Regulatory Guide 1.115

Rev 1 Protection Against Low-Trajectory Turbine Missiles

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Issue date:	07/31/1977
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RG-1.115, Rev 1
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Revisom 1 U.S. NUCLEAR REGULATORY COMMISSION July 1977 x REGULATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.115 PROTECTION AGAINST LOW-TRAJECTORY TURBINE MISSILES

A. INTRODUCTION

• mal operation. Missiles resulting from destructive

-overspeed -failures would be generated7-iftie General Design Criterion 4, "Environmental and overspeed protection system malfunctions and the Missile Design Bases," of Appendix A, "General turbine speed increases to a point at which the low Design Criteria for Nuclear Power Plants," to 10 pressure wheels or rotor will undergo ductile failure.

CFR Part 50, "Licensing of Production and Utiliza The kinetic energy of ejected missiles can b'e sufficient tion Facilities," requires, in part, that structures, to damage even substantial reinforced concrete slabs systems, and components important to safety be ap and panels. Thus turbine missiles have the potential propriately protected, against the effects of missiles for damaging safety-related structures, systems, and that might result from equipment failures. Failures components of the plant.

that could occur in the large steam turbines of the main turbine-generator sets have the potential for Missiles from a turbine failure can be divided into producing large high-energy missiles. This guide two groups: "high-trajectory" missiles, which are describes methods acceptable to the NRC staff for ejected upward through the turbine casing and may protecting safety-related structures, systems, and cause damage if the falling missile strikes an essential components against low-trajectory missiles resulting system and "low-trajectory" or "direct" missiles, from turbine failure by appropriate orientation and which are ejected from the turbine casing directly placement of the turbine-generator set. The Advisory toward an essential system. This guide outlines accep Committee on Reactor Safeguards has been con table methods of protection against low-trajectory sulted concerning this guide and has concurred in the turbine missiles.

regulatory position.

Consideration of turbine missile protection is rele

B. DISCUSSION

vant for essential systems, i.e., those structures, systems, and components necessary to ensure:

Although there is little information available on failures of large turbines, cumulative failure data 1. The integrity of the reactor coolant pressure based on operating history for conventional plants' boundary, indicate that the protection of safety-related portions of nuclear power plants from turbine missiles is an 2. The capability to shut down the reactor and appropriate safety consideration. The two broad maintain it in a cold shutdown condition, or categories of turbine failures are usually referred to as design overspeed failures and destructive overspeed 3. The capability to prevent accidents that could failures. Missiles resulting from design overspeed result in potential offsite exposures that are a signifi failures are the result of brittle fracture of turbine cant fraction of the guideline exposures of 10 CFR

blade wheels or portions of the turbine rotor itself. Part 100, "Reactor Site Criteria."

Failures of this type Ican occur dduring startup or nor The potential consequences of turbine missiles in clude direct effects (e.g., damage to the spent fuel

• • Linesindicate substantive changes from previous issue. storage pool) as well as indirect effects (e.g., impair

'Bush, S. H., "Probability of Damage to Nuclear Components," ment of vital control room functions). In either case, Nuclear Safety, Vol. 14, No. 3, May-June, 1973. it is necessary to show that the risk from turbine mis- USNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission, U.S. Nuclear Regu latory Commission, Washington, D.C. 20555, Attention: Docketing and Service Regulatory Guidets 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 sOecific problems The guides are issued in th? 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 differ.ent from those 4bt out in the guides will be accept- 2. Research and Test Reactors

7. Transportation

3. Fuels and Materials Facilities 8. Occupational Health able if they provide a basis for the findings requisite to the issuance or continuance 9. Antitrust Review it or of a apermit orolicens t Commission.

by the ommissin. 4. Environmental and Siting liense by 5. Materials and Plant Protection 10. General 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.

siles is acceptably small, either because design additional evidence, use of the historical failure rate features ate provided to prevent damage or because is appropriate. Assurance of low failure rates can be the probability of a strike by a turbine missile is suf enhanced by an inservice inspection program.

ficiently low. Turbine orientation and placement, Tradeoffs between frequency and level of testing and shielding, quality assurance in design and fabrication, improvements in reliability are currently under study inspection and testing programs, and overspeed by the NRC staff.

protection systems are the principal means of safeguarding against turbine missiles. The first of these, turbine orientation and placement, provides a A more difficult protection problem is presented high degree of confidence that low-trajectory missiles by runaway turbine failures that may result in turbine resulting from turbine failures will not damage essen speeds of 180% to 190% prior to destructive failure of tial systems. the turbine wheels or shaft. Again, historical data in dicate a destructive overspeed failure rate of about The probability of damage by low-trajectory tur 10" per turbine year. The staff's view is, however, bine missiles is large enough to warrant design that significant reduction in the rate of destructive precautions in future plants. The historical failure overspeed failures may be obtained by the applica data on conventional units indicate that an incidence tion of improved overspeed protection systems, rate of 10' per turbine year is appropriate for redundant turbine steam valving, improved valve material failures at speeds up to design overspeed design, and frequent valve testin

g. The degree of

(120% to 130% of turbine operating speed). There is credit for improved systems and procedures appears reason to believe that improvements in turbine to be limited primarily by the reliability of turbine design, particularly in materials selection, will reduce steam valving. Many of the destructive overspeed the design overspeed failure rate. However, an failures of recent years were caused by the failure of operating history of the length required to permit es turbine steam valves to close and stop the flow of timates of very low failure rates, even in the absence steam even though a trip signal was generated. A

of any failures, has not been accumulated. This, and definitive study of turbine valve failure modes is not the recurrence of disc or rotor degradation due to available in the published literature, but the subje& is other causes, leads the staff to conclude that without currently being investigated by the NRC staff.

I~

Figure 1 Low-Trajectory Turbine Missile Strike Zone

1.115-2 I I

Evidence currently available' indicates that low by'the Modified National Defense Research Council trajectory turbine missile strikes will be concentrated (NDRC) formula.' Predictions of backface scabbing within an area bounded by lines inclined at 25 degrees due to missile impact were not as good. For metal to the turbine wheel planes and passing through the structures, application of the Ballistic Research end wheels of the low-pressure stages (see Figure 1). Laboratory (BRL) formula5 should give conservative This applies to the low-pressure :stage shrunk-on results for large missiles.

wheels of the 1800-rpm turbines generally used with light-water-cooled reactors. Essential systems within If multiple barriers are counted on to protect essen this area and close to the turbine axis are most tial systems, the protection is deemed adequate if the vulnerable; those further removed from the turbine last barrier will stop the missile without generating axis are less likely to be hit by a missile. Systems out secondary missiles that could damage any essential side this area are not endangered by high-energy low system. For calculating residual velocities after the trajectory missiles. missile has perforated a barrier, the following relationship is conservative:

The staff has concluded that protecting essential systems by excluding them from the low-trajectory vr = (vi

2

-

2 vp)

V2 hazard zone has less associated uncertainty than other methods and thus is the preferred method of where vr = residual missile velocity after perfora protection. However, plants with less favorable tur tion, bine orientation have been found acceptable. The protection of an essential system within the low Vi= incident missile velocity, and trajectory missile strike zone is considered adequate against low-trajectory turbine missiles if the system is Vp = incident missile velocity required to just either small enough or far enough removed from the perforate the barrier, calculated by con turbine that the probability of its being struck by a servative use of penetration data.

turbine missile is less than 1NO-. If more than one es sential system is so located, the sum of the This guide addresses only large missiles that might probabilities of their being struck should be less than be ejected in the event of a turbine failur

e. The in

"iO(. This criterion is a conservative way to ensure herent protection provided in most plants (generally that the hazard rate due to low-trajectory turbine 1 1/2/ to 2 feet of reinforced concrete) ensures that missiles is less than 10- per year, which the NRC minor missiles, which could be ejected in significant staff considers an acceptable risk rate for the loss of numbers and in widely scattered directions once the an essential system from a single event. Combina casing is breached, would not result in damage to es tions of such measures as care in the placement of es sential systems.

sential systems, separation of redundant equipment, Since turbine missile hazards may arise from non and special attention to turbine valve reliability have nuclear as well as other nuclear units on the site, con been shown, through -detailed strike and damage sideration should be given to the placement of pre analyses, to have accomplished the objective of en sent and, to the extent possible, future units on the suring a low risk of damage from turbine missiles. site. It should be recognized that the placement of currently proposed plants may affect the future place Some degree of protection against low-trajectory ment of additional units.

turbine missiles may be provided by barriers. There is no body of experimental evidence on the impact ef

C. REGULATORY POSITION

fects of large irregularly shaped missiles similar to those observed in turbine failures on either steel or 1. Essential systems of a nuclear power plant reinforced concrete structures. Considerable uncer should be protected against low-trajectory turbine tainty attends the current practice of using damage missiles due to failure of main turbine-generator sets.

predictions based on ordnance data, particularly in Consideration may be limited to the structures, the choice of an "effective impact area." However, systems, and components listed in the Appendix to conservative damage predictions can be made by us Regulatory Guide 1.117, "Tornado Design Clas-.

ing results of similar tests and conservative assump sification." The effect of physical separation of tions. Some recent data3 were motivated by protec redundant or alternative systems may also be con tion against tornado missiles. For cases in which the sidered. Each essential system and its location should impact was normal and the impact area known, there be identified on dimensioned plan and elevation was good correlation 3 between actual penetration dis layout drawings.

tances into reinforced concrete and those predicted

"Kennedy, R. P., "A Review of Procedures for Analysis and

2 Design of Concrete Structures to Resist Missile Impact Effects,"

Ibid. Nuclear Engineering and Design, 1976.

3

""Full-Scale Tornado-Missile Impact Tests," EPRI NP-148, "5"Fundamentals of Protective Design," TM-5-855-1, Department Electric Power Research Institute, April 1976. of the Army, July 1965.

1.115-3

2. Protection of essential systems or structures table if, in the event of a turbine failure, the against direct strikes by low-trajectory turbine mis probability of damage summed over all such systems I

siles can be provided by appropriate placement and is less than, 10.3 orientation of the turbine units. The protection of an essential system is acceptable if the system and any 5. Turbine designs significantly different from cur protecting structure are located outside the low rent 1800-rpm machines will be reviewed on a case trajectory missile strike zones, which are defined by by-case basis to determine the applicability of the

+/-25-degree lines emanating from the centers of the 'strike zone.

first and last low-pressure turbine wheels as measured from the plane of the wheels (see Figure 1). The strike zones associated with the turbines of all present and future nuclear and nonnuclear units at the site should

D. IMPLEMENTATION

be considered.

The purpose of this section is to provide informa

3. When protection of essential systems is tion to applicants regarding the NRC staff's plans for provided by barriers, dimensioned plan and elevation using this regulatory guide.

layout drawings should include information on wall or slab thicknesses and materials of pertinent struc This guide reflects current NRC staff practice.

tures. The protection is considered acceptable if no Therefore, except in those cases in which the appli missile can compromise the final barrier protecting cant proposes an acceptable alternative method for an essential system. Steel barriers should be thick complying with specified portions of the Commis enough to prevent perforation. Concrete barriers sion's regulations, the method described herein is be should be thick enough to prevent backface scab ing and will continue to be used in the evaluation of bing. submittals in connection with operating license or construction permit applications until this guide is

4. The protection of essential systems located revised as a result of suggestions from the public or within the low-trajectory missile strike zone is accep additional staff review.

I

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