ML053140225
| ML053140225 | |
| Person / Time | |
|---|---|
| Issue date: | 01/17/2006 |
| From: | Buslik A, Guo J NRC/NRR/ADES/DSS/SBPB, NRC/RES/DRAA/PRAB |
| To: | |
| Buslik A. 415-6184; Guo J. 415-1816 | |
| References | |
| DG-1143 RG-1.076, Rev 1 | |
| Download: ML053140225 (14) | |
Text
U.S. NUCLEAR REGULATORY COMMISSION January 2006 OFFICE OF NUCLEAR REGULATORY RESEARCH Division 1 DRAFT REGULATORY GUIDE Contacts: A.J. Buslik, (301) 415-6184 J. Guo, (301) 415-1816 DRAFT REGULATORY GUIDE DG-1 1 4 3 (Proposed Revision 1 of Regulatory Guide 1.7 6, dated April 19 74 )
DESIGN-BASIS TORNADO AND TORNADO M ISSILES FOR NUCLEAR POW ER PLANTS A. INTRODUCTION The U.S. Nuclear Regulatory Commission (NRC) proposes this draft regulatory guide as an update to Regulatory Guide 1.76, Design Basis Tornado for Nuclear Power Plants.
Tow ard that end, this draft regulatory guide provides licensees and applicants with new guidance that t he NRC staff considers acceptable for use in selecting the design-basis tornado and design-basis tornado-generated missiles that a nuclear pow er plant should be designed to w ithstand in each of the three regions w ithin the contiguous United States to prevent undue risk to the health and safety of the public. This guide does not address the determination of the design-basis tornado and tornado missiles for sit es located in Alaska, Haw aii, or Puerto Rico; such determinations w ill be evaluated on a case-by-case basis. This guide also does not identify the specific structures, syst ems, and components that should be designed to w it hstand the effects of the design-basis tornado or should be prot ected from tornado-generated missiles and remain functional. In addition, this guide does not address the missiles att ributable to ext reme w inds, such as hurricanes, w hich w ill be considered on a case-by-case basis w hen identif ied.
This regulatory guide is being issued in draft form to involve the public in the early stages of t he development of a regulatory position in this area. It has not received staff review or approval and does not represent an official NRC staff position.
Public comments are being solicit ed on this draft guide (including any implementation schedule) and its associated regulatory analysis or value/impact statement . Com ment s should be accompanied by appropriate support ing dat a. Writ ten comment s may be submit ted to the Rules and Directives Branch, Off ice of Administration, U.S. Nuclear Regulatory Commission, Washington, DC 20555 -0001.
Comm ent s may be submit ted electronically through the NRCs int eract ive rulemaking Web page at htt p://w w w .nrc.gov/w hat-w e-do/regulatory/rulemaking.html. Copies of comments received may be examined at t he NRC Public Document Room, 11555 Rockville Pike, Rockville, MD. Comments w ill be most helpful if received by March 27, 2006.
Requests f or single copies of draft or act ive regulatory guides (w hich may be reproduced) or for placement on an automatic dist ribution list f or single copies of f uture draft guides in specific divisions should be made to t he U.S. Nuclear Regulatory Commission, Washington, DC 20 55 5, At tention: Reproduction and Distribution Services Section, or by fax to (301)4 15 -22 89 ; or by email to Distribution@nrc.gov. Electronic copies of t his draft regulatory guide are available through the NRCs int eract ive rulemaking Web page (see above); the NRCs public Web sit e under Draft Regulatory Guides in t he Regulat ory Guides document collect ion of the NRCs Electronic Reading Room at ht tp:// w w w .nrc. gov/reading-rm/doc-collections/; and the NRCs Agencyw ide Documents Access and Management Syst em (A DAMS) at htt p://w w w .nrc.gov/reading-rm/adams.html, under A ccession No. ML053140225.
General Design Criterion (GDC) 2, Design Bases for Protection A gainst Nat ural Phenomena, of Appendix A, General Design Criteria for Nuclear Pow er Plants, to Title 10, Part 50, of the Code of Federal Regulations (10 CFR Part 50), requires that structures, syst ems, and components t hat are import ant t o safety must be designed to w ithstand the effects of natural phenomena such as tornadoes w ithout loss of capability t o perform their safety funct ions. GDC 2 also requires that the design bases for t hese structures, syst ems, and components shall reflect (1) appropriate consideration of the most severe of the nat ural phenomena that have been historically reported for the site and surrounding area, w it h sufficient margin f or the limit ed accuracy, quant it y, and period of time in w hich the historical data have been accumulated, (2) appropriate combinations of t he effects of normal and accident conditions w it h t he ef fects of the natural phenomena, and (3) the import ance of the safety funct ions to be performed.
GDC 4, Environmental and Dynamic Effects Design Bases, of Appendix A to 10 CFR Part 50 requires, in part, t hat structures, systems, and components that are import ant t o safety must be protected against the eff ects of missiles from events and condit ions outside the plant.
For stationary power reactor site applications submitt ed before January 10, 1997, Paragraph 100.10(c)(2) of 10 CFR Part 100, Reactor Site Criteria, states that meteorological condit ions at the site and in the surrounding area should be considered in determining the acceptability of a site for a pow er reactor.
For stationary pow er reactor site applications submit ted on or after January 10, 1997, Paragraph 100.2 0(c)(2) of 10 CFR Part 100 requires that meteorological characteristics of the site that are necessary f or safety analysis or may have an impact upon plant design (such as maximum probable w ind speed) must be considered in determining the acceptability of a site for a nuclear pow er plant. In addition, Paragraph 100.21 (d) of 10 CFR Part 10 0 requires that the physical characteristics of the site, including meteorology, must be evaluated and site parameters established such that potential threats from such physical characteristics w ill pose no undue risk to t he t ype of facilit y proposed t o be located at the site.
The NRC issues regulat ory guides t o describe t o t he public met hods that the st aff considers acceptable for use in implementing specif ic part s of the agency s regulations, to explain techniques that the staff uses in evaluating specific problems or postulated accidents, and to provide guidance to applicant s. Regulatory guides are not substit ut es for regulations, and compliance w it h regulat ory guides is not required. The NRC issues regulat ory guides in draft form to solicit public comment and involve the public in developing the agencys regulatory positions. Draf t regulatory guides have not received complete st aff review and, t herefore, t hey do not represent official NRC staff posit ions.
This regulatory guide contains information collections that are covered by the requirements of 10 CFR Part 50 w hich t he Of fice of Management and Budget (OMB) approved under OMB control number 3150-0011. The NRC may neither conduct nor sponsor, and a person is not required to respond t o, an informat ion collect ion request or requirement unless the requesting document displays a currently valid OMB control number.
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B. DISCUSSION Regionalization of Tornado Wind Speeds Nuclear pow er plants must be designed so that t he plants remain in a safe condition in the event of the most severe tornado that can reasonably be predicted to occur at a site as a result of severe meteorological conditions. The original version of Regulatory Guide 1.76, published in April 1 974, w as based on WASH-1300 (Ref. 1). WA SH-1300 chose the design-basis tornado w ind speeds so t hat the probabilit y of occurrence of a tornado t hat exceeded the design-basis w as on the order of 10 !7 per year per nuclear power plant.
WA SH-1300 used 2 years of observed tornado intensity data (1971 and 1972 ) to derive design-basis tornado charact eristics f or t hree regions w it hin t he cont inental United States.
The design-basis tornado w ind speeds present ed in this draft regulatory guide are based on Revision 1 to NUREG/CR-4461 (Ref. 2). The tornado database used in t he revised NUREG/CR-4461 includes information recorded for more than 46,8 00 tornado segments occurring from January 1, 1 950, t hrough A ugust 31, 2 003. M ore than 39 ,6 00 of those segment s had suf ficient information on location, intensity, lengt h, and w idth t o be used in the analysis of tornado strike probabilities and maximum w ind speeds. The methods used in this analysis are similar to those used in the analysis of the init ial tornado climatology leading to init ial publication of NUREG/CR-4461 in 1986 , w ith the addition of a term to account for f inite dimensions of structures (sometimes called t he lif eline term),
as w ell as consideration of the variation of w ind speeds along and across the tornado footprint.
The term associated w ith the finite dimensions of struct ures w as discussed in detail by R.C. Garson et al. (Ref. 3). The basic idea is that, for finite struct ures, a tornado striking any point on t he structure can cause damage. The original NUREG/CR-4461 used a point model, w here the nuclear pow er plant w as assumed to be a point struct ure. Therefore, including the finite dimensions of st ructures increases the tornado strike probability.
The basic model of a tornado footprint is a rectangle charact erized by t he w idth and length of the tornado path. The analysis accounts for the variation of w ind speeds w ithin the rectangle area, w hereas the model in the original version of NUREG/CR-4461 did not.
Meteorological and topographic condit ions, w hich vary significantly w ithin the continental United States, influence the frequency of occurrence and intensity of tornadoes.
The NRC staff has determined that the design-basis tornado w ind speeds for new reactors should be such that the best estimat e of the exceedance frequency is 10 !7 per year, retaining the same exceedance frequency as in the original version of this regulatory guide.
The results of the analysis indicated that a maximum w ind speed of 134 m/s (300 mi/h) is appropriate for t ornadoes for the central portion of the United States; a maximum w ind speed of 116 m/s (260 mi/h) is appropriate for a large region of the United States along the east coast , the northern border, and w est ern great plains; and a maximum w ind speed of 89 m/s (200 mi/h) is appropriate for the w estern United States. These geographic w ind speed regions are defined by observed tornado occurrence within 2° latit ude and longitude boxes in the cont iguous United States. Figure 1 show s the three tornado intensity regions for the contiguous United St ates f or the 10 !7 per year probabilit y level, in w hich the abscissa is t he longitude (degrees West) and the ordinate is the latit ude (degrees North).
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Figure 1. Tornado Int ensity Regions f or the Contiguous United St ates for Exceedance Probabilities of 10 -7 per Year Tornado Characteristics Tornadoes can be charact erized by a mutually consistent set of parameters including maximum t otal w ind speed; radius of maximum t angential (rotational) w ind speed; tornado tangential, vertical, radial, and translational w ind speeds; and associated atmospheric pressure changes w it hin t he core.
In order to estimate the pressure drop and rate of pressure drop associated w ith the design-basis tornado, this draft regulatory guide models the tornado as a single Rankine combined vort ex, as in the original version of Regulatory Guide 1.7 6. A single Rankine combined vortex is a simple model possessing only azimut hal velocity. The w ind velocit ies and pressures are assumed not to vary w it h t he height above the ground. Theref ore, the flow field is tw o-dimensional. The flow field of a Rankine combined vortex is equivalent to that of a solid rot ating body w it hin t he core of radius Rm . Out side t he core, the rot ational speed falls off as 1/r. That is to say, the rotational speed V R is given by (1a)
(1b)
Here, V Rm is the maximum rot ational speed, occurring at radius r = Rm . Moreover, the Rankine combined vort ex moves w ith the translational speed V T of the tornado.
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The pressure drop from normal atmospheric pressure to t he center of t he Rankine combined vortex is computed by balancing the pressure gradient and the centrifugal force (cyclostrophic balance), and integrating from infinity to the cent er of the vortex. It is given by (2) )p = D V Rm 2 , w here D is the air density, taken as 1.226 kg/m3 (0.0765 4 lbm/ft 3 ).
The maximum rate of pressure drop is given by t he follow ing equation:
(3) (dp/dt)max = (V Rm /Rm ) ) p The NRC staff chose the Rankine combined vort ex model for it s simplicity, over the model developed by T. Fujita (Ref. 4). Fujitas model has a tornado w ith an inner core and an annulus (outer core) w here the vertical motions are concentrated. In the annulus betw een the inner core radius and the outer core radius, suct ion vort ices form in strong tornadoes. These suction vort ices rotate around the center of t he parent tornado.
In the Fujit a model, t he tornado radius Rm is larger than the 45.7 met ers (150 feet) assumed in the original version of Regulatory Guide 1.7 6. In fact, t he tornado radius of maximum rotational w ind speed for a 134 m/s (300 mi/h) tornado is 157.5 meters (517 feet).
How ever, the suction vort ices have their maximum rotational w ind speed at a radius of 33 meters (108 feet). Despite the fact that the pressure drop associat ed with a suction vortex (t hat is, the pressure drop f rom ambient pressure t o t he cent er of the suction vortex) is somew hat less than for the parent tornado, the maximum rat e of pressure drop is greater, because the maximum time rat e of change of pressure is inversely proportional to the Rankine combined vortex radius and is directly proportional to the translational speed of the Rankine combined vortex. The radius for the suction vortex is smaller than that for the parent tornado, and the maximum translational speed for a suction vortex is the sum of the t ranslational speed of the tornado, and the speed with w hich the suction vortex rotates around the center of the parent tornado. In order to avoid a nonconservative maximum t ime rate of change of pressure, this draft regulatory guide retains the 45.7-meter (150-foot) radius of maximum w ind speed for the tornado used in the original version of Regulatory Guide 1.76. In addition, this draft regulatory guide retains the definit ion of the tornado maximum rotational w ind speed V Rm as the difference betw een the maximum t ornado w ind speed V and the t ranslational speed V T . The tornado translational speed for the tornado is one fifth of the maximum tornado w ind speed, w hich is consist ent w it h t he t ornado t ranslational speeds in t he original version of Regulatory Guide 1.76.
Design-Basis Tornado Characteristics In the original version of Regulatory Guide 1.76, tornadoes in each geographical region w ere characterized by (1) maximum w ind speed, (2) t ranslational speed, (3) maximum rotational speed, (4) radius of maximum rot ational speed, (5) pressure drop, and (6) rate of pressure drop. Because the model used in this draft regulatory guide is based on a single Rankine combined vort ex, t he same parameters are used herein. If a tornado model w ith suction vortices w ere used, addit ional paramet ers w ould have had t o be included. Table 1 summarizes t he design-basis tornado characterist ics f or this draft regulatory guide.
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Table 1. Design-Basis Tornado Characteristics M aximum Radius of M aximum rotat ional max imum w ind Translat ional speed rotat ional Pressure Rate of speed speed m /s speed drop pressure drop Region m/ s (mi/h) m/ s (mi/h) (mi/h) m (ft) mb (psi) mb/ s (psi/s)
I 1 3 4 (3 0 0 ) 2 7 (6 0 ) 107 4 5 . 7 (1 5 0 ) 1 4 1 (2 . 0 ) 8 3 (1 . 2 )
(2 4 0 )
II 1 1 6 (2 6 0 ) 2 3 (5 2 ) 9 3 (2 0 8 ) 4 5 . 7 (1 5 0 ) 1 0 6 (1 . 5 ) 5 4 (0 . 8 )
III 8 9 (2 0 0 ) 1 8 (4 0 ) 7 2 (1 6 0 ) 4 5 . 7 (1 5 0 ) 6 3 (0 . 9 ) 2 5 (0 . 4 )
Tornado-Generated Missile Characteristics To ensure t he saf ety of nuclear pow er plants in t he event of a tornado st rike, NRC regulations require that nuclear power plant designs must consider the impact of tornado-generated missiles (i.e., objects moving under t he action of aerodynamic forces induced by the tornado w ind), in addition to the direct action of the tornado w ind and the moving ambient pressure field. Wind velocities in excess of 34 m/s (75 mi/h) are capable of generating missiles from objects lying w ithin the path of the tornado w ind and from t he debris of nearby damaged struct ures.
The tw o basic approaches used to charact erize tornado-generated missiles are (1) a standard spect rum of tornado missiles, and (2) a probabilistic assessment of the tornado hazard. No definitive guidance has been developed for use in charact erizing site-dependent tornado-generated missiles by hazard probabilit y met hods. The damage t o safety-related st ructures by tornado or ot her w ind-generated missiles implies the occurrence of a sequence of random event s. That event sequence typically includes w ind based occurrence in the plant vicinit y in excess of 34 m/s (75 mi/h), existence and availability of missiles in the area, injection of missiles into the w ind field, suspension and flight of those missiles, impact of the missiles with safety-related structures, and resulting damage to critical equipment. Given defense-in-depth considerations, the uncertainties in these events preclude the use of a probabilistic assessment as the sole basis for assessing the adequacy of prot ection against tornado missile damage.
Protection from a spectrum of missiles (exemplified by a massive missile that deforms on impact at one end of the spectrum and a rigid penet rat ing missile at the other) provides assurance that the necessary st ructures, syst ems, and components w ill be available to mitigate the potential effects of a tornado on plant safety. Given that the design-basis tornado w ind speed has a very low frequency, to be credible, the represent ative missiles must be common around the plant sit e and must have a reasonable probability of becoming airborne within the tornado w ind field.
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In order t o evaluate the resist ance of barriers to penet rat ion and gross f ailure, the t ornado missile speeds must also be def ined. Est imates of tornado-generated missile speeds for nuclear plant design purposes are presented in Wind Eff ects on Structures, by E. Simiu and R.H. Scanlan (Ref. 5). One of the assumpt ions on w hich t hese estimates w ere based w as that t he missiles start t heir motion from a point located on the tornado translation axis, at a distance dow nw ard of the tornado center equal to the radius of maximum circumf erential w ind speeds. In addition, it w as assumed that the speed w ith w hich a missile hits a target is equal to the maximum speed (V max ) that the same missile w ould attain if it s t rajectory w ere unobstructed by the presence of any obst acle.
The tornado w ind field model used in the calculational method for the maximum missile velocit ies differs somewhat f rom the tornado w ind field model used in the discussion of tornado characteristics (above) to obtain the tornado pressure drop and maximum time rat e of change of the pressure. The tornado w ind field model (w hich includes a radial component for the tornado w ind speed) and the equations of motion used for the maximum missile velocit ies are given in Chapter 16 of Reference 5. A computer program w as written to calculate the maximum horizontal missile speeds by solving the equations of motion given in Chapter 16 of Reference 5.
Design-Basis Tornado Missile Spectrum In accordance w ith 10 CFR 50.3 4, GDC 2, and GDC 4, st ruct ures, systems, and components that are import ant t o safety must be designed to w ithstand the eff ects of natural phenomena w ithout losing the capability t o perform their safety f unct ion. Tornado missiles are among t he most extreme effects of credible natural phenomena at nuclear pow er plant sites. The selected design-basis missiles for nuclear pow er plants include at least (1) a massive high-kinetic-energy missile that deforms on impact , (2) a rigid missile that test s penetrat ion resist ence, and (3) a small rigid missile of a size suf ficient to pass through any opening of protective barriers. The NRC staff determined that a 15.24 -cm (6-inch) Schedule 40 steel pipe and an automobile are acceptable as the penetrating and massive missiles, respectively, for use in the design of nuclear pow er plants as common object s near the plant site. In order to test the configuration of openings in the protect ive barriers, the missile spect rum also includes a 2.54 -cm (1-inch) solid steel sphere as a small rigid missile. The charact eristics of these missiles are based on methods described in Reference 5. Table 2 summarizes the design-basis tornado missile spect rum and maximum horizont al speeds.
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Table 2. Design-Basis Tornado Missile Spectrum and Maximum Horizontal Speeds M issile Type Schedule 4 0 Pipe Aut om obile Solid St eel Sphere 0 . 1 6 8 m dia x 4 . 5 8 m 5 m x 2 m x 1.3 m 2 .5 4 cm dia Dimensions long (1 6 . 4 ' x 6 .6 ' x 4 . 3 ' ) (1 inc h d ia)
(6.6 25 " dia x 15 ' long) 1 3 0 kg 18 10 kg 0.0 66 9 kg M ass (2 8 7 lb) (4 0 0 0 lb) (0.1 47 lb) 0 . 0 0 4 3 m 2 /kg 0 . 0 0 7 0 m 2 /kg 0 . 0 0 3 4 m 2 / kg C DA/ m (0 . 0 2 1 2 f t 2 /lb) (0 . 0 3 4 3 f t 2 /lb) (0 . 0 1 6 6 f t 2 /lb) 4 7 m /s ec 52 m/sec 41 m/sec Region I (1 5 5 f t /sec ) (1 7 0 f t /sec ) (1 3 4 f t /sec )
3 8 m /s ec 45 m/sec 21 m/sec V Mhmax Region II (1 2 3 f t /sec ) (1 4 9 f t /sec ) (68 ft /sec )
8 m /sec 34 m/sec 7 m /sec Region III (2 7 ft /sec) (1 1 3 f t /sec ) (23 ft /sec )
The missiles list ed in Table 2 are considered to be capable of striking in all directions w ith horizontal velocities of V M h max and vertical velocities equal to 67 percent of V M h max .
Barrier design should be evaluated assuming impact normal to the surface for the Schedule 40 pipe and the aut omobile missile.
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C. REGULATORY POSITION The NRC staff has established the follow ing regulatory positions for licensees and applicant s t o use in selecting t he design-basis tornado and design-basis tornado-generated missiles that a nuclear pow er plant should be designed to w ithstand to prevent undue risk to the health and safety of the public:
(1) Nuclear power plants should be designed to w ithstand the design-basis tornado.
The paramet er values specif ied in Table 1 for the appropriat e regions ident if ied in Figure 1 are generally acceptable to the NRC staff for defining the design-basis tornado for a nuclear pow er plant. Sites located near the general boundaries of adjoining regions may involve additional considerations. The radius of maximum rotational speed of 45 .7 meters (15 0 f eet) is used for all three tornado intensity regions.
(2) If a design-basis tornado proposed for a given site is characterized by less-conservative paramet er values than the regional values in Table 1, a comprehensive analysis should be provided to justif y the selection of the less-conservative design-basis tornado.
(3) The design-basis tornado-generated missile spectrum in Table 2 is generally acceptable to the staff for the design of nuclear pow er plants.
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D. IMPLEMENTATION The purpose of this section is t o provide information t o applicant s and licensees regarding the NRC staff s plans for using this draft regulatory guide. No backf itting is intended or approved in connection w it h its issuance.
The NRC has issued this draft guide to encourage public participation in its development.
Except in those cases in w hich an applicant or licensee proposes or has previously established an acceptable alternative method for complying with specified portions of the NRCs regulations, the methods to be described in the active guide w ill reflect public comment s and w ill be used in evaluating (1) submitt als in connection with applications for construction permit s, standard plant design cert if icat ions, operating licenses, early site permits, and combined licenses; and (2) submit tals from operat ing react or licensees w ho voluntarily propose to initiat e syst em modifications if there is a clear nexus betw een the proposed modifications and the subject for w hich guidance is provided herein.
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REFERENCES
- 1. U.S. Atomic Energy Commission, Technical Basis for Interim Regional Tornado Criteria, WASH-1300, May 1974. 1
- 2. J.V. Ramsdell, Jr., Tornado Climatology of t he Contiguous United States, NUREG/CR-4461, Revision 1, PNNL-15112, U.S. Nuclear Regulatory Commission, April 2005.2
- 3. R.C. Garson et al., Tornado Design Winds Based on Risk, Journal of the St ructural Division, Proceedings of the American Society of Civil Engineers, Vol. 101, No. 9 , pp.
1883-1897, September 1975.3
- 4. T. Theodore Fujita, Workbook of Tornadoes and High Winds for Engineering Applications, SMRP Research Paper No. 165, September 19 78 . 4
- 5. Emil Simiu and Robert H. Scanlan, Wind Effects on Struct ures: Fundamentals and Applications to Design, 3 rd Edit ion, John Wiley & Sons, August 1996. 5 1
Copies are available for inspection or copying f or a fee from the NRCs Public Document Room at 1 15 55 Rockville Pike, Rockville, MD; the PDR s mailing address is USNRC PDR, Washingt on, DC 20 555; t elephone (3 01) 415-4737 or (800 ) 397-420 9; fax (301) 415 -35 48 ; email PDR@nrc.gov.
2 Copies are available at current rates from the U.S. Government Print ing Of fice, P.O. Box 37082, Washingt on, DC 20 40 2-9 32 8 (t elephone (202) 512 -18 00 ); or from t he National Technical Information Service (NTIS) by w riting NTIS at 528 5 Port Royal Road, Springfield, VA 22161; http:// w w w .nt is.gov; t elephone (703) 487-4650. Copies are available for inspection or copy ing for a f ee from the NRCs Public Document Room at 11555 Rockv ille Pike, Rockville, MD; the PDR s mailing address is USNRC PDR, Washingt on, DC 20 555; t elephone (3 01) 415-4737 or (800) 397-4209; f ax (3 01) 415-3548; email is PDR@nrc.gov. This document is also available elect ronically through the NRCs public Web sit e at http://w w w .nrc. gov/reading-rm/doc -collect ions/nuregs/cont ract/cr44 61 /.
3 Copies may be purchased f rom the American Society for Civ il Engineers (A SCE), 1801 Alexander Bell Drive, Reston, VA 20 19 0 [phone: 800 -54 8-ASCE (2723 )]. Purchase information is available through the ASCE Web site at ht tp: //w w w .pubs.asce.org/ WWWdisplay.cgi?5011559.
4 Copies are available for inspection or copying f or a fee from the NRCs Public Document Room at 1 15 55 Rockville Pike, Rockville, MD; the PDR s mailing address is USNRC PDR, Washingt on, DC 20 555; t elephone (3 01) 415-4737 or (800 ) 397-420 9; fax (301) 415 -35 48 ; email PDR@nrc.gov. This document is also available through the NRCs Agenc yw ide Document s Access and Managem ent System (A DAMS) at htt p://w w w .nrc.gov/reading-rm/adams.html, under Accession No. ML052650410.
5 Copies may be purchased from the publisher, John Wiley & Sons, 1 11 River Street, Hoboken, NJ 0 7030-5774
[phone: 20 1-7 48 -60 00 ]. Purchase informat ion is available through the publishers Web site at htt p://w w w .w iley.com/WileyCDA/WileyTitle/productCd-047 112 157 6.html.
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REGULATORY ANALYSIS
- 1. Statement of the Problem The U.S. Nuclear Regulatory Commission (NRC) issued the original version of Regulat ory Guide 1.76 in April 1974 to describe a design-basis t ornado t hat the NRC staff considered accept able for use in selecting t he design-basis tornado t hat a nuclear pow er plant should be designed to w ithstand in each of the three regions w ithin the contiguous United States to prevent undue risk to the health and safety of the public. The crit erion used then, and still used in this version of t his guide, is t hat t he exceedance frequency for the design-basis tornado should be 10 !7 per year. How ever, more data are available now than w hen the original version of this guide w as developed, and the methods used to est imate the frequency of exceedance of tornado w ind speeds have improved. A new analysis show s that the t ornado design-basis w ind speeds corresponding t o t he exceedance frequency of 10 !7 per year are low er than those given in the original version of this guide. Therefore, a revision to this regulatory guidance is necessary t o include updated information.
- 2. Objective The objective of t his regulatory action is to update the NRC s guidance w ith respect to the definit ion of the design-basis tornado and tornado missiles. This w ill give applicants and licensees the opportunit y to t ake advant age of the reduced w ind speeds of the revised design-basis tornado, w hich should lead to increased regulatory eff ectiveness by avoiding unnecessary conservatism that offers litt le safety benefit.
- 3. Alternative Approaches The NRC staff considered the follow ing alternative approaches to the problem of out dated guidance regarding the design-basis tornado and tornado missiles:
(1) Do not revise Regulatory Guide 1.76.
(2) Update Regulatory Guide 1.76.
3.1 Alternative 1: Do Not Revise Regulatory Guide 1.76 Under this alt ernative, the NRC w ould not revise this guidance, and licensees w ould continue to use the original version of this regulatory guide. This alternative is considered the baseline or no action alternative and, as such, involves no value/impact considerations.
3.2 Update Regulatory Guide 1.76 Under t his alt ernative, the NRC w ould updat e Regulat ory Guide 1.76 w it h new tornado data to reflect the new est imates of the f requency of exceedance of tornado w ind speeds. Tornado design-basis w ind speeds corresponding to the exceedance frequency of 10 !7 per year are low er t han those given in t he original version of this guide.
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The benefit of this act ion w ould be saving resources on the part of licensees and applicants building new nuclear pow er plants, w ith the latt er realizing the predominant savings.
The costs to the NRC w ould be the one-time cost of issuing the revised regulatory guide (that is, relatively small), and applicants and licensees w ould incur little or no cost.
Other possible consequences of this action include the possibility of underestimating the frequencies of exceedance of tornado w ind speeds. How ever, considering t he conservatism of struct ural design for other loads, it is likely that a nuclear pow er plant could w ithstand higher tornado w ind speeds. It appears very unlikely that the core damage frequency from tornadoes could be much greater than 10 !7 , and it is even more unlikely t hat a core damage accident with a large early release w ill occur. Therefore, any adverse consequences of adopt ing t his alt ernative are considered extremely remot e.
- 3. Conclusion Based on this regulatory analysis, the staff recommends that the NRC should revise Regulatory Guide 1.76. The staff concludes that the proposed action w ill reduce unnecessary conservatism in the specification of the design-basis tornado, leading to cost savings for industry, especially with regard to applications for standard plant design certifications and combined licenses.
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BACKFIT ANALYSIS This draft regulatory guide provides licensees and applicants w ith new guidance that t he NRC staff considers acceptable for use in selecting the design-basis tornado and design-basis tornado-generated missiles that a nuclear pow er plant should be designed to w ithstand in each of the three regions w ithin the contiguous United States to prevent undue risk to the health and safety of the public. The application of t his guide is voluntary.
Licensees may continue to use the original version of this regulatory guide if they so choose.
No backf it, as defined in 10 CFR 50.1 09, is either intended or implied.
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