Regulatory Guide 1.115

Rev 1 Protection Against Low-Trajectory Turbine Missiles

ML003739456
Person / Time
Issue date:	07/31/1977
From:	Office of Nuclear Regulatory Research
To:
References
RG-1.115, Rev 1
Download: ML003739456 (4)

U.S. NUCLEAR REGULATORY COMMISSION

Revisom 1 July 1977 x REGULATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.115 PROTECTION AGAINST LOW-TRAJECTORY TURBINE MISSILES

A.

INTRODUCTION

General Design Criterion 4, "Environmental and Missile Design Bases," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10

CFR Part 50, "Licensing of Production and Utiliza tion Facilities," requires, in part, that structures, systems, and components important to safety be ap propriately protected, against the effects of missiles that might result from equipment failures. Failures that could occur in the large steam turbines of the main turbine-generator sets have the potential for producing large high-energy missiles. This guide describes methods acceptable to the NRC staff for protecting safety-related structures, systems, and components against low-trajectory missiles resulting from turbine failure by appropriate orientation and placement of the turbine-generator set. The Advisory Committee on Reactor Safeguards has been con sulted concerning this guide and has concurred in the regulatory position.

B.

DISCUSSION

Although there is little information available on failures of large turbines, cumulative failure data based on operating history for conventional plants'

indicate that the protection of safety-related portions of nuclear power plants from turbine missiles is an appropriate safety consideration. The two broad categories of turbine failures are usually referred to as design overspeed failures and destructive overspeed failures. Missiles resulting from design overspeed failures are the result of brittle fracture of turbine blade wheels or portions of the turbine rotor itself.

Failures of this type Ican occur dduring startup or nor

• • Lines indicate substantive changes from previous issue.

'Bush, S. H., "Probability of Damage to Nuclear Components,"

Nuclear Safety, Vol. 14, No. 3, May-June, 1973.

mal operation. Missiles resulting from destructive

-overspeed -failures would be generated7-iftie overspeed protection system malfunctions and the turbine speed increases to a point at which the low pressure wheels or rotor will undergo ductile failure.

The kinetic energy of ejected missiles can b'e sufficient to damage even substantial reinforced concrete slabs and panels. Thus turbine missiles have the potential for damaging safety-related structures, systems, and components of the plant.

Missiles from a turbine failure can be divided into two groups: "high-trajectory" missiles, which are ejected upward through the turbine casing and may cause damage if the falling missile strikes an essential system and "low-trajectory" or "direct" missiles, which are ejected from the turbine casing directly toward an essential system. This guide outlines accep table methods of protection against low-trajectory turbine missiles.

Consideration of turbine missile protection is rele vant for essential systems, i.e., those structures, systems, and components necessary to ensure:

1. The integrity of the reactor coolant pressure boundary,

2. The capability to shut down the reactor and maintain it in a cold shutdown condition, or

3. The capability to prevent accidents that could result in potential offsite exposures that are a signifi cant fraction of the guideline exposures of 10 CFR

Part 100, "Reactor Site Criteria."

The potential consequences of turbine missiles in clude direct effects (e.g., damage to the spent fuel storage pool) as well as indirect effects (e.g., impair ment of vital control room functions). In either case, 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 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 it orolicens by t ommissin.

4. Environmental and Siting

9. Antitrust Review apermit or liense by the Commission.

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 features ate provided to prevent damage or because the probability of a strike by a turbine missile is suf ficiently low. Turbine orientation and placement, shielding, quality assurance in design and fabrication, inspection and testing programs, and overspeed protection systems are the principal means of safeguarding against turbine missiles. The first of these, turbine orientation and placement, provides a high degree of confidence that low-trajectory missiles resulting from turbine failures will not damage essen tial systems.

The probability of damage by low-trajectory tur bine missiles is large enough to warrant design precautions in future plants. The historical failure data on conventional units indicate that an incidence rate of 10'

per turbine year is appropriate for material failures at speeds up to design overspeed

(120% to 130% of turbine operating speed). There is reason to believe that improvements in turbine design, particularly in materials selection, will reduce the design overspeed failure rate. However, an operating history of the length required to permit es timates of very low failure rates, even in the absence of any failures, has not been accumulated. This, and the recurrence of disc or rotor degradation due to other causes, leads the staff to conclude that without additional evidence, use of the historical failure rate is appropriate. Assurance of low failure rates can be enhanced by an inservice inspection program.

Tradeoffs between frequency and level of testing and improvements in reliability are currently under study by the NRC staff.

A more difficult protection problem is presented by runaway turbine failures that may result in turbine speeds of 180% to 190% prior to destructive failure of the turbine wheels or shaft. Again, historical data in dicate a destructive overspeed failure rate of about

10" per turbine year. The staff's view is, however, that significant reduction in the rate of destructive overspeed failures may be obtained by the applica tion of improved overspeed protection systems, redundant turbine steam valving, improved valve design, and frequent valve testing. The degree of credit for improved systems and procedures appears to be limited primarily by the reliability of turbine steam valving. Many of the destructive overspeed failures of recent years were caused by the failure of turbine steam valves to close and stop the flow of steam even though a trip signal was generated. A

definitive study of turbine valve failure modes is not available in the published literature, but the subje& is 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 trajectory turbine missile strikes will be concentrated within an area bounded by lines inclined at 25 degrees to the turbine wheel planes and passing through the end wheels of the low-pressure stages (see Figure 1).

This applies to the low-pressure :stage shrunk-on wheels of the 1800-rpm turbines generally used with light-water-cooled reactors. Essential systems within this area and close to the turbine axis are most vulnerable; those further removed from the turbine axis are less likely to be hit by a missile. Systems out side this area are not endangered by high-energy low trajectory missiles.

The staff has concluded that protecting essential systems by excluding them from the low-trajectory hazard zone has less associated uncertainty than other methods and thus is the preferred method of protection. However, plants with less favorable tur bine orientation have been found acceptable. The protection of an essential system within the low trajectory missile strike zone is considered adequate against low-trajectory turbine missiles if the system is either small enough or far enough removed from the turbine that the probability of its being struck by a turbine missile is less than 1NO-. If more than one es sential system is so located, the sum of the probabilities of their being struck should be less than

"iO(. This criterion is a conservative way to ensure that the hazard rate due to low-trajectory turbine missiles is less than 10- per year, which the NRC

staff considers an acceptable risk rate for the loss of an essential system from a single event. Combina tions of such measures as care in the placement of es sential systems, separation of redundant equipment, and special attention to turbine valve reliability have been shown, through -detailed strike and damage analyses, to have accomplished the objective of en suring a low risk of damage from turbine missiles.

Some degree of protection against low-trajectory turbine missiles may be provided by barriers. There is no body of experimental evidence on the impact ef fects of large irregularly shaped missiles similar to those observed in turbine failures on either steel or reinforced concrete structures. Considerable uncer tainty attends the current practice of using damage predictions based on ordnance data, particularly in the choice of an "effective impact area." However, conservative damage predictions can be made by us ing results of similar tests and conservative assump tions. Some recent data3 were motivated by protec tion against tornado missiles. For cases in which the impact was normal and the impact area known, there was good correlation3 between actual penetration dis tances into reinforced concrete and those predicted

2Ibid.

"3"Full-Scale Tornado-Missile Impact Tests," EPRI NP-148, Electric Power Research Institute, April 1976.

by'the Modified National Defense Research Council (NDRC) formula.' Predictions of backface scabbing due to missile impact were not as good. For metal structures, application of the Ballistic Research Laboratory (BRL) formula5 should give conservative results for large missiles.

If multiple barriers are counted on to protect essen tial systems, the protection is deemed adequate if the last barrier will stop the missile without generating secondary missiles that could damage any essential system. For calculating residual velocities after the missile has perforated a barrier, the following relationship is conservative:

2

2 V2 vr = (vi - vp)

where vr = residual missile velocity after perfora tion, Vi= incident missile velocity, and Vp = incident missile velocity required to just perforate the barrier, calculated by con servative use of penetration data.

This guide addresses only large missiles that might be ejected in the event of a turbine failure. The in herent protection provided in most plants (generally

1 1/2/ to 2 feet of reinforced concrete) ensures that minor missiles, which could be ejected in significant numbers and in widely scattered directions once the casing is breached, would not result in damage to es sential systems.

Since turbine missile hazards may arise from non nuclear as well as other nuclear units on the site, con sideration should be given to the placement of pre sent and, to the extent possible, future units on the site. It should be recognized that the placement of currently proposed plants may affect the future place ment of additional units.

C.

REGULATORY POSITION

1. Essential systems of a nuclear power plant should be protected against low-trajectory turbine missiles due to failure of main turbine-generator sets.

Consideration may be limited to the structures, systems, and components listed in the Appendix to Regulatory Guide 1.117, "Tornado Design Clas-.

sification." The effect of physical separation of redundant or alternative systems may also be con sidered. Each essential system and its location should be identified on dimensioned plan and elevation layout drawings.

"Kennedy, R. P., "A Review of Procedures for Analysis and Design of Concrete Structures to Resist Missile Impact Effects,"

Nuclear Engineering and Design, 1976.

"5"Fundamentals of Protective Design," TM-5-855-1, Department of the Army, July 1965.

1.115-3

2. Protection of essential systems or structures against direct strikes by low-trajectory turbine mis siles can be provided by appropriate placement and orientation of the turbine units. The protection of an essential system is acceptable if the system and any protecting structure are located outside the low trajectory missile strike zones, which are defined by

+/-25-degree lines emanating from the centers of the 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 be considered.

3. When protection of essential systems is provided by barriers, dimensioned plan and elevation layout drawings should include information on wall or slab thicknesses and materials of pertinent struc tures. The protection is considered acceptable if no missile can compromise the final barrier protecting an essential system. Steel barriers should be thick enough to prevent perforation. Concrete barriers should be thick enough to prevent backface scab bing.

4. The protection of essential systems located within the low-trajectory missile strike zone is accep UNITED STATES

NUCLEAR REGULATORY COMMISSION

WASHINGTON, D. C. 20555 OFFICIAL BUSINESS

PENALTY FOR PRIVATE USE, $300

table if, in the event of a turbine failure, the probability of damage summed over all such systems I

is less than, 10.3

5. Turbine designs significantly different from cur rent 1800-rpm machines will be reviewed on a case by-case basis to determine the applicability of the

'strike zone.

D.

IMPLEMENTATION

The purpose of this section is to provide informa tion to applicants regarding the NRC staff's plans for using this regulatory guide.

This guide reflects current NRC staff practice.

Therefore, except in those cases in which the appli cant proposes an acceptable alternative method for complying with specified portions of the Commis sion's regulations, the method described herein is be ing and will continue to be used in the evaluation of submittals in connection with operating license or construction permit applications until this guide is revised as a result of suggestions from the public or additional staff review.

POSTAGE AND FEES PAID

U.S. NUCLEAR REGULATORY

i COMMISSION

I