ML20049J328

Forwards Response to Rl Tedesco 811110 Ltr to Hg Parris Re Tornado Missile Protection for 480-volt Transformers. Positive Protection from 1-inch Diameter Missile for Intake & Exhaust Openings Over Transformers Will Be Provided

ML20049J328
Person / Time
Site:	Sequoyah
Issue date:	03/05/1982
From:	Mills L TENNESSEE VALLEY AUTHORITY
To:	Adensam F Office of Nuclear Reactor Regulation
References
NUDOCS 8203150071
Download: ML20049J328 (3)

Text

, ,

.be t TENNESSEE VALLEY AUTHORITY CHATTANOOGA TENNESSEE 37401 N

400 Chestnut Street Tower II ,

• yen S C"fl, March 5, 198 .'/ , , ,.g

., . a,v..-

~

\g3?7 ~-

- Director of Nuclear Reactor Regulation J g }

S; . Attention: Ms. E. Adensam, Chief A ,

% Licensing Branch No. 4 Division of Licensing  % "

U.S. Nuolear Regulatory Commission /h M Washington, DC 20555

Dear Ms. Adensam:

( In the Matter of Tennessee Valley Authcrity

)

)

Docket Nos. 50-327 50-328 1 Enclosed is our response to R. L. Tedesco's November 10, 1981 letter to H. G. Parris regarding tornado missile protection for the 480-V I transformers ati Sequoyah Nuclear Plant. In summary, because NRC has not accepted our previous probabilistic evaluation, we agree to provide positive protection froin the one-inch diameter missile for the intake and exhaus' openings over the 480-V transformers and for the exhaust vents over

- the 125-V battery rooms. The additional protection will be floor grating which will equal or exceed that previously accepted by the NRC for use in

- ' the west main steam valve room at Sequoyah Nuclear Plant.

I >

If you have any questions concerning this matter, please get in touch with

e J. E. Wills at FIS 858-2683 Very truly yours, TENNESSEE VALLEY AUTHORITY ie

y ^

s .

~ QL.~M. f Q Mills, Mahager Nuclear Regulation and Safety Sworn to and subscribed be ore me

~

thisd day o 1982

. - Notary Public

~'

• ~ My Commission Expires Enclosure oc: U.S. Nuclear Regulatory Commission D Df Region II $

Attn: Mr. James P. O'Reilly, Regional Administrator

- 101 Marietta Street, Suite 3100 Atlanta, Georgia 30303 / /

eg '-

t 920E3150071 82030S I DR ADOCK 05000 An Equal Opportunity Employer LA

___ _2 . _ - - _ _ . 2 , - - 3 3 , .

> s.

ENCLOSURE e

RESPONSE TO R. L. TEDESCO'S NOVEMBER 10, 1981 LETTER TO H. G. PARRIS TORNADO MISSILE PROTECTION FOR 480-VOLT TRANSFORMERS AND 125-VOLT BATTERIES SEQUOYAH NUCLEAR PLANT In the supplemental safety evaluation report input for Sequoyah Nuclear Plant, NRC has concluded that the 480-volt shutdown transformers are not adequately protected from tornado missiles entering the intake and exhaust vents. The issue was expanded in a telecon on December 15, 1981, to include the 125-volt battery room exhaust vents. In reaching the latter conclusion, NRC rejected a probabilistic assessment of the potential for damage to the 480-volt transformers at Sequoyah Nuclear '

Plant which TVA submitted on August 10, 1981. A response to NRC Auxiliary Systems Branch's concerns on Watts Bar Nuclear Plant submitted in a letter dated October 28, 1981, noted that TVA considered the probabilistic assessment for Sequoyah Nuclear Plant to be valid for Watts Bar Nuclear Plant. TVA also noted.in the October 28, 1981 response that the potential for damage to the 125-volt battery rooms was even lower than for the 480-volt transformers due to the size and number of openings and to partial shielding provided by an adjacent parapet wall.

As noted in the supplemental safety evaluation report input for i Sequoyah Nuclear Plant, TVA's probabilistic assessment calculated a total event probability of impact by a missile on an intake or exhaust vent on the roof of the auxiliary building as the product of several conditional probabilities. The P *P value was taken from Electric Power Research Institute (EPRI) NE-768n TVA still considers the conclusions presented in the probabilistic assessment to be valid.

The total event probability presented is conservatively calculated.

TVA did not include conditional probabilities to reflect the following:

1. A missile which strikes an opening will not always impact a transformer or battery. A conditional probability based on the angle of entry through a vent could be formulated.

2. Due to the proximity of the turbine building to the vents, potential missiles would not likely be accelerated to the velocities of missile spectrum A of Final Safety Analysis Report section 3.5. Therefore, a conditional probab'ility which considered a range of likely velocities and an associated potential for damage to a component could be formulated.

, Theassessmentcalculagedtheprobabilityofamissileimpactinga vent to be 1.56 X 10 peryearoyoneorderofmagnitudeofmargin with respect tc a criterion of 10 per year. The consideration of the additional probabilities discussed abois would certainly increase that margin. It is TVA's judgment that tne conservative nature of the probabilistic assessment was sufficient to compensate for the differences between Sequoyah Nuclear Plant (and Watts Bar Nuclear Plant) and the generic plant discussed in EPRI NP-768.

- _ , _. ,n., .-. -- _ --. . . - . , . . , .

. .=. . _ . . ..

Page 2 i

TVA could perform a plant specific probabilistic analysis for the Sequoyah Nuclear Plant and Watts Bar Nuclear Plant sites using the TORMIS code of EPRI NP-768. Such an approach has not been used previously so its acceptance is not assured. Therefore, TVA proposes to resolve this concern by providing positive protection from the 1-inch-diameter missile for the intake and exhaust openings over the 480-volt transformers and for the exhaust vents over the 125-volt

^

battery rooms. .The additional protection will be floor grating which will equal or exceed that previously accepted by NRC for use in the west main steam valve room at Sequoyah Nuclear Plant. As noted in the Sequoyah Nuclear Plant safety evaluation report, the grating in the

, main steam valve room is sufficient to stop the 1-inch rod at vertical i

velocities up to 252 feet per second. That velocity exceeds the '

, design velocities of the 1-inch rod for Sequoyah Nuclear Plant and Watts Bar Nuclear Plants. .

G 9

9

?

1

.- - , . - . .