ML20248L372

Combined Annual Frequency of Tornado-Generated Missile Strike Per Unit Area of Exposure at Songs,Units 2 & 3

ML20248L372
Person / Time
Site:	San Onofre
Issue date:	11/05/1997
From:	M'Lita Carr, Chung G, Hook T SOUTHERN CALIFORNIA EDISON CO.
To:
Shared Package
ML20248L371	List: ML20248L369 ML20248L372
References
NSG-97-06, NSG-97-6, NUDOCS 9803200040
Download: ML20248L372 (5)

Text

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1 NUCLEAR SAFETY GROUP

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PROBABILISTIC RISK ASSESSMENT REPORT Combined Annual Frequency of a Tornado-Generated Missile Strike Per Unit Area of Exposure at San Onofre Units 2 and 3 November 5, 1997 REPORT NSG-97-06 1

l Prepared by: / , Nd (M. P. Carr) i i

Reviewed by: A ng)

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Approved by:

I (T. G. Hook) f l

l 9803200040 990313 PDR ADOCK 0S000361 P PDR

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NSG-97-06 PURPOSE The purpose of this analysis is to determine the total combined annual frequency of a tornado-generated missile strike per unit area of exposure for San Onofre Units 2 & 3 for two distinct categories: Category 1, "S t ructures , systems and components (SSCs) damaged by missiles generated by tornados classified as F'1 through F'5 (windspeeds of greater than 72 mph)"; and Category 2, "SSCs damaged by missiles generated by tornado classes F'3 through F'S (windspeeds of greater than 157 mph)".

BACKGROUND SCE committed by letter to the NRC dated 11/24/80 and Updated Final Safety Analysis Report (UFSAR) Section 3.5 (Table 3.5-12) to protect SONGS Units 2 &3 from tornado-generated missiles. An internal audit determined that tornado-generated missile barriers for certain areas of the plant do not fully satisfy this licensing commitment for the worst case tornado-generated missiles.

Although the existing barriers may not be adequate to protect against the worst case tornado-generated missiles, the frequency that an adequate barrier would be required is very low. NUREG-0800, " Standard Review Plan," (SRP) Section 3.5.1.4, provides a conservatively acceptable threshold for safety due to damage caused by postulated missile strikes. Per SRP Section 3.5.1.4, Rev. 1, tornado-generated missile protection is not required if the aggregate probability of a missile strike is less than 10" per year.

The total annual frequency of a tornado-generated missile strike per unit area of exposure that is calculated herein for Category 1 and Category 2 will be used as input to determine if the aggregate probability of a missile strike on critical components (as identified in UFSAR Table 3.5-12) at San Onofre is indeed less than 10" per unit per year.

ASSUMPTIONS The following assumptions are used in this analysis:

1. Tornados with windspeeds in Fujita category F'6 (windspeeds greater than 318 mph) are deemed to be incredible and are not considered in this' study. Tornados classified as less severe than F'1 are not of sufficient size nor duration to present a significant hazard. [2]

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NSG-97-06

2. It has been determined that tornados with Fujita F-Scale categories F'1 and F'2 (windspeeds up to 157 mph) do not adversely affect the operability of certain SSCs like the AFW system [1] . Therefore, this analysis calculates the frequency per unit area of missile strike exposure for two categories of SSCs based on damage susceptibility. Category 1 encompasses all SSCs whose operability is affected by missiles generated by tornado classes F'1 through F'S (windspeeds of greater than 72 mph) while Category 2 contains all SSCs which are only vul'nerable to missiles generated by tornados with windspeeds of greater than 157 mph (F'3 through F'5).

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All credible missiles are " worst case" missiles as described in UFSAR Table 3.5-6.

4. The missile strike probability used in the SONGS 1 tornado study is assumed to be applicable to SONGS 2/3 [2] . A walkdown during the SONGS 1 study conservatively estimated that the number of potential missiles within a 2,000 ft radius of the missile targets was 16,040. This number is conservative due to the margins applied to the original count of potential missiles. The distribution (type and quantity) of potential missiles at SONGS 2/3 would be expected to be approximately the same as that of SONGS 1 since the administrative, maintenance, and operational practices which govern the number of potential missiles are largely site-wide rather than unit-specific practices. In addition, the missile pickup / transport area is approximately the same size for all three units. Therefore, the SONGS 1 missile inventory and thus the missile strike probability are assumed to be applicable to Units 2 and 3.

5. It is assumed that each missile has the same probability of striking a given area.

ANALYSIS The frequency of a tornado-generated missile strike per unit area is described by the following general equation:

F=E ( f t (tornado)

• P1 (missile strike) )

where the equation is summed over all applicable Fujita tornado classifications for each category and l

F= Frequency of a tornado-generated missile strike / Unit Area of Exposure 2

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NSG-97-06 i- Fujita-tornado category (i = 1, 2, 3, 4 and 5 for Category 1 and (i = 3, 4, and 5 for Category 2) f3 - frequency of tornado class i [yr"]

Pi- probability of a missile strike per square foot l given tornado class i (ft-2) l The tornado frequencies used are from the Tornado Hazard Review -

San Onofre Unit 1 (2) , completed by ERIN Engineering and Research, Inc.. In the study, three sources of tornado frequencies were referenced. The study applied the most conservative hazard curve available (NRC). These conservative frequencies are replicated in Table 1.

Table 1

, Frequency / Unit Area:

Probability f fs L* Pi ;

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'f Missile

-Strike Per-Win'dspeed' Category:1y (category'2 Fujita Fr ency  : Unit Area

• F ' 1 ; - F ' S -.

1F'3'- F'5:

(mph)

Scale 1 f) . (Pi )

F' 1 73-112 3.5E-05/yr 1.0E-09 3.5E-14 --

j F' 2 113-157 9.0E-06/yr 5.7E-07 5.1E-12 --

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F' 3 158-206 1.5E-06/yr 1.5E-06 2.3E-12 2.3E-12 T' 4 207-260 3.5E-07/yr 1.8E-06 6.3E-13 6.3E-13 F' S 261-318 8.0E-08/yr 2.2E-06 1.8E-13 1.8E-13 '

Total = E f i *Pi= 8E-12/yr*ft.2 3E-12/yr*ft 2 I

The missile strike probability is based on a SONGS Unit 1 missile population of 16,040 (2).

! RESULTS AND CONCLUSIONS The total combined annual frequency of a tornado-generated '

' missile strike per unit area of exposure at San Onofre Units 2 &

3 as determined by this analysis is,approximately 8E-12/yr*ft 2 for Category 1 SSCs and 3E-12/yr*ft' for Category 2 SSCs.

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f NSG-97-06 AEFERENCES

[1] E-mail from T. Yee to G. Chung, " Tornado Missile Barriers l for AFW Piping", December 10, 1993. .

[2] Tornado Hazard Review, San Onofre Unit 1, Final Report, Prepared by Erin Engineering and Research, Inc., June 1990.

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