ML20209C897

Forwards Independent Assessment of Probabilities of Extreme Wind Speed from Tornado & Nontornado Phenomena.Applicant Reasonably Estimated Probability Associated W/Tornadoes,But Has Not Considered Probabilities for Nontornado Phenomena

ML20209C897
Person / Time
Site:	Seabrook
Issue date:	04/12/1984
From:	Gammill W Office of Nuclear Reactor Regulation
To:	Parr O Office of Nuclear Reactor Regulation
Shared Package
ML20209C800	List: ML20209C798 ML20209C897 ML20209D266 ML20209D278 ML20209D286 ML20209D327 ML20209D479 ML20209D512 ML20209D547 ML20209D870 ML20209E015 ML20209E274 ML20211C165 ML20211C259 ML20211C363 ML20211C389 ML20211C430 ML20211C484 ML20211C558 ML20211C632 ML20211C641 ML20211C679 ML20211C769 ML20211C792 ML20211C841 ML20211C904 ML20211C920 ML20211C942 ML20211D042 ML20211D068 ML20211D076 ML20211D092 ML20211D109 ML20211D129 ML20211D140 ML20211D169 ML20211D178 ML20211D191 ML20211D198 ML20211D208 ML20211D226
References
FOIA-87-6 NUDOCS 8404250235
Download: ML20209C897 (4)

Text

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APli IE BfM Docket Nos. 50-443/444 MEMORANDUM FOR: Olan D. Parr, Chief

.. Auxiliary Systems Branch, D51 FROM: William P. Gammill, Chief Meteorology and Effluent Treatment Branch, DSI SUSJECT: TORNADO OCCURRENCE FREQUENCY FOR SEABROOK In response to your reouest of March 2,1984, we have performed an inde-pendent assessment of the probability of tornado occurrences ir, the vicinity of the Seabrook plant. We have also reviewed the repcrt entitled, "Seabrook Nuclear Power Plant Tornado Missile Analysis," dated September 1983, prepared by Applied Research Associates, Inc. for the applicant.

Enclosed is our independent assessment of the probabilities of extreme wind speeds f rom tornadoes and non-tornado phenomena (includine hurricanes).

We conclude that the applicant has reasonably estimated the prcbabilities of extreme wind speeds associated with tornadoes. However, the applicant has not considered the probabilities of extreme winds from non ornaco phenomena.

Our assessment of tornado wind speeds and probabilities is bassa on an examination of tornadoes in a one-degree latitude-longitude box centered at the plant site. Because of the proximity of the plant site to the Atlantic Ocean, the area of consideration was limited to the land area contained in the one-degree box, approximately 60% of the total area, or about 2081 square miles. The period of record examined was the 32-year period, 1951-1982. Information on tornado occurrences, includingorecast path area and intensity, was compiled by the National Severe Storms .r Center of the National Oceanic and Atmospheric Administration. This data base represents the most complete compilation of tornado statistics avail-able to us at this time. Generally, information on tornado occurrences before 1954 is not considered to be as reliable as that compiled since then.

However, several significant tornadoes (Fujita tornado scale F2 and F3) were reported in the Seabrook region in the period 1951-1953. Theref ore, we extended our normal data base to include these events. The inclusion of tornado statistics for 1983 should not appreciably alter our estimates of tornado probabilities, sa kfNS _

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t O. D. Farr M 12 k In the period 1951-1932, 37 tornadoes wert reported in the one-degree latitude-longitude box (excluding overwater area) containing the plantThe site, resulting in a mean annual frequency of about 1.2 tornadoes per year.

expected path area associated with these tornadcas is 0.142 square miles.

• Considering the mean annual frequency, tornado path area, ano geographic l l

area (2081 square milts), the probabili site was computed to be aboutper 7.9year x 10 using gytheofmethod a tornado. strike at th described by Thom. We also examined tornado occurrences in larger geographic areas, a two-degree latitude-longitude box and within 50 miles of the plant site (also excluding overwater area). One of the largest tornadoes reported in liew England, the so-called " Worcester Tornado" The probabilities

{Fujita class F4), was included in the two-degree box. .

of a tornado strike at the plant site computed using data fro = these larger geographic areas arc slightly lower than that calculated above.

The enclosed distribution of tornado wind speed versus probability has been developed by multiplying the tornado strike probability with the probability of exceeding tornado wind speeds from Figure 5 of WASH-1300.

The applicant apparently examined tornado occurrences within a 50 mile

' radius of the plant for the period 1950-1977 anc computed a probability of strike of 7.0 x 10-5 per year. The applicant used a mean annual frequency of about 2.5 tornadoes per year with a mean path area of 0.124 square miles for the computation of strike probability. Some dis-

' crepancies exist b:tueen our data base and that used by the applicant for the same geographic area; however, these differences do not appear i

i to be significant for the computation of strike probability.

Tha enclosed .listribution of extreme wind speeds from non-tornado pheno ena is based on observations made at Boston, Massachusetts for the period 193G-1977. This information is presented in a report by Simiu, et al, entitled "Extre:ne Wind Speeds at 129 Stations in the Contiguous United States," flBS Building Science Series 118 (March 1979). .

This evaluation was prepared by J. Fairobent, with assistance frcra E.

Markee and I. Spickler. Any questions should be directed to l'.r. Fairobent l

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i llilliam P. Ga nill, Chief MeteoroloDy and Effluent Treatment Cranch Division of Systems Integration

Enclosure:

DISTRIBUTION:

As stated t1ETB Reading File .,

Docket File 50-443 Docket File 50-444 (w/o encl) WPGannill cc: See next page f.:ETB Docket File f

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