Regulatory Guide 1.117

Rev 1,Tornado Design Classification

ML003739346
Person / Time
Issue date:	04/30/1978
From:	Office of Nuclear Regulatory Research
To:
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Reg Guide 1.117, Rev 1
Download: ML003739346 (3)

Revision 1 U.S. NUCLEAR REGULATORY COMMISSION April 1978 REGULATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.117 TORNADO DESIGN CLASSIFICATION

A. INTRODUCTION

considered.

A basic provision of tornado protection criteria is General Design Criterion 2, "Design Bases for that those structures, systems, and components whose Protection Against Natural Phenomena," of Appen failure could result in conservatively calculated expo dix A, "General Design Criteria for Nuclear Power sures comparable to the guideline exposures of 10

Plants," to 10 CFR Part 50, "Licensing of Produc CFR Part 100, "Reactor Site Criteria," should be tion and Utilization Facilities," requires, in part, that protected against DBT effects to prevent such failure.

structures, systems, and components important to This provision by itself, however, would not provide safety be designed to withstand the effects of natural protection for certain other structures, systems, and phenomena such as tornadoes without loss of capabil ity to perform their safety functions. Criterion 2 also components which could be damaged by a less se vere, but more likely, tornado. In order to ensure pro requires that the design bases for these structures, tection for more probable events having less severe systems, and components reflect (1) appropriate consequences, the selection of structures, systems, combinations of the effects of normal and accident and components to be protected against the effects of conditions with the effects of natural phenomena and

(2) the importance of the. safety functions to be per a DBT is based on not allowing offsite exposures to formed. exceed an appropriate fraction of 10 CFR Part 100

guidelines.

This guide describes a method acceptable to the Protection of designated structures, systems, and NRC staff for identifying those structures, systems, components may generally be accomplished by de and components of light-water-cooled reactors that signing protective barriers to preclude tornado dam should be protected from the effects of the Design age. For example, the primary containment, reactor Basis Tornado (see Regulatory Guide 1.76, "Design building, auxiliary building, and control structures Basis Tornado for Nuclear Power Plants"), including should be designed against collapse and should pro tornado missiles, and remain functional. The Advis vide an adequate barrier against missiles. However, ory Committee on Reactor Safeguards has been con the primary containment need not necessarily main sulted concerning this guide and has concurred in the tain its leaktight integrity. If protective barriers are regulatory position. not installed, the structures and components them selves should be designed to withstand the effects of

B. DISCUSSION

the tornado, including tornado missile strikes. The Nuclear power plants should be protected from the physical separation of redundant or alternative struc effects of tornado strikes. The likelihood of a credi tures or components required for the safe shutdown ble tornado strike varies from about 10'- per year to of the plant is generally not considered acceptable by values several orders of magnitude higher. Physical itself for protecting against tornado effects, including design parameters of tornado protection provisions tornado-generated missiles. This is because of the are such that designated structures, systems, and large number and random direction of potential mis components will be able to maintain their necessary siles that could result from a tornado as well as the capabilities in the event of a Design Basis Tornado need to consider the single failure criterion.

(DBT), as defined in Regulatory Guide 1.76. This It is not necessary to maintain the functional capa ensures that protection of the designated items bility of all Seismic Category I structures, systems, against all credible tornadoes has been adequately and components because the probability of the joint

-Lines indicate substantive changes from previous issue. occurrence of low-probability events (loss-of-coolant USNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission, US. Nuclear Regu latory Commission. Washington, D.C. 20555, Attention Docketing and Service Regulatory Guides are issued to describe and make available to the public methods Branch.

acceptable to the NRC staff of implementing specific parts of the Commission's regulations. to delineate techniques used by the staff in evaluating specific problems The guides are issued in the following ten broad divisions or poslulated accidents, or to provide guidance to applicants. Regulatory Guides are not substitutes for regulations, and compliance with them is not required 1. Power Reactors 6. Products Methods and solutions different from those set out in the guides will be accept 2. Research and Test Reactors 7. Traosporlation able if they provide a basis for the findings requisite to the issuance or continuance 3 Fuels and Materials Facilities 8. Occupational Health of a permit or license by the Commitsion. 4 Environmental and Siting

9. Antitrust

5. Materials and Plant Protection 10. General Review Comments and suggestions for improvements in these guides are encouraged at all Requests for single copies of issued guides (which may be reproduced) or for place times, and guides will be revised, as appropriate. to accommodate comments and ment on an automatic distribution list for single copies of future guides in specific to reflect new information or experience. This guide was revised as a result of divisions should be made in writing to the US. Nuclear Regulatory Commission.

substantive comments received 'from the public and additional staff review. Washington. D.C. 20555, Attention Director. Division of Document Control.

accident with DBT or smaller tornado, or earthquake The appendix to this guide lists structures, sys with DBT or smaller tornado) is sufficiently small. tems, and components, which together with their However, equipment used to provide long-term core foundations and supports, should be protected from cooling following a LOCA should be protected, the effects of a DBT (see Regulatory Guide 1.76),

including tornado missiles, without loss of capability Similarly, it is not generally necessary to protect to perform their safety functions. Those structures, the radioactive waste systems since, even in the event systems, and components that should be protected of gross failure, offsite exposures would remain well may require reevaluation for designs that differ sub below the guideline exposures of I0 CFR Part 100 stantially from those now in use.

because of the limited inventory allowed in these systems.

D. IMPLEMENTATION

Structures, systems, and components important to safety that should be protected from the effects of a The purpose of this section is to provide informa Design Basis Tornado are: tion to applicants regarding the NRC staff's plans for using this regulatory guide.

1. Those necessary to ensure the integrity of the reactor coolant pressure boundary; Except in those cases in which the applicant pro poses an acceptable alternative method for complying

2. Those necessary to ensure the capability to shut with specified portions of the Commission's regula down the reactor and maintain it in a safe shutdown tions, the method described herein will be used in the condition (this includes both hot standby and cold evaluation of construction permit applications doc shutdown capability); and keted after May 30, 1978.

3. Those whose failure could lead to radioactive If an applicant wishes to use this regulatory guide release.3 resulting in calculated offsite exposures in developing submittals for applications docketed on greater than 25% of the guideline exposures of 10 or before May 30, 1978, the pertinent portions of the CFR Part 100 using appropriately conservative ana application will be evaluated on the basis of this lytical methods and assumptions. guide.

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APPENDIX

STRUCTURES, SYSTEMS, AND COMPONENTS OF LIGHT-WATER-COOLED

REACTORS TO BE PROTECTED AGAINST TORNADOES

I. The reactor coolant pressure boundary. 1 could result in potential offsite exposures in excess of the criterion given in subitem (3) of the regulatory

2. Those portions of the main steam and main position.

feedwater systems 2 in PWRs up to and including the outermost isolation valves. 9. Systems or portions of systems that are required for monitoring, actuating, and operating tornado

3. The reactor core and individual fuel assemblies, protected portions of systems listed in items 4, 6,.7, at all times, including during refueling. and 13.

4. Systems or portions of systems that are required 10. All electric and mechanical devices and cir for (1) attaining safe shutdown, (2) residual heat re cuitry between the process sensors and the input ter moval, (3) cooling the spent fuel storage pool, (4) minals of the actuator systems involved in generating mitigating the consequences of a tornado-caused signals that initiate protective actions by tornado PWR steam line break, 3 (5) makeup water for the protected portions of systems listed in items 4, 6, 7, primary system, and (6) supporting the above sys and 13.

tems, e.g., cooling water, ultimate heat sink, air sup ply, auxiliary feedwater, and ventilation. 11. Those portions of the long-term emergency core cooling system that would be required to main

5. The spent fuel storage pool, to the extent neces tain the plant in a safe condition for an extended time sary to preclude significant loss of watertight integ after a loss-of-coolant accident.

rity of the storage pool and to prevent missiles from contacting fuel within the pool. 12. Primary reactor containment and other safety related structures, such as the control room building

6. The reactivity control systems, e.g., control rod and auxiliary building, to the extent that they not col drives and boron injection system. lapse, allow perforation by missiles, or generation of secondary missiles, any of which could cause unac

7. The control room, including all equipment ceptable damage to tornado-protected items. How needed to maintain the control room within safe ever, the primary containment need not necessarily habitability limits for personnel and safe environmen maintain its leaktight integrity.

tal limits for tornado-protected equipment.

13. The Class 1E electric systems, including the

8. Those portions of the gaseous radwaste treat auxiliary systems for the onsite electric power ment system whose failure due to tornado effects supplies, that provide the emergency electric power needed for the functioning of plant features included in items 1 through 11 above.

SAs defined in §50.2 of 10 CFR Part 50.

2 The system boundary includes those portions of the system re 14. Those portions of structures, systems, and quired to accomplish the specified safety function and connecting components whose continued function is not required piping up to and including the first valve (including a safety or but whose failure could reduce to an unacceptable relief valve) this is either normally closed or capable of automatic closure when the safe function is required. safety level the functional capability of any plant fea Alternatively, the main steam system, up io and including a sec tures included in items I through 13 above or could ond isolation valve such as a redundant series MSIV, qr a turbine result in incapacitating injury to occupants of the con stop valve, may be protected. trol room.

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