ML20077R891
| ML20077R891 | |
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|---|---|
| Site: | South Texas  |
| Issue date: | 08/31/1983 |
| From: | BECHTEL GROUP, INC. |
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| NUDOCS 8309210044 | |
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ATTACHMENT 2 ST-HL-AE-1003 South Texas Project Probabilistic Evaluation of Hurricane-Generated Missile Hazard to the Containment Isolation Valve
- Compartment Equipment 14926-001 Risk / Reliability Group Los Angeles Power Division Bechtel Power Corporation l
August 1983 l
' 8309210044 830913 PDR ADOCK 05000498 A
PDR - - - - - - - - - ~ - - ~ ~ ~
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M SOUTH TEXAS PROJECT HURRICANE MISSILE EVALUATION REPORT j I. Introduction l This study uses Probabilistic Risk Assessment (PRA) methodology to 8 evaluate the probability that equipment located in the containment isolation valve cubicle (IVC) might be damaged by hurricane generated missiles. Hurricane-generated missiles include objects on or near the plant site that eculd become airborne during a hurricane and be transported to the top of the IVC.
I The study includes an evaluation of the likelihood of a hurricane 8 occurrence, as well as the probability of potential missiles becoming airborne and being transported to the top of the IVC. The extent of l damage is not evaluated, but is conservatively assumed to be certain t and total for all missile strikes.
II. Acceptance Criteria The NRC's acceptance criteria are contained in the " General Design Criteria (GDC) for Nuclear Plants" [1]. Specifically, GDC 2 and 4 apply to this evaluation and are summarized below:
- a. GDC 2 requires that " Structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as - hurricanes - without loss of capability to perform their safety functions . . ."
- b. GDC 4 requires that ". . . structures, systems, and components shall be appropriately protected against dynamic effects, includ-ing the effects of missiles,. . . from events and conditions outside the nuclear power unit."
The Standard Review Plan (SRP) [2] Section 3.5.1.4 provides further guidance in meeting GDC 2 and 4 requirements. Specifically, SRP Section 3.5.1.4 refers to the acceptance criteria of SRP 2.2.3, which states ". . . design basis events include each postulated type of accident for which the expected rate of occurrence of potential exposures inexcessofthe10CFRPart100guidelgesisestimatedtoexceedthe NRC staff objective of approximately 10 per year . . . expected rate
! of occurrence of potential expogures in excess of the 10 CFR 100 guidelines of approximately 10 per year is acceptable if, when combined with reasonable qualitative arguments, the realistic E probability can be shown to be lower. . ."
III. Summary The probability of failure of the equipment in the IVC to perform its safety function in the event of a hurricane is evaluated using PRA E methodology. The study quantifies the probability of hurricane generated missiles hitting the top of the IVC. The results are compared to the NRC 1
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acceptance criteria. The results indicate that hurricane-generated L missiles are not a significant threat to the IVC equipment. The results further indicate that no physical barriers are required at N the top of the IVC.
I.
~1 IV. Analysis Approach d The probability of damage to equipment located in the IVC depends on
-q three factors:
y~ .
A. The burricane occurrence rate at the plant site f(w).
] B. The conditional probability of one or more hurricane generated a missiles striking the top of any one of four IVCs, given the hurricane occurrence, PH I")*
C. The conditional probability of IVC equipment being damaged, given that the hurricane-generated missile or missiles have entered the IVC, PD.
So:
The hurricane occurrence rate is based on an NRC-sponsored study [4).
The cumulative probability of exceeding a given hurricane wind speed i at the plant site is shown in Table I.
I The conditional probability of the missile strike, given the hurricane occurrence, depends on the following subfactors:
A. The number of potential missiles.
B. The conditional probability of the potential missiles becoming I airborne (or injected), given the occurrence of a hurricane.
C. The conditional probability of missiles being transported from their origin to the target, given that they become airborne.
l D. The target area.
l The number of potential missiles is based on data from Electric Power n4 Research Institute (EPRI) surveys at seven nuclear power plant sites [5].
The probability of the potential missiles becoming airborne is calculated using a missile model developed at Jet Propulsion Laboratory (JPL) [6].
The conditional probability of missiles being transported from their origin to the target is based on a statistical mechanics model [7), [8).
The area of the top of each of the four IVCs is 745 square feet (total target area is 2980 square feet). The IVC height is 55 feet above grade and the grade elevation does not vary significantly E within 300 feet of the IVC. The number of potential missiles and the missile density incorporated into this study are shown in Table II.
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l The conditional probability of IVC equipment being damaged, given that the hurricane-generated missile or missiles have entered the IVC, is conservatively taken to be certain and total. That is, the conditional probability is taken as unity.
The result of this very co9servativg estimate is compared with the NRC acceptance criteria of 10 and 10 per year. Because of the uncertainty of some factors, we use the median as the "best estimate" or
" recommended" value [9).
V. Assumptions and Conservatisms ,
1 The following assumptions are used in this study: I A. A hurricane missile strike on the top of any one IVC represents failure (see conservatisms A and D, below).
B. The distribution of potential missiles by number and length is based on an EPRI survey of seven nuclear plants [5].
C. The conditional probability of a missile striking the target, given the hurricane occurrence, is adopted from the tornado missile model [7, 8, 11].
D. The frequency of hurricane wind speed is fitted with a Weibull distribution.
Conservatisms incorporated in this study are:
l A. The strike probability, in comparison to the activity release frequency acceptance criteria, assumes:
- 1. Missile-inflicted damage is certain and total.
B. The potential missile model assumes:
lE i 1. A missile distribution based on EPRI survey maximum.
- 2. A missile density increased by a factor of 2.5 over the i EPRI survey.
l3 3. All the missiles are distributed up to 20 feet above -
!3 grade.
- 4. The number of unrestrained missiles used in this analysis Il l
equals the total number of missiles in the EPRI survey (restrained and unrestrained).
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C. Eurricane speed is determined for the standard elevation 30 m (about 100 ft). This speed is significantly higher than the wind speed at the 20-foot elevation where potential missiles are located.
D. Geometric factors that result in further conservatisms are:
1.' ' Sheltering by other structures is neglected.
- 2. A missile strike in any IVC opening results in failure. 1
- 3. The area of safety-related equipment inside the IVC is less l than the IVC top area.
VI. Results and Conclusions The result of the analysis is the probability of hurricane missile damage to the IVC equipment. The median (50th percentile) value is reported in Table III. The medi g or "best estimate" value of strike
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probability is lass than 1 x 10 per year. Thisisverys9allcogared to the NRC activity release acceptance criteria value of 10 to 10 per year.
The above results indicate that hurricane-generated missiles are not a significant threat to the IVC equipment. These results further indicate that no physical barriers are required at the top of the IVC to protect IVC equipment from potential hurricane-generated missiles.
I VII. REFERENCES
[1] 10 CFR Part 50, Appendix /., " Design Basis for Protection Against Natural Phenomena."
[2] Standard Review Plan, U.S. Nuclear Regulatory Commission, i NUREG-75087.
[3] Nuclear Regulatory Commission, " Design Basis Tornado for Nuclear l Power Plants," Regulatory Guide 1.76, April 1974.
[4] Batts, M. E., "Probabilistic Description of Hurricane Wind 1 Speeds," Journal of the Structural Division, Proceedings of the American Society of Civil Engineers, Vol 108, No. ST7, July 1982.
[5] Twisdale, L. A., et al., " Tornado Missile Risk Analysis," EPRI NP-768, May 1978, EPRI NP-769, May 1978.
[6] Redmann, G. M., et al., " Wind Field and Trajectory Models for Tornado-Propelled Objects," EPRI 308, Technical Report 1, February 1976.
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[7] Goodman, J. and Koch., J. E., " Conditional Probability of the Tornado Missile Impact Given a Tornado Occurrence," Proceedings of the International ANS/ ENS Topical Meeting on Probabilistic Risk Assessment, Port Chester, New York, September 20-24, 1981, pp. 419-424.
[8] Goodman, J. and Koch, J. E., "The Probability of a Tornado Missile Hitting a Target," Nuclear Engineering and Design 74,(1983).
[9] Apostolakis, G., et al., " Data Specialization for Plant Specific Risk Studies," Nuclear Engineering and Design 56 (1980), pp. 321-329.
[10] Historical Extreme Winds for the United States - Atlantic and Gulf of Mexico Coastlines, prepared by M. J. Changery, NUREG/CR-2639
[11] Goodman, J. and Koch, J. E., "The Assessment of Tornado Missile Hazard to Nuclear Power Plants," Proceedings of International Conference on Numerical Methods in Nuclear Engineering, Montreal, Quebec, Canada, September 6-9, 1983.
[12] Simiu, E. and Scanlan, R. M., Wind Effects on Structures: An Introduction to Wind Engineering, John Wiley and Sons, Inc.,
New York, 1978. ,
[13] Batts, M. E., et al., " Hurricane Wind Speeds in the United States," i Journal of the Structural Division, Vol.106, No. ST10, Proc. 1 Paper 15744, Oct. 1980, pp. 2001-2016. ,
[14] Simiu, E. and Filliben, J. J., "Weibull Distributions and Extreme Wind Speeds," Journal of the Structural Division, Vol. 106, No.
ST12, Proc. Paper 15909, Dec. 1980, pp. 2365-2374.
[15] Georgiou, P. N., et al., " Design Wind Load in Regions Dominated l by Tropical Cyclones, " Proceedings of Sixth International i Conference on Wind Engineering, Gold Coast, Australia, 1
March (1983).
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l TABLE I TE PROBABILITY OF EXCEEDING A GIVEN WIND SPEED, w; NEAR TE STP PLANT SITE PER YEAR Milepost 300 Milepost 350 0 (m/s) w (mph) Weibull Distribution Weibull Distribution
-0 -0 50 112 3.4 x 10 3.7 x 10
-5 55 123 4.2 x 10 -5 5.1 x 10 60 134 3.1 x 10
-6 4.6 x 10 -6 65 145 1.2 x 10 -7 2.5 x 10 -7 70 157 2.1 x 10 -9 7.1 x 10 -9
-11 75 168 1.3 x 10 -11 9.8 x 10
-13 2.5 x 10 -10 80 179 5.7 x 10 TABLE II DISTRIBUTION OF POTENTIAL MISSILES Median (50thPercentile)
Number of Potential Missiles on 6,000 Site and Vicinity (2.5 x 107 ft 2), N
-4 LocalSurfaceDens$tyof 2.40 x 10 Potential Missiles Near the IVC (ftr),n p
Table III MEDIAN VALUE FOR TE PROBABILITY OF DAMAGE TO IVC FROM HURRICANE-GENERATED HISSILES
-10 Weibull wind speed distribution (milepost 350), <1 x 10 per year 100% potential unrestrained missiles up to 20-foot elevation i
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APPENDIX A GENERAL METHOD A.1 Introduction Probabilistic evaluation of hurricane generated missile hazard to the contain-ment isolation valve cubicle equipment is based on histories 1 data of hurricane eccurrences along the Gulf of Mexico and on a theoretical model for the c:nditional prchability of hitting a target given the hurricane occurrence.
A.2 Hurricane Frequency of Striking the STP Nuclear Power Plant Site According to [10], the cumulative distribution of extreme wind occurrence can be described in two ways:
I" ~ ") (A.1)
Fy (v) = exp < - exp ,
, 4 F2 (W) = exP
/ " h ))) y (A.2) {
where F (w) and F, (w) are cumulative functions of Type I and Type II dis-i tributi8ns, w is In extreme wind speed in sph, and a, o, and y are distribu-tion parameters.
Because the wind speed, w, in the formula for conditional probability of hit-ting is given in m/s, we introduce conversion coefficient S = 2.2374145, which converts m/s into uph.
We will use formulae (A.1) and (A.2) in the as y totic area where F1 (w) and F, (w) differ from unity by a small number (10 or less). In this area, f5mulae A.1 and A.2 can be simplified:
(
(A.3) l Fy (w) = 1 - exp
- (0" ~ ")} -
/" ) -y 0 (A.4)
F2 (w) = 1 - f i I
Batts [4] propo' sed the use of a Weibull distribution, which gives the best fit for a tail:
f i I
- (0" } 8 +y (A.5)
F ,(w) = 1 - exp The best parameters a, c, and y for Type I and Weibull distributions for all -
cileposts on the Gulf of Mexico are given in [4].
A-1
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- l The density functions for hurricane occurrence for all three types of l cumulative distribution given by formulae (A.3), (A.4), and (A.5) are: !
. . l i g(w)=hexp -
0"}" (A.6)
~
f2 (v) = (A.7)
Sw - a N Sw - a fw (w) = f g ,,p ,
g (A.8)
Therefore, the probability, dP , that a hurricane with maximum wind speed in the range (w, w + dw) will Etrike the plant site is:
dP,= f (w) dw (A.9) where f(w) is one of density functions (A.6), (A.7), or (A.8).
Now we have to choose between these three distributions. The Type II distri-bution was designed for nontropical storms. According to [12), the applica-tion of Type II distribution to hurricanes leads to a ridiculous result.
Forexample,fgrtheestimated1000-yearwind(i.e.,theprobabilityof occurrence 10 per year), a value of 1250 mph is predicted by (A.7).
It is much higher than a maximum possible wind speed for hurricanes.
According to [12), the gradient wind, w, can be evaluated according to the formula:
(A.10) w=hh where:
h = gradient of pressure p = sir density f = Coriolis parameter
( = coefficient depending on ratio of geostrophic wind velocity and Coriolis velocity. The range for cyclonic winds is:
0.5 < ( < 1 (A.11) 6 A-2
n Assuming the maximum pressure drop of 1 sta and minimum distance of this pressure drop of 1 km, we obtain the maximum value for the pressure gradient:
b=101.3h da as (A.12)
Maximum hurricane wind speed, w , corresponding to this gradient of pressure at the site latitude is in the fange:
120 mph < w, < 240 mph (A.13)
Analysis performed in [12, 15] indicates that Weibull distribution is more appropriate for high wind prediction than Type I distribution. Therefore, the Weibull distribution was incorporated in this study.
The STP site location is shown on Figure A-1. It is located between mileposts 300 and 350. The parameters of Weibull distributions for both mileposts are presented in [4]. Cumulative distributions are shown in Table I.
A.3 Conditional Probability of Hitting a Target, Given the Hurricane Occurrence at Plant Site The conditional probability, P , of a missile hitting a target given a hurricane occurrenceattheplantsitewNsadoptedfromthetornadomissilestudy[7,8]:
PH (") * "p A q (w) $ (w, z) (A.M) where:
n = local density of potential missiles AP = area of target z = target elevation over the ground q(w) = probability of injection of potential missiles E $ (w, z) = height distribution of airborne missiles In this study, the scope is limited to estimating the median value of the Probability of missile strike. Multiplication of medians for all factors is a good estimate for median value of PH (")*
'" The probability of injection, q(w), was calculated according to the method
.> described in [8]. The parameters of height distribution, $ (w, z), were calculated according to the formulae given in [11]. The results of.these I calculations are shown in Table A.I.
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A.4 Damage Probability to IVC Per Year The annual damage probability, TP , t the IVC can be calculated by formula:
a F
PT* . f(w) PE (w) dw (A.15)
"o since PD = 1 f r all strikes, where PH (w) is given by fomula (A.14) and f(w) by fomula (A.8).
The damage probability is less than 1 x 10 -10 per year.
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TABLE A.1 HEIGHT DISTRIBUTION $(z,F), INJECTION PROBABILITY q(w) AND CONDITIONAL PROBABILITY OF HITTING PH (w) AS FUNCTIONS OF WIND SPEED w FOR STP PLANT SITE w -
q(w) PH I")
$(z,w) Unrestrained Restrained Unrestrained Restrained (m/s) (mph) (z = 55 ft) Missiles Missiles Missiles Missiles 81 181 .1940 0 0 -0 0 86 192 .2264 .0719 0 .0116 0 91 204 .2587 .1479 .0470 .0274 96
.0087 215 .2906 .2075 .0982 .0431 101
.0204 226 .3218 .2603 .1451 .0599 .0334 106 237 .3521 .3065 .2311 .0772 .0582 111 248 .3814 .3518 .2790 .0960 .0761 116 260 .4094 .3793 .3009 .1111 .0881 121 271 .4363 .4092 .3429 .1277 .1070 126 282 .4619 .4443 .3920 .1468 .1295 131 293 .4864 .4606 .4249 .1602 .1478 136 304 .5095 .4992 .4424 .1819 .1612 141 315 .5316 .5237 .4611 .1991 .1753 146 327 .5524 .5436 .4650 .2148 .1837 151 338 .5722 .5661 .5258 .2317 .2152 156 349 .5909 .5849 .5291 .2472 .2236 161 360 .6085 .5989 .5480 .2607 .2385 166 371 .6253 .6077 .5541 .2718 .2478 171 383 .6411 .6242 .6041 .2862 .2770 l
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