diameter, since the equation is reasonably insensitive to this pa rame ter in the normal range of turbine missile diameters.

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METHOD

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The probability of unacceptaole missile damage is computed from:

2 3 21 T S M

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1 L jns (P 2 P3 )ijntse P P lij n S jn i=lj=In=1 t- 1s= 1 m= 1 where P = Total probability for incurring damage during period between inspections p

= Disc failure probability for inspection 1 interval considered P = Strike probability, assuming disc failure 3 - Probability that the impacting missile will cause unacceptable damage i = Unit number of turbine being considered j = Operating condition at which failure occurs

= 1 = 100 percent rated speed

= 2 = 120 percent ove rs pe ed

= 3 = 191 percent destructive overspeed n = Turbine disc under consideration. There are a total of 21 discs per turbine, 20 low pressure wheels, and one high pressure rotor.

t = A target T = Total number of targets l

M = Number of fragment types a.e ociated with dise n )

(i.e. , disc, cyli1de r, and clade ring fragments pe r i Ref. 4) .

I L = Number of fragments of each type generated by a disc failure

= 8 for high pressure rotor

3 or 4 for low pressure wheels S

Number of variations in L considered for disc n

= 1 for high pressure rotor

= 2 for low pressure wheel 3

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An analysis using the above equation has been done twice to find the

. probability using two dif ferent criteria for unacceptable missile damage:

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Crite rion "A" -

Using the modified NDRC formulas for scabbing and perforation, a missile will be considered to produce unacceptable damage if it perforates the final concrete barrier if iined by steel or produces backface scabbing if unlined.

Criterion "B" -

Using the CEA-EDF formula for perforation (con-sidered the best of current pe rf oration formu-las, Ref. 6), a missile will produce unacceptable damage only if it perforates the final barrier (i.e., scabbing is not considered).

The analysis usi'g the above equation and Criterion "A" has six major steps:

1. Define target areas and locations with respect to the tu rbine ,

hoods. This is done by defining the target, usually as an  ;

entire structure containing essantial equipment, but occasionally as uniy a portion of a structure. The a re a s , locations, and structural protection of the targe ts are derived ii v.a Stone &

Webster drawings of the North Anna Power Station.

2. Using the modified National Defense Research Council (NDRC) formula and the missile properties from Ref. 4, calculate the minimum concrete thickness required to prevent backface scabbing (T and the minimum concrete thickness required to prevent pef)f oration (T ) for each missile.

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3. Eliminate targets for which P = 0 or for which redundant or alternative systems can be idhntified for every essential system within the target. This is straightforward for high trajectory missiles since the missiles come approximately straight dean and hit the target structures directly without interacting with any other barriers. For low trajectory mis-siles, essential targets may be in the shados of barriers.

Important barriers include the turbine support, the moisture separators, roof s, walls, and the . turbine hall operating floor (at shallow impact angles) . These barriers and the possible range of missile trajectories are evaluated by manual analysis to determine which structures defined in step 1 (or which portions of stroctures) are actually targets.

4 Calculate the probability of perforating the containment dome and cylinder for each missile based on its velocity normal to the targe t surf ace at impact. The containment liner has baen conservatively neglected in this analysis.

5. Calculate strike probabilities using the MA-057 computer code.

6. Manually sum the terms in the probability equation. De t a iled analysis at this step may consider refinement in target areas. l 4

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The analysis was then rept sted using Criterion "B" as follows:

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1. Since disc 2 has a Pg value which comprises more than 99 per-cent of the total P value for the ta rbine for the 100 pe rcent and 120 percent cases, only the prnhm 111ty due to disc 2 is modified. Probabilities for other otscs are conservatively assumed to be identical to those calculated in the analysis using Crite rion "A."

2. The probability at the 191 pe rcent speed case is not reanalyzed, but is reduced by the probability found for disc 1 which only produces scabbing damage. Disc 1 accounts for approximately 75 percent of the total probability for high trajectory missiles

_ad approximately 25 percent of the total for '.aw traj ec to ry missiles.

3. It is not necessary to repeat steps 1, 3, or 5, as these are identical for both analyses.

4. The CEA-EDF perfort tion formula is used to calculate the minimum concrete thickness required to prevent perforation for disc 2 and its associated fragments.

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PENETRATION EQUATIONS I. BALLISTIC RESEARCH LABORATORY (BRL) FORMULA For steel barriers, calculate the perforation thickness T using E

the Ballistic Research Laboratory (BRL) formula:

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p = 17,400 k d where T - Steel wall thickness required P to prevent perforation (in)

E = Energy of the missile normal to barrier (f t-lb) k = Constant depending on the steel (k e 1) d = Diameter of the missile (in)

Since t. e turbine missile is not cylindrical, the diameter may be approxin ated by d =

h 4A/7r

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where A is the missile cross-sectional area.

II. MODIFIED NATIONAL DEFENSE RESEARCH COUNCIL (NDRC) FORMULA For concrete barriers, use the modified National Defense Research Council (NDRC) formula to calculate the penetration depth:

• G(x/d) = K Nd

• D (V/1000)

(* , for x/d .5 2.0 where G(x/d) = (x/d-1), for x/d > 2.s K = Concrete penetrability factor = 180ff N = Missile shape f actor = 0.84 (blunt-nosed) ,

1 d =

f4A/P , missile diameter (in) 1 D = W/d , the calibre density of the missile (Ib/in )

V = Velocity (f t/sec) x = Penetration depth (in)

W = Missile weight (lb) 6

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TO CALCULATE SCABBING THICKNESS The penetration depth, x, can be converted to T8 , the concrete wall thickness required to prevent scabbing, using:

Ts /d - 7.91 x/d - 5.06 (x/d) , for x/d'5E 0.65 Ts /d = 2.12 + 1.36 x/d , for x/d := 0.65 TO CALCULATE PERFORATION THICKNESS The NDRC perforation thickness can be found using the NDRC penetra-tion depth, x, and TP /d = 3.19 x/d -0.718 (x/d) , for x/d 25 1.35 T /d = 1. 3 2 + 1. 24 x /d , for x/d :> 1.35 P

III. CEA - EDF PERFORATION FORMULA (REF. 6)

For concrete barriera, use the CEA-EDF formula to calculate the minimun thickness required to prevent perforation by solid missiles:

Tp = 0.765 (fc ') ~! E I V 3/4 i

where T = Concrete wall thickness required to prevent P

perforation (in) f ' = Concrete strength (psi)

'a' = Missile weight (1b) d = Missile diameter (in)

V = Missile velocity (f t/sec)

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CALCULATION CONSERVATISMS

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In order to demonstrate the conservative naturg6 * * * " " " ""

defend the use of an acceptance criteria of 10 per year, as allowed by S.R.P. 2. 2.3 (Ref. 9), the conservatisms are presented below.

a. The analysis did not take full advantage of the separation of redundant systems due to the extensive research required to properly generate this input.

b. The cross-sectional areas of the entire safety-related structures were used in calculating the strike probability values, P2. For perforation considerations this is extremely conservative, since only strikes to the cross-sectional area of the actual safety-related equipment and components would result in damage. For scabbing considerations, equipment not in the direct path of a missile might still be af fected by secondary missiles, but secondary missiles have a much lower damage probability P3, so using the area of the entire structures instead of just the safe ty-related parts is definitely conse rvative,

c. The perforation protection provided by tank walls and the containment liner has been neglected. The containment cylinder is lined with 3/8 inch of steel and the dome has a 1/2-inch liner. For a typical missile with a diameter of 20 inch and a weight of 2,500 lb, the velocity required to perf orate the liner acting alone is:

119 f t/sec for the dome and 96 f t/sec for the cylinder (using B RL f o rmula , see method section)

d. Current analyttcal methods to predict scabbing and perforation are known to be conse rvative. This is most noticeable in the scabbing p re d ictions.

e. Using the 23-day compressive strength of concrete, fc', to calculate penetration distances is conservative. Concrete continues to gain strength throughout its life and typically achieves a strength of 120 percent fe' by the end of its first year.

f. Due to the complexity of evaluating the numerous shielding structures and components between the turbine and the targets, only the moisture l

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separator, turbine pedestal, and turbine room floor were considered.

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DESIGN INPUT

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The following areas contain systems essential to shut down the plant, maintain it in c safe shutdown condition, and/or limit off-site exposures.

This list is based on the assumption that no earthquake or pipe rupture occurs concurrently with the postulated turbine failure:

Reac tor Containment Main Steam Valve Area Control Room Relay Room Auxiliary Building Fuel Building (portions only)

Cable Vault Cable Tunnel Fuel 011 Pump House and Tanks Decay Tanks - Waste Gas Condensate Storage Tank Service Water Pump House and Piping Auxiliary Feedwa ter Pipe Tunnel Other conceivable targets were eliminated.

Turbine missile properties are provided in Ref. 4.

Missile generation probabilities are provided in Ref. 5 and 8.

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CONCLUSIONS Using the more conservative of the tao analytical methods in this calcula-tion, North Anna Units 1 and 2 have acceptable probabilities of damage due to turbine failure, if the turbines are inspected at an interval not to exceed one year ,f actual operation. This acceptable inspection interval can be increased by as much as eight months if the less con servative approach (Criterion "B") is used.

It should be noted that the single most important factor influencing the total probability values is the P1 value for the destructive overspeed case. 'Jestinghouse currently provides a ,alue of 1.7 x 10- for this quantity. If this value can be further reduced by periodic valve testing, it should be possible to extend the acceptable interval between turbine inspections.

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SU'DtARY OF RESULTS, Numerical results based on one year of continuous operation are summarized in Tables I and II for the following two criteria:

Cri te rion "A" (Tab le I) Criterion "B" ' Tab le II)

1. The initiation of back-face scabbing Scabbing is neglected, as the cons titutes unacceptable damage, probability of resulting (Uses modified NDRC formula for damage is considered small scabbing)

2. Uses 2S-day concrete strergth Since concrete has aged for (f_') of 3,000 psi. several years a 20 percent increase in fe' is used.

fe' = 3,600 psi.

3. Uses modified NDRC formula for Uses more accurate perfora-pe rf oration as recommended by tion formula recently NRC in Ref. 1. developed by CEA-EDF.

Based on the conse rvatism inherent in the method of analysis (see list of calculation _gonservatisms at end of methods section), the acceptance criteria of 10 per year should be compared to the total probability value for each unit-trajectory combination.

Nunerical results based on two years of continuous operation are sunnt rized in Tables III and IV. Use acceptance criteria of 2 x 10 " for these

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Some rough guidelines, rela t ive to the ef fect of future units at the North Anna site on the re sults of this calculation, are given in Table V.

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TABLE I BASED ON: CRITERION "A" AND P1 VALUES FR0bi WESTINGHOUSE (REF. 5 AND 14)

TOTAL DAMAGE PROBABILITY FOR ONE YEAR

. CONTINUOUS OPERATION PERCENT OF RATED SPEED UNIT - TRAJECTORY 100% 120% 191% TOTAL

-6 Unit 1 - Low Traj. 2.536 x 10 ~7 4.23 x 10

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6. 5 46 x 10~ 0.912 x 10 Unit 1 - High Traj.

-10 Due to Unit 1 Turbine 1. 35 x 10-7 8. 4 2 x 10 5.229 x 10~

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-9 Due to Unit 2 Turbine 9.72 x 10 ' 2.12 x 10~ 2.395 x 10

~9 Total, 1.45 x 10 ~7 1.054 x 10 5.469 x 10~ 2.00 x 10~

-9 Unit 2 - Low Traj. 2.91 x 10~ 4.25 x 10 6.432 x 10~ 0.933 x 10~

Unit 2 - High Traj .

-10 Due to Unit 1 Turbine 1.09 x 10 -8 2.33 x 10 3.602 x 10

-9 Due to Unit 2 Turbine 1.79 x 10~ 1.44 x 10~ 2.802 x 10~

-8 Total 1. 90 x 10~ 1. 68 x 10~ 3_.1,6 2 x 10 2.232 x 10~

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TABL2 II BASED ON: CRITERION "B" AND F 1 VALUES FROM WESTINGHOUSE (R EF . 5 AND 14)

TorAL DAMAGE PROBABILITY FOR ONE YEAR CONTINUOUS OPERATION PERCENT OF RATED SPEED UNIT - TRAJECTORY 100% 120% 191% TOTAL 1.94 x 10

-13 1.059 x 10- 4.763 x 10- 4.763 x 10~

Unit 1 - Low Traj .

Unit 1 - High Traj.

-11 1.019 x 10 -9

-9 Due to Unit 1 Turbine 9.355 x 10 1.554 x 10 Due to Unit 2 Turbine 3.90 x 10~ 6.686 x 10- 1.591 x 10~

-9 -10 ~9 ~9 Total 1.409 x 10 1.604 x 10 3.145 x 10 4. 714 x 10 Unit 2 - Low Traj. 2. 85 7 x 10

-1 1.059 x 10 -13 4.706 x 10- 4.706 x 10-Unit 2 - High Traj.

Due to Unit

• Turbine 2.283 x 10 ~9 1. 03 2 x 10

-10 2.860 x 10

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Due to Unit 2 Turbine 2.923 x 10 7.336 x 10~ 3.360 x 10~

3.151 x 10 ~0 6. 22 x 10 -9

-10 Total 8.368 x 10 3.857 x 10~

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BASED ON: CRITERION "A" AND P1 VALUES- FROM WESTINGHOUSE (R EF . 5 AND 14)

TOTAL DAMAGE PROBABILITY FOR IWO YEARS CONTINUOUS OPERATION PERCENT OF RATED SPEED UNIT - TRAJECTORY 100% 120% 191% TOTAL

-6 Unit 1 - Loa Traj. 1.37 x 10

-5 1.727 x 10

-7 1.309 x 10 1.513 x 10 -5 Unit 1 - High Traj.

~0 Due to Unit 1 Turbine 7.66 x 10 3. 5 3 x 10~ 1.046 x 10~

Due to Unit 2 Turbine 0. 5 2 x 10~ 0.87 x 10~ 4.79 x 10~

-6 ~0 -6 Total 8.18 x 10 4.40 x 10 1.094 x 10~ 8.333 x 10 Unit 2 - Los Traj. 1.56 x 10~ 1. 71 x 10~ 1.236 x 10~ 1.706 x 10~

Unit 2 - High Traj.

Due to Unit 1 Turbine 6.46 x 10~ 1. 03 x 10~ 7.024 x 10 ~9 Due to Unit 2 Turbine 1.00 x 10~ 6.03 x 10~ 5.604 x 10~

Total 1. 07 x 10 -5 7.06 x 10

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6. 3 24 x 10

~0 1.033 x 10 -5

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Note: Acceptance criteria is 2 x 10 i

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J TABLE IV BASED ON: CRITERION "B" AND P1 VALUES FROM WESTINGHOUSE (R EF . 5 AND 14)

TOTAL DAMAGE PROBABILITY FOR 'lWO YEARS CONTINUOUS OPERATION PERCENT OF RATED SPEED UNIT - TRAJECTORY 100% 120% 191% TOTAL

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1. 21 x 10-10

-II Unit 1 - Low Traj . 4.37 x 10 9.526 x 10~ 9.5 28 x 10 Unit 1 - High Traj.

-0 -9 -9 Due to Unit 1 Turbine 8.585 x 10 3.818 x 10 3.103 x 10

-9 Due to Unit 2 Turbine 2.194 x 10~ 2. 7 7 2 x 10 3.182 x l'[

1. 078 x 10 -7 -9 6.29 x 10

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1. 207 x 10~

Total 6.59 x 10

~9 -1 Unit 2 - Law Traj . 1.723 x 10 4.37 x 10 9.412 x 10" 9.430 x 10~

Unit 2 - High Traj .

1.349 x 10" -9 ~9 Due to Unit 1 Turbine 4.418 x 10 5.72 x 10

-6 Due to Unit 2 Turbine 3.369 x 10 3.099 x 10~ 6. 7 2 x 10~

-6 -8 -8 -6 Total 3.504 x 10 3. 5 4 x 10 1. 244 x 10 3. 5 5 2 x 10

-6 Note: Acceptance criteria is 2 x 10

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TABLE V EFFECTS OF FUTURE UNITS ON CALCULATION RESULTS Ef fect of Missiles From On To tal Probability For: Unit 3 Unit 4 Total Unit 1 - Low Trajectory Case None None None Unit 2 - Low Trajectory Case Increase None Increase of t 1% of v 1%

Unit 1 - High Trajectory Case Inc rea se Inc rea se Increase of v 4% of e 1% of = 5%

Unit 2 - High Trajectory Case Increase Inc rea se Increase of = 9% of e 4% of = 13%

Note: Prior to the operation of a future unit, it has zero probability of damaging the existing units.

l Assumptions:

1. The missiles generated from future units were assumed to be identical ,

to those of the existing units.

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2. Ibrbine hood configtration and disc trajectories were also considered l identical to the existing units, except future units were considered to have three low pressure hoods instead of cao.

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REF ERENC ES:

1. Regulatory Guide 1.115, Rev. 1, " Protection Against Lms-Trajectory Turbine Missiles," USNRC, July 1977.

2. Standard Review Plan 3. 5.1. 3, Rev. 1, " Turbine Missiles,"

USNRC.

3. Regulatory Guide 1.117, Rev. 1, " Tornado Design Classification,"

USNRC, April 1978.

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4. CT-24321 " Turbine Missile Report," Westing'..ouse Electric Corporation, August 1980.

5. CT-24822 "Results of Probability Analyses of Disc Rupture and Missile Generation," Westinghouse Electric Corporation, August 1980.

6. George E. Sliter, " Assessment of Empirical Concrete Impact Formulas," Journal of The Structural Division, May 1980.

7. MA-057 Computer Code, " Strike Probability Analysis for Turbine Missiles," User Manual, Stone & Webster Engineering Corporation, Reissued October 1979.

8. " Analysis of The Probability of The Generation and Strike of Missiles from a Nuclear Turbine," Westinghouse Electric Corporation, March 1974.

9. Standard Review Plan 2. 2.3, " Evaluation of Potential Accidents,"

USNRC.

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