ML20217B196

Discusses Review of Farley Nuclear Plant,Units 1 & 2 Licensing & Design Basis for Protection Against Tornado Missiles at Farley in Accordance w/970402 Memo from RM Gallo

ML20217B196
Person / Time
Site:	Farley
Issue date:	10/15/1997
From:	Marsh L NRC (Affiliation Not Assigned)
To:	Berkow H NRC (Affiliation Not Assigned)
Shared Package
ML20217B144	List: ML20217B137 ML20217B181 ML20217B196 ML20217B211
References
TAC-M98372, TAC-M98373, NUDOCS 9804220443
Download: ML20217B196 (6)

Text

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,. UNITED STATES

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NUCLEAR REGULATORY COMMISSION WASHINGTON. D.C. 3086H001

\ October 15, 1997 MEMORANDUM T0: Herbert N. Berkow, Director Project Directorate II-2 Division of Reactor Projects I/II Office of Nuclear Reactor Regulation FROM: Ledyard B. Marsh, Chief Plant Systems Branch h Division of Systems Safety and Analysis Office of Nuclear Reactor Regulation

SUBJECT:

'FARLEY NUCLEAR PLANT, UNITS 1 AND 2 -

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LICENSING / DESIGN BASIS FOR TORNADO MISSILE PROTECTION (TAC NOS. M98372 AND M98373)

The Plant Systems Branch has reviewed the licensing and design basis for i protection against tornado missiles at Farley in accordance with the April 2,1997 memorandum to you from Robert M. Gallo, then Chief of the l Special Inspection Branch (PSIB). In that memorandum, PSIB identified a '

tornado missile protection issue related to the exhaust silencers for the emergency diesel generators (EDGs) of both units. The exhaust silencers for the EDGs were installed outside, on the roof of the EDG buildings. According to the memorandum, the equipment appears to be protected from horizontal tornado generated missiles by building walls. However, the equipment is susceptible to vertical and other non-horizontal missiles. In response to an inspection question, the licensee stated that their licensing / design basis did not require the equipment to be protected from vertical and other non-horizontal missiles. PSIB identified that, at the time of the inspection, the licensee could not provide any documentation supporting their statement.

The licensee also stated that not all plants in their vintage considered the vertical and non-horizontal missiles in their design for tornado missile protection. Based on this information, PSIB specifically requested the staff to determine whether the tornado missile protection design / licensing basis ,

included missile spectra other than horizontal missiles and any generic implications.

This issue was identified as an unresolved item (URI 97-201-9) in NRC Inspection Report Nos. 50-348/97-201 and 50-364/97-201 dated May 13, 1997. By {

1etter dated May 28, 1997, the licensee provided information intended to  !

support their position that to reevaluate or modify the licensing and design  !

bases to include non-horizontal missile spectra is unnecessary and would be a l new requirement.

l The licensee's submittal references and quotes excerpts from a number of differer.t sources that were used for the Farley design basis tornado criteria.

The licensee interprets these documents as clearly supporting their claim that only horizontal tornado missiles need to be considered for protection of safe CONTACT: W. LeFave, SPLB/DSSA/NRR 301 415-3285 9804220443 980421 l PDR ADOCK 05000348 P PDR

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t Herbert N. Berkow 2 shutdown equipment. Our review of these same documents does not lead us to

' the same conclusion. Because of the gravitational effects, all horizontal

• missiles have vertical components as they leave the influence of the tornado funnel. However, the highest velocity of a given missile occurs while still directly influenced by the tornado. The staff, therefore, requires the calculated velocity of missiles at the leading edge of a tornado to be used when determining the thickness of structural walls to prevent missile penetration. However, in general the staff usually only asked licensees' to specifically perform vertical missile (includes all non-horizontal missiles) analyses for structures, systems, and components susceptible to vertical missiles, such as cooling (SSC) that buried tower fans, were thought pi ~ to be cabling, and some ventilation openings. In the case of EDG discharge' ping, piping / silencers, the staff sometimes accepted that only horizontal or nearly horizontal missiles had the capability to damage them in a manner (such as severe crimping to block flow) that would make them inoperable. The most likely type of damage to the EDG silencers would involve perforation and denting that would not increase backpressure to the extent of preventing the EDG from performing its function. Also, the probability of a vertical missile strike to such horizontally protected components is significantly reduced because of the increasingly reduced missile density away from the tornado funnel. The angle of incidence of non-horizontal missiles is less than that of horizontal missiles which will also reduce the damage potential from any identified missile.

Vertical missiles in general were not discussed in the staff's safety evaluation reports unless there was a specific plant vulnerability involving specific components. The staff's reviews for protection against vertical missiles were performed on a case-by-case basis when specific vulnerabilities were identified. Since-there would be an infinite number of possible impact energies (below the horizontal energy), licensees were not required to identify; " vertical or non-horizontal spectra" unless there was a known plant-specific vulnerability. Therefore, vertical missile spectra were only required to be evaluated (and identified in the FSAR) in those specific plant roviews where a particular vulnerability was identified. To simplify the  !

vertical missile analysis, the licensee usually just used the horizontal missile spectra and impact energies in their. evaluation. In some cases the horizontal spectra may have been used with reduced impact energies (usually about 80% of the horizontal energy for each missile). However, in the case of Farley, the licensee incorrectly interprets the absence of specific non-horizontal missile spectra from the FSAR and other licensing documents to be verification that vertical missiles do not have to be considered in tornado missile protection analyses for their plant.

During.the licensing review of all plants for which tornado missile protection was required, the staff usually verified that the thickness of roofs for safety-related structures and the depth of buried safety-related piping were adequate to protect the potentially affected equipment' from tornado missiles.

Without the consideration for non-horizontal missiles, there would be no minimum depth for buried piping. For these reviews the impact energies of the horizontal missile spectra were usually assumed, and the missiles were assumed capable of striking in any direction. In the Farley FSAR Section 3.5.1.2 on missile protection, the licensee describes the adequacy of tornado missile

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Herbert N. Berkow 3 protection for Category I structures. The licensee states that "These structures, having at least 2-ft thick concrete exterior walls and roof slabs,

- constitute barriers against missile penetration. Calculations show that the deepest missile penetration of the concrete barriers would be 10 in.

Therefore, the 2-ft thick slabs provide ample protection." The fact that roof slabs are considered missile protection is confirmed in the discussion of missile barrier design procedures in FSAR Section 3.5.4. Additionally, FSAR Section 9.2.1.3 describes that all safety-related (service water) piping located outdoors is covered by a minimum of 3 feet,10 inches of soil for missile protection. In response to a staff request for additional information during the licensing review (see Attachment 6 of the licensee's submittal),

the licensee again identified that the roofs of safety-related structures were considered to provide adequate missile protection. All of the FSAR information supports the existence of tornado generated vertical missiles.

Staff reviewers would have reviewed this information and, given the fact that there are no obvious vertical missile vulnerabilities (e.g., cooling tower fans or spray pond piping), concluded that adequate missile protection against all tornado missiles was provided. In SER Section 3.5, the staff specifically identifies that the roof thicknesses and buried piping depths provide missile protection. However, the FSAR plan and layout drawings also clearly show that the EDG silencers had only horizontal missile protection. Thus, the staff may have considered this limited vulnerability acceptable based on engineering judgement. Or, this limited vulnerability (detectable from drawings but not discussed in FSAR text) may have been missed by the staff during its review.

The staff performs an audit type of review during the licensing process and it is not expected each and every vulnerability could be determined by the staff.

It is the licensee's responsibility to provide adequate tornado missile protection regardless of whether certain vulnerabilities may have been overlooked by the staff. It is also the responsibility of the licensee to describe and justify such vulnerabilities during the licensing process.

From a generic standpoint, the issue of tornado missile protection was most recently addressed in Generic Letter (GL) 95-04 " Final Disposition of the Systematic Evaluation Program Lessons-Learned Issues,," and by Generic Safety Issue (GSI) 156.1.5 " Tornado Missiles." In GL 95-04, the staff identified that protection from tornado missile for plants licensed before 1975 would be adequately addressed by the Individual Plant Examination for External Events (IPEEE) analysis required by GL 88-20, Supplement No. 4. The IPEEE was also the basis for dropping GSI 156.1.5. GSI 156.1.5 (see NUREG-0933) was concerned with missile protection for plants licensed before 1972, and in particular those reviewed prior to 1968 when criteria on tornado protection were first developed. As identified in NUREG-0933, the staff believes or expects that any vulnerability associated with tornado missiles will be evaluated and reported in the IPEEE submittals.

Based on the above, it is our position that the postulation of vertical missiles is within the design basis for Farley, and all other plants before and after that vintage (either as part of the licensing basis or as a result of IPEEE). While we agree with the licensee that, in general, horizontal missiles are bounding (from an energy standpoint), this does not preclude that adequate protection from vertical missiles was not required by the original

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Herbert N. Berkow 4 licensing and design bases. However, it is not apparent that there is a significant enough vulnerability to the EDG silencers such that the staff

" would have required a specific analysis to be performed and may have accepted (based on engineering judgement) a limited vertical missile vulnerability for i these components at the time of licensing.

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We, therefore, conclude that requiring additional specific protection for "

these components should be considered a backfit. However, the licensee should consider non-horizontal missiles and the vulnerability of the EDG silencers and exhaust piping during the performance of the IPEEE for Farley. I As additional support for our conclusions that vertical missiles'are within the licensing / design basis for Farley, we are providing the attached (Attachment 1) as sort of a rebuttal to some of the licensee's interpretation of various licensing support documents referenced in the May 28, 1997 submittal.

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We consider our efforts on TAC Nos. M98372 and M98373 to be complete. If you i have any questions or comments on this memorandum or its attachment please call Bill LeFave of my staff at 301-415-3285.

Attachment:

As stated '

cc: J. Zimmerman R. Matthew l D. Jackson

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ATTACHMENT 1 PLANT SYSTEMS BRANCH DISCUSSION OF LICENSEE'S INTERPRETATION OF DOCUMENTS

, TORNADO MISSILE DESIGN BASIS FOR FARLEY, UNIT 1 AND 2 ITEM 1 1

• The licensee references Bechtel Power Corporation (Bechtel) Topical Report l l B-TOP-3 " Design Criteria for Nuclear Power Plants Against Tornadoes," and the l FSAR in defining their tornado design criteria to include tornado wind force, i

pressure differential, and tornado missile impingement. The FSAR specifies that these three parameters are " applied concurrently ir, combinations producing the most critical conditions." These documeats'in essence ~ provide a

! characterization of the tornado wind which describes a horizontal force cpplied across the full surface of the side of a building or structure.

Therefore, according to the licensee, by definition, the tornado wind force does not bear down on the roof or top surface (s) of a structure, rather it strikes horizontally against the side surfaces of any obstacle in its path.

The design basis tornado-generated missiles set in motion and propelled by the tornado wind t.nd, as such, travel in the same horizontal direction.

We basically agree with the licensee's discussion but do not find it germane i

to whether protection is required from vertical type tornado missile. The staff does not consider vertical missiles to be vertically ejected from the

. tornado, but to be horizontal missiles initially with a vertical component of velocity at it-leaves the direct influence of the tornado. They would strike  !

with reduced energy depending on the angle of trajectory or penetration to the l target. The above description by the licensee are the limiting conditions for '

which a structure must be designed with respect to tornado forces but cannot in.any way be. interpreted to mean that vertical missiles are not postulated.

ITEM 2 The licensee states the fact that two of the bounding mii.sile cases for Farley are described as " travelling end on" and all are assumed not to tumble, establishes that the design basis missiles do not rotate in flight; but rather travel and impact head on into the structure. Further, the fact that the automobile bounding missile case is described as being carried and impacting "not more than 25 feet above ground" clearly demonstrates a horizontal approach, parallel to the ground and side wall impact.

Again, the licensee does not go far enough in their interpretations. We agree that the automobile has a horizontal approach that must be considered up to a height of 25 feet and at that point has its highest impact energy. However, once reaching 25 feet the automobile must eventually come down and then, becomes a vertical missile threat with lower impact energy. The licensee's l

interpretations tend to ignore the actual physics of any airborne missile once it is ejected from the tornado funnel. Also, the licensee's own quote that the bounding missiles are horizontal implies that they bound any non-horizontal missile which we believe gives evidence that they were considered in the licensee's analysis but thought to be bounded. If it is not the non-horizontal missile that is bounded by the horizontal missile considered in the analyses, what is bounded'l

ITEM 3 i

The licensee identifies that it is important to note that Farley was desi ned and received its Safety Evaluation Report (SER) before the staff issued t$e

• Standard Review Plan (SRP) in November 1975. Thus, having been issued after l the staff's review and ap not applicable to Farley, proval of the Farley design, the SRP was not and is identify that missiles were[ capable of striking in all directions.) At the Note that t time of the review the only official regulatory guidance available to the industry was' Regulatory Guide 1.76 which established the parameters of the design basis tornado, but did not address tornado-generated missiles. 4 l

Using the licensee's logic, it would appear that the staff did not require I protection from vertical missiles until the SRP was issued. In fact, the criteria put forth in the SRP (SRP Section 3.5.1.4) were based on current staff practice that had been in effect for a number of years. For the most

! part, the staff did not develop new criteria / requirements when developing the SRP. The licensee's FSAR discussions on buried piping and roof thicknesses i

for Category I structures, we believe, verifies that the staff did consider i vertical tornado missiles during its review of plants during the review of i Farley vintage plants. This also appears to be supported by the information '

provided in GL 95-04 and GSI 156.1.5.

l l ISSUE 4 i l

The licensee quotes a number of Bechtel sources in a long discussion entitled I

" Design for Spectra Other Than Horizontal Missiles was Not Required for Compliance with GDC 2 and 4." The gist of this discussion is that "No particular missile criteria need to be applied to the roofs of the Emergency Diesel Generator Building and the Control Room." The staff agrees in general l with the licensee's discussions. However, we differ on the reasoning behind the conclusions reached. Generally sseaking, the thicknesses of seismic Category I structures (for reasons otier than missiles), including the roofs, are known to provide adequate protection from tornado missiles even when assuming the bounding horizontal impact energy. Therefore, the staff does not require a specific vertical missile spectra analysis.

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