Regulatory Guide 1.91
| ML003740286 | |
| Person / Time | |
|---|---|
| Issue date: | 02/28/1978 |
| From: | Office of Nuclear Regulatory Research |
| To: | |
| References | |
| RG-1.91, Rev 1 | |
| Download: ML003740286 (6) | |
U.S. NUCLEAR REGULATORY COMMISSION
Revision 1 February 1978
,'REGULATORY
GUIDE
OFFICE OF STANDARDS DEVELOPMENT
REGULATORY GUIDE 1.91 EVALUATIONS OF EXPLOSIONS POSTULATED TO OCCUR
ON TRANSPORTATION ROUTES NEAR NUCLEAR POWER PLANTS
A. INTRODUCTION
General Design Criterion 4, "Environmental and Missile Design Basis," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10
CFR Part 50, "Licensing of Production and Utiliza tion Facilities," requires that nuclear power plant structures, systems, and components important to safety be appropriately protected against dynamic ef fects resulting from equipment failures that may occur within the nuclear power plant as well as events and conditions that may occur outside the nuclear power plant. These latter events include the effects of ex plosion of hazardous materials that may be carried on nearby transportation routes. This guide describes methods acceptable to the NRC staff for determining whether the risk of damage due to an explosion on a nearby transportation route is sufficiently high to warrant a detailed investigation. Acceptable methods for evaluating structural adequacy when an investiga tion is warranted are also describe
d. This guide is
-
limited to solid explosives and hydrocarbons liquified under pressure and is not applicable to cryogenically liquified hydrocarbons, e.g., LNG. It considers the effects of airblasts on highway, rail, and water routes but excludes pipelines and fixed facilities.
B. DISCUSSION
In order to meet General Design Criterion 2, "De sign Basis for Protection Against Natural Phenomena," of Appendix A to 10 CFR Part 50 with respect to tornadoes, the structures, systems, and components important to safety of a nuclear power plant must be designed to withstand the effects of a design basis tornado, including wind, pressure drop, and the effects of missiles, without causing an acci dent and without damage that would prevent a safe and orderly shutdown. In addition, those structures, systems, and components must be designed to ac commodate the vibratory ground motion associated with the Safe Shutdown Earthquake.
The effects of explosives that are of concern in analyzing structural response to blast are incident or reflected pressure (overpressure), dynamic (drag)
pressure, blast-induced ground motion, and blast generated missiles. It is the judgment of the NRC
staff that, for explosions of the magnitude considered in this guide and the structures, systems, and compo nents that must be protected, overpressure effects are controlling. Drag pressure effects will be much smaller than those due to the wind loading assumed for the design basis tornado. The effects of blast generated missiles will be less than those associated with the blast overpressure levels considered in this guide. If the overpressure criteria of this guide are exceeded, the effects of missiles must be considered.
The effects of blast-induced ground motion at the overpressure levels considered in this guide will be less than those of the vibratory ground motion as sociated with the Safe Shutdown Earthquake.
This regulatory guide describes a method for de termining distances from critical plant structures to a railway, highway, or navigable waterway beyond which any explosion that might occur on these trans portation routes is not likely to have an adverse effect on plant operation or to prevent a safe shutdown.
Under these conditions, a detailed review of the transport of explosives on these transportation routes would not be required.
A method for establishing the distances referred to above can be based on a level of peak positive inci dent overpressure (designated as Ps, in Ref. 1) below which no significant damage would be expected. It is USNRC REGULATORY GUIDES
Comments should be sent to the Secretary of the Commission, US. Nuclear Regu.
Regulatory Guides are issued to describe and make available to the public methods latory Commission, Washington, D.C.
20555. Attention Docketing and Service acceptable to the NRC staff of implementing specific parts of the Commission's Branch.
regulations, to delineate techniques used by the staff in evaluating specific problems The guides are issued in the following ten broad divisions or postulated 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. Transportation 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 Commission.
4. Environmental andSiting
9. Antitrust Review
5. Materials and Plant Protection
10. General Comments and suggestions for improvements in these guides are encouraged at all times, and guides will be revised, as appropriate, to accommodate comments and Requests for single copies of issued guides (whch may be reproduced) or for place to reflect new information or experience.
However, comments on this guide,if ment on an automatic distribution list for single copies of fut-ire guide% in specilic received within about two months after its issuance, will be particularly useful in divisions should be made in writing to the U.S. Nuclear Regulatory Commission, evaluating the need for an early revision.
Washington, D.C.
20555. Attention.
Director. Division of Document Control
the judgment of the NRC staff that, for the structures, systems, and components of concern, this level can be conservatively chosen at 1 psi (approximately 7 kPa). Based on experimental data on hemispherical charges of TNT cited in Reference 1, a safe distance can then be conservatively defined by the relationship R > kW/
3 where R is the distance in feet from an exploding charge of W pounds of TNT. When R is in feet and W in pounds, k = 45. When R is in meters and W in kilograms, k = 18.
The concept of TNT equivalence, i.e., finding the mass of substance in question that will produce the same blast effect as a unit mass of TNT, has long been used in establishing safe separation distances for solid explosives. A test program is required to estab lish that equivalence (Ref. 2). For solid substances more efficient in producing blast effects than TNT,
equivalents are known by the manufacturers. For solid substances not intended for use as explosives but subject to accidental detonation, it is conservative
- o use a TNT equivalence of one in establishing safe
- tandoff distances, i.e.. use the cargo mass in Equa ion (1).
Application of the TNT equivalence concept to
)ossible detonations of vapor clouds formed after an iccidental release of hydrocarbons is not as well Jocumented. However, investigations of accidents
- hat resulted in blast damage have used this concept n attempts to estimate, based on blast damage, the zffective yield of the explosion (Ref. 3). Most as sessments of this type have led to estimates that less than one percent of the calorific energy of the substance was released in blast effects. Since the ratios of heat of combustion of hydrocarbons to that of TNT are typically about 10, this corresponds to an equivalence on a mass basis of 10 percent. However, there have been accidents in which estimates of the calorific energy released were as high as 10 percent.
The blast energy realized depends, in great measure, on phenomena that are accident specific, i.e., the rate of release of the substance and the way in which the cloud is ignited. A reasonable upper bound to the blast energy potentially available based on experi mental detonations of confined vapor clouds is a mass equivalence of 240 percent (Ref. 4). A detailed analysis of possible accident scenarios for particular sites, including consideration of the actual cargo, site topography, and prevailing meteorological conditions may justify a lower effective yield. But, when estab lishing safe stand-off distances independent of site conditions, use of an upper bound is prudent.
Determination of the maximum probable quantity of hazardous cargo is dependent on both the transpor tation mode and the vehicles utilized. The maximum probable hazardous solid cargo for a single highway truck is 50,000 pounds (23,000 kg). Similarly, the maximum explosive cargo in a single railroad box car is approximately 132,000 pounds (60,000 kg). The largest probable quantity of explosive material trans ported by ship is approximately 10,000,000 pounds
(4,500,000 kg). For illustrative purposes, the safe distances, as defined by inequality (1), are shown in Figure 1 for these quantities of TNT. When ship ments are made in connected vehicles such as rail road cars or barge trains, an investigation of the pos sibility of explosion of the contents of more than one vehicle is necessary.
In cases where the distances from the transporta tion route to the structures, systems, and components that must be protected are not sufficiently great to allow a conclusion (based on conservative assump tions) that the peak positive incident overpressure would be less than I psi (approximately 7 kPa), an analysis of the frequency of hazardous cargo ship ment may show that the attendant risk is sufficiently low. It is the judgment of the NRC staff that, if the exposure rate, r, defined in Equation (2) can be shown to be less than 10r per year, the risk of dam age due to explosions is sufficiently low.
r = nfs where r = exposure rate,
(2)
n = explosion rate for the substance and transportation mode in question in explosions per mile, f = frequency of shipment for the sub stance in question in shipments per year, and s = exposure distance in miles (see Fig ure 2).
If the substance in question is shipped on more than one transportation mode near the plant, exposure rates calculated for the modes should be summed.
If an adequate data base for estimating the explo sion rate for a substance is lacking, an estimate can be made by utilizing nationwide statistics for the par ticular transportation mode, i.e.,
n = nln 2
(3)
where ni = accidents per mile for the transporta tion mode, and n2 = cargo explosions per accident for the transportation mode.
Because of the low frequency of occurrence of the events under consideration, estimates based on aver-
1.91-2 Ii
age frequency may have wide confidence bands, and conservative estimates may be preferred. If estimates of explosion rate, frequency of shipment, and expo
~
sure distance are made on a realistic or best estimate basis, an exposure rate less than 10' per year is suf ficiently low. If conservative estimates are used, an exposure rate less than 10-6 per year is sufficiently low.
If it cannot be shown that the distance to the trans portation route is great enough or that the exposure rate is low enough to render sufficiently low the risk of damage to a structure housing a system or compo nent that must be protected, an analysis of the blast load effects may be made. The loading combination to be considered may be limited to:
C =D + L + T, + R, + B
(4)
where C = combined load effect, D
-
dead load effect, L
- live load effect (not including wind or snow loads),
T,, = thermal load effect during normal operating or shutdown conditions, R,, = pipe reaction effect during normal operating or shutdown conditions, and B = blast load effect, with the explosion source positioned to maximize the load combination for the structural element under consideration. Only the incident (or, if appropriate, re flected) pressure loading need be considered.
Either a static analysis using twice the appropriate pressure loading or an elastic analysis using dynamic load factors (Ref. 5) is acceptable for computing blast load effects. The blast pressure should be considered to act both inward and outward in order to account for dynamic stress reversal. Overturning and sliding stability as well as the ability of supporting structures to carry loads transmitted from the directly loaded ex terior surfaces must be assessed.
C. REGULATORY POSITION
In the design of nuclear power plants, the ability to withstand the possible effects of explosions occurring on nearby transportation routes should be considered.
The following methods are acceptable to the NRC
staff for ensuring that the risk of damage due to an explosion on a nearby transportation route is suffi ciently low.
I. When carriers that transport explosives can approach vital structures of a nuclear facility no closer than the distances computed using Figure 1, no further consideration need be given to the effects of blast in plant design. In calculating TNT equivalents, assumptions of 100 percent TNT (mass) equivalence for solid energetic materials and 240 percent TNT
(mass) equivalence for substances subject to vapor phase explosions are acceptable upper bounds when effective yields generated from test data do not exist.
Lower effective yields may be justified by analyses accounting for reacti.on kinetics, site topography, and prevailing meteorological conditions when the hazardous cargos can be identified.
2. If transportation routes are closer to structures and systems important to safety than the distances computed using Figure 1, the applicant may show that the risk is acceptably low on the basis of low probability of explosions. A demonstration that the rate of exposure to a peak positive incident overpres sure in excess of 1 psi (7 kPa) is less than 10-6 per year, when based on conservative assumptions, or
10-7 per year, when based on realistic assumptions, is acceptable. Due consideration should be given to the comparability of conditions on the route to those of the accident data base.
3. If transportation routes are closer to structures and systems important to safety than the distances computed using Figure 1, the applicant may show that the risk to the public is acceptably low on the basis of capability of the safety-related structures to withstand blast and missile effects associated with de tonation of the hazardous cargo. In assessing the capacity of structures to resist blast loads, a simplified quasi-static analysis of blast effects using the load combination of Equation (4) is acceptable.
Effective yields based on analyses accounting for reaction kinetics, site topography, and prevailing meteorological conditions can be used when justified.
D. IMPLEMENTATION
The purpose of this section is to provide guidance to applicants and licensees regarding the NRC staff's plans for utilizing this regulatory guide.
Except in those cases in which the applicant pro poses an alternative method for complying with spec ified portions of the Commission's regulations, the method described herein will be used in the evalua tion of construction permit applications docketed on or after February 24, 1978.
If an applicant wishes to use this regulatory guide in developing submittals for applications docketed on or before February 24, 1978, the pertinent portions of the application will be evaluated on the basis of this guide.
1.91-3
REFERENCES
1. Department of the Army Technical Manual TM
5-1300, "Structures to Resist the Effects of Acci dental Explosions," June 1969.
2. Napadensky, H. S., and L. Jablansky, "TNT
Equivalency Investigations,"
Proceedings of the
16th Annual Explosives Safety Seminar, Depart ment of Defense Explosives Safety Board, Wash ington, D.C., September 1974.
3. Strehlow, R. A., and W. E. Baker, "The Charac terization and Evaluation of Accidental Explo sions," NASA CR-134779, June 1975.
4. Eichler, T. V., and H. S. Napadensky, "Acciden tal Vapor Phase Explosions on Transportation Routes near Nuclear Power Plants," Final Report J 6405, IIT Research Institute, Chicago, Illinois, April 1977.
5. Biggs, J. M., "Introduction to Structural Dynamics," McGraw-Hill, New York, 1964.
1.91-4 I
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