ML20217B181

Discusses Design Insp of Farley Units 1 & 2,for Jan-March of 1997,identified Tornado Missile Protection Issue Re Exhaust Piping & Silencers for EDG for Both Units (Including Station Blackout DG)

ML20217B181
Person / Time
Site:	Farley
Issue date:	12/18/1997
From:	Berkow H NRC (Affiliation Not Assigned)
To:	Richards S NRC (Affiliation Not Assigned)
Shared Package
ML20217B144	List: ML20217B137 ML20217B181 ML20217B196 ML20217B211
References
NUDOCS 9804220438
Download: ML20217B181 (2)

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% UNITED STATES g NUCLEAR REGULATORY COMMISSION  ;

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+, December 18, 1997 MEMORANDUM TO: Stuart A. Richards, Chief Events Assessment, Generic Communications and SpecialInspection Branch Division of Reactor Program Management Office of Nuclear Reactor Regulation FROM: Hert>ert N. Berkow, Director Project Directorate ll-2

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Division of Reactor Projects - 1 (/ I Office of Nuclear Reactor Reg 6

SUBJECT:

FARLEY NUCLEAR PLANT UNITS 1 AND 2  :

LICENSING / DESIGN BASIS FOR TORNADO MISSILE PROTECTION )

REFERENCE:

Memoranda from Ledyard B. Marsh to Herbert N. Berkow, dated October 15,1997, and from Southem Nuclear Operating ,

Company to USNRC, dated June 28,1995 l The design inspection of the Farley Units 1 end 2, January through March 1997, identified a tomado missile protection issue regarding the exhaust piping and silencers for the emergency diesel generators (EDG) for both units (including the station blackout diesel generator). The j exhaust piping and silencers in question are outside, on the roof of the EDG buildings, and as such are Seismic Category I and safety related. The issue identified during your design inspection was susceptibility of the EDG exhaust to a spectrum of nonborizontal missiles. The l inspec. tion identified sections of the Farley FSAR design basis that state that Category I ,

equipment and piping outside the containment building are either housed in Category l l structures or buried underground. Your question centered on whether the Farley plant conformed to General Design Criterion 2 if the design / licensing basis requires consideration of 7

protection of the EDG exhaust equipment from vertical and other nonborizontal missiles.

The absence of requirements for design or analysis of structures, systems, or components for nonhorizontal missile spectra in the SRP, FSAR, or other design / licensing documents does not mean that nonborizontal missiles do not have to be considered in the licensing basis for a plant.

The staff did require the licensee to perform specific vertical missile analyses for structures, systems, or components thought to be particularly vulnerable, such as cooling tower fans, buried piping, cabling, and some ventilation openings. In other cases, where the staff did not specifically request vertical missile analyses, the staff used engineering judgement during the bconsing review to determine if adequate protection existed from tomado missiles, e.g., for review of the EDG building roof thickness Without consideration for nonhorizontal missiles, there would have been no minimum thickness for roofs or for buried piping. However, for Farley, the staff accepted that only horizontal or nearly horizontal missiles could damage the EDG exhaust (such as a severe crimp or dont that would substantially block fow) to render the system inoperable. The staff views the most likely damage to the EDG exhaust piping or 9804220438 980421 PDR ADOCK 05000348 P PDR

S. A. Richards silencers to be perforation and denting, which may reduce their efficiency, but not increase backpressure sufficently to prevent the EDG from performing its function. For Farley, the EDG j buildings have short walls that protrude above the roofline that would eliminate or reduce this threat for a large portion of higher energy tomado missiles.

Also, for the missile to have sufficient voitical velocity to be a hazard, it would also have to be an ample distance from the tomado funnel so that its vertical velocity was a significant fraction of its velocity vector. However, if the missile is sufficently far from the tomado to have a significant vertical velocity, the overall threat is reduced due to the increasingly reduced missile density with distance from the tomado funnel.

It is the stars opinion that (1) Fariey's design and licensing bases do indeed require safety-related structures, systems, and components to be protected from vertical missiles, although the criteria for specific nonhorizontal missile spectra in tomado missile analyses are not discussed in the FSAR or other design / licensing documents. The stars licensing review included consideration for sufficient protection from nonborizontal missiles and, in cases where specific vulnerabilities were identified, required analyses for protection from vertical missiles; (2) sufficient protection from nonhorizontal missiles is provided due to the location of the Farley emergency and station blackout diesel generator exhaust piping and silencers so that no specific analysis as to their vulnerability is or was required; (3) the impact most damaging to ,

the exhaust system or silencer would be from a horizontal or near-horizontal missile from a )

nearty tomado, which would likely be reduced or eliminated due to the extended walls of the EDG building; and (4), nonborizontal tomado missiles that did manage to strike the exhaust piping or silencers would likely either be glancing blows or perforations, which would not render the system incapable of supporting EDG functions.

The licensee has considered vulnerability of the plant, including the EDGs and supporting equipment to wind-carried missiles. The second reference, dated June 28,1995, from the Southem Nuclear Operating Company to address Generic Letter 88-20 Supplement 4, states that no specific vulnerabilities were identified for Farley for the case of high winds or tomadoes.

Additionally, no other plant-unique extemal event was identified that poses any significant threat of severe accident within the context of the screening approach for "High Winds, Floods, and i Others," provided by NUREG-1407.

This issue was identified as an unresolved item (URI 97-201-9) in NRC Inspection Report Numbers 50-348/97-201 and 50 364/97-201 dated May 13,1997. We recommend that this unresolved item and design inspection open issue be closed with no action necessary based on the referenced technical staff review and this memorandum. j i

Docket Nos. 50-348 and 50-364

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