ML19346A123
| ML19346A123 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 06/01/1981 |
| From: | Mills L TENNESSEE VALLEY AUTHORITY |
| To: | Adensam E Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8106050183 | |
| Download: ML19346A123 (6) | |
Text
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TENNESSEE VALLEY AUTHORITY l
s CHATTANOOG A. TENNESSEE 37401 400 Chestnut Street Tower II o
June 1, 1981 F
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Dimotor of Nuclear Reactor Regulation N's 4
u Attention: Ms. E. Adaname, Chief a 7s Lios.asing Branch No. 4 Q,
Division of Licensing
/s, U.S. Nuclear Regulatory Commiv'cn 4
p Washington, DC 20555 Dear Ms. Adensas In the Matter of the Application of
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Docket Nos. 50-327 Tennessee Valley Authority
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50-328 As requested by NRC, TVA has movaluated the issue concerning the potential for tornado generated missiles to compromise plant safety at Sequoyah Nuclear Plant by damaging the 480-V shutdown board transformers in the Auxiliary Building.
In order to place this issue in proper prospootive, a study has been done to determine an upper limit for the probability of tornado missiles damaging the subject transformers. A writeup discussing this study is enclosed. The conclusion of this study is that the probability of a tornado missile damaging tNs subject transformers is extremely low and is not a credible event worthy of further study.
Concurrently, postulating the coeurrence of this highly improbable eve::c, it is our position, based on the results of tha aletlated loss of all ao power test performed during the Sequoyah unit 1 nm:,aral circulation test, ', hat site shutdown could be achieved.
Very truly yours, t
TENNESSEE VALLEY AUTHORITY L. M. Mills, Manager Nuclear Regulation and Safety 4
Sworn and'subse bed before me th day.o h 1981 Y
My sion Expires 7
Enclosure I
8106050183 An Equal Opportunity Employs I.. - _. _ _, _
ENCLOSURE ASSESSMENT OF THE POTENTIAL FOR DAMAGE TO THE 480V SHUTDOWN TRANSFORMERS FROM TORNADO MISSILES The total event probability, P,
f a vertical tornado missile t
impacting any safety-related roof surface can be calculated as the product of three component probabilities:
Pt=P*P*P a
n v
where P is the probability of a tornado striking the plant 3
P is the probability of a missile striking a s"arety-related barrier P is the probability of a vertical strike on a horizontal y
roof surface Tornado Strike Probability (P )3 The total event probability, P, is computed using the informatilon contained in references 1 and 3. The tornado stgike probability for the Sequoyah area is given in WASH-1300 as 1x10 per year for any tornado intensity assuging an average path area of 2.8 square miles (approximately 3.6x10- per year per square mile). This value is confirmed in reference 2 which gives tge p"obability of a tornado
- per year per.quare mile.
strike in the Sequoyah area ar 4.0x10 Missile Strike Probability (P )n The estimation of missile strike probabilities is based on the data presented in reference 2.
Table 1 summarizes the calculation of the missile strike probabilities. Column C presents the probability of impact on any barrier at a representative one unit nuclear plant site for a sampling population of 6000 NRC-spectrum missiles and a particular tornado intensity. Reference 2 considered a range of from F2 to F6 Only tornado intensities 2 through 6 (greater than 113 mi/h windspeed) were considered." Column D is the weighted probability of impact on a safety-related barrier when the frequency of occurrence of each tornado classification is considered. The frequency of occurrence of the tornadoes intensities considered in the studies discussed in reference 2 are taken from Table 2.
The weighted total probability of an intensity 2 through intensity 6 intensity tornado impacting any 5
safety-related barrier is 1.68x10 per year.
Vertical Strike Probability (P )y In order for a missile to pass through a roof opening and damage a transformer, its orientation requires a vertical strike of the missile on a roof opening. The missile histories developed in reference 2 show that approximately ten percent of all barrier impact events are for vertical missiles on roof surfaces. From this, one can conclude thag the probability of a vertical strike on a roof opening, P, is 1x10 y
' Table 2 gives a frequency distribution based on observed tornadoes. No intensity 6 tornadors have been observed to date. However, for the purposes of this study a conservative frequency of 0.1 percent of the total.
Applicability to Sequoyah It is recognized that the studies presented in reference 2 are for a representative one-unit nuclear plant site and that the probability values presented in reference 2 are not developed specifically for Sequoyah. However, the representative site layout used in referer.ce 2 does consider the basic safety features of the Sequoyah plant with similar sizes and distances. Also, the representative site is oriented with respect to the wind field in order to maximize potential missile traj ec tories. This is a conservatism due to the fact that not all tornado strikes at a specific site will be oriented in a similar manner.
The studies in reference 2 further estimate the probability of a missile strike for a representative two-unit plant. The results for this site configuration show only a five to ten percent increase over the values for a representative one-unit site. This demonstrates the strike probabilities are not sensitive to variations in the specific plant target area. Therefore, the calculation of sit 3-specific probability values for Sequoyah would not differ radically from those found in the representative site study.
Conclusion l
For the conditions discussed above, the total event probability, P '
t is:
P = (4x10- ) (1.68x10-5) (1.0x10-l)
-10
= 6. /x10
/ year (missile striking any roof surface in typical l
plant) l l
l l
This is the probability of a vertical strike for a one-unit representative site given a sample missile population of 6000.
I The probabil}gy of a vertical missile striking the opening is much less than 6.7x10-, the probability of a strike on a safety-related roof surface. The probability of the missile entering the opening at the exact trajectory to hit a safety-related comonent is extremely unlikely. Postulating a double strike event occuring in such a manner as to damage redundant transformers would reduce the probability even further; therefore, this is not considered to be a credible event for the Sequoyah Nuclear Plant.
We recently completed a review of this concern regarding the presence of roof openings over the transformer rooms. The review focused on the damage which could result from the passage of a single missile through one of the roof openings. Due to the presence of separation barriers between transformers and separation of electrical cables and conduits, no interaction of losses were found whicP could result in the loss of both trains of safety-related systems. The impact of a second missile in another transformer room was not considered credible for the reasons discussed above.
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TABLE 1 Column A
B(
C(
D Intensity Frequency of Probability of Weighted Probability of Tornado Multiple of Multiple Missile Tornado Class (%)
Missile Strike (Pn)
Strike
-5
-5 2
26.6 4.71x10 1.25x10
-5
-6 3
7.2 5.52x10 3 97x10
-5
-7 4
2.1 1.61x10 3 04x10
-6 5
.12 6.68x10 8.02x10-9
-6
-9 6
.1 2.17x10 2.17x10
-5 Total 1.68x10 (1) WASH-1300, " Technical Basis for Interim Regional Tornado Criteria" (2) L. A. Twisdale, Tornado Missile Rink Study, EPHI NP-768, May 1978 4
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TABLE 2(I WINDSPEED DISTRIBUTION -
No. of Percent Windspeed Classification Tornadoes of Total 6 (windspeed-318 mi/h)
(.1)**
5 (windspeed 260 to 318 mi/h) 2 0.12 4 (207 to 260 mi/h) 34' 2.11 3 (158 to 206 mi/h) 115 7.13 2 (113 to 157 mi/h) 430 26'.68 1 (73 to 112 mi/h) 710 44.05 0 (40 to 72 mi/h) 321 19.91
- None has been observed.
- Assumed. See note on page 2.
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