Regulatory Guide 1.115: Difference between revisions

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{{Adams
{{Adams
| number = ML003739456
| number = ML13350A282
| issue date = 07/31/1977
| issue date = 03/31/1976
| title = Rev 1 Protection Against Low-Trajectory Turbine Missiles
| title = Protection Against Low-Trajectory Turbine Missiles.
| author name =  
| author name =  
| author affiliation = NRC/RES
| author affiliation = NRC/OSD
| addressee name =  
| addressee name =  
| addressee affiliation =  
| addressee affiliation =  
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| license number =  
| license number =  
| contact person =  
| contact person =  
| document report number = RG-1.115, Rev 1
| document report number = RG-1.115
| document type = Regulatory Guide
| document type = Regulatory Guide
| page count = 4
| page count = 3
}}
}}
{{#Wiki_filter:U.S. NUCLEAR REGULATORY  
{{#Wiki_filter:U.S. NUCLEAR REGULATORY  
COMMISSION
COMMISSION
Revisom 1 July 1977 x REGULATORY  
March 1976 REGULATORY  
GUIDE OFFICE OF STANDARDS  
GUIDE OFFICE OF STANDARDS  
DEVELOPMENT  
DEVELOPMENT
REGULATORY  
REGULATORY  
GUIDE 1.115 PROTECTION  
GUIDE 1.115 PROTECTION  
Line 25: Line 25:


==A. INTRODUCTION==
==A. INTRODUCTION==
* General Design Criterion  
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.
4, "Environmental and speed, at which point the low-pressure wheels or rotor Missile Design Bases," of Appendix A, "General Design will undergo ductile failure. The kinetic energy of Criteria for Nuclear Power Plants," to 10 CFR Part 50, ejected missiles can be sufficient to cause penetration of"Licensing of Production and Utilization Facilities," several feet of reinforced concrete.


Failures that could occur in the large steam turbines of the main turbine-generator sets have the potential for producing large high-energy missiles.
ThitS, turhine missiles requires.


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.
in part. that structures.


==B. DISCUSSION==
systems, and compo- have the potential for damaging safety-related structures, nents of nuclear power plants important to safety be systems, and components of the Olant."ý:aC.
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.
appropriately protected against the effects of missiles that might result from equipment failures;
This guide describes a method acceptable to the NRC staff for Missiles from a turbie if'lhurd cani be divided into two protecting such components against low-trajectory mis- groups: "high-traj~iory" missiles, which are ejected siles resulting from turbine failure by appropriate orien- uppward through theturb'ine casing and may cause tation and placement of the turbine. Another guide is damage by filliig back:d6Wvn on an essential system (see under preparation with regard to protection against regulatory jýillbn C'l) and "low-trajectory" or high-trajcctory (lob shot) missiles resulting from turbine is es, w0ich are ejected from the turbine 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.
cing 'd: I
an essential system. This guide outlites a'eptable methods of protection against low-


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.
==B. DISCUSSION==
..rtia ctoy turbine missiles.Cumulative failure data based on turbine operti" .history for conventional plants' indicate that tlheprotec Consideration of turbine missile protection is relevant tion of safety-related portions of nuclear poiA plant for those plant systems and components necessary to from turbine missiles is an appropriatee ik-ty sdc .. i ft shut down a plant safely. The potential consequences of ation, although there is little informiion avaiab_'lI'on turbine nmssiles include direct effects (e.g., damage to failures of large turbines.


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.
The two biad cat ries of the spent fuel storage pool), as well as indirect effects turbine failures are usually referred tolfý design over- (e.g., impairment of vital control room functions).  
In speed failures and destructive overspeed failures.


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).
Missiles either case, it is necessary to show that the risk from resulting from design ovefpcLed failures are the result of turbine missiles is acceptably small, either because design brittle fracture of tw 6'ine. *Iade wheels, or portions of features are provided to prevent damage or because the the turbine rotor itse ( .-IM " or this type can occur probability of a strike by a turbine missile is sufficiently during startu wOirmi operation.
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.
Usually they are low. Turbine orientation and placement, shielding,*characte as occurring at about 120 quality assurance in design and fabrication, inspection to 130, e Cf no ,__al speed. Missiles resulting from and testing programs, and overspeed protection systems destru~
;failures would be generated if the are the principal means of safeguarding against turbine oversptcr a
system malfunctions and the tur- missiles.


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
The first of these, turbine orientation and bine speefl~iocreases to about 180 percent of normal placement, provides a high degree of confidence that low-trajectory missiles resulting from turbine failures Bush S. [i.. "Probability of Damnage to Nuclear Components," will not cause damage that would prevent a safe Nuclear Safety. Vol. 14, No. 3, May-June 1973. shutdown of the reactors on a sit
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.
====e. USNRC REGULATORY ====
GUIDES Ctimr,,ents should he sent to the Secrtairy of the Commission.


5. Materials and Plant Protection
U.S. Nuclear Regulatory Guides ate ised to describe and make avilabl to the public Regulatory Commissionr, Washington.
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:
D.C 205M. Attention Doclketing and matthods acceptable to the NRC stall Ot irrmplementing specific paris of the S rVicO Section.Commitsron,'
Director.
regu a ions, tO delineate techniques used by the iteft in evalu The guides are issued in the following ten broad divisions sting specitrc oroblems or postulated accidents.


Division of Document Control.
oat to provilde guidance to appli Catns. Regulatory Guides sae not Subslitutes for regulation$.
and compliance
1, Power Reactors 6 Products wIth them is not required Methods and solutions different from those set Out in 2. Research and Test Reactors 1 T1tSpornotion the guides will be acceptable of they opovide a basis for the findings requisite to 3 Fuels and Materials Facilities
9. Occupational Health the issuance or continuance of a permit or license by the Commission
4 Enviaonmenleand Siting 9 Antitrust Review Comments and suggestion%
for improvements in these guides ate encouraged S Matetials and Plant Protection
10. General at all times, dand guides wilt he revised, as apotootirate.


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.
to accommodate corn means and to reflect new atrmafio tr etpetlence However, comments on Copies of published guides may be obtained by written request indicatinq the this guide, it received within abioul two months abter its issuance.


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.
wil be poa divisions desired to the U S Nucleat Regulatory Commission.


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
Washington.
(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.
0 C.ticultaty useful m evaluating the noeed tot in early tevision 20566. Attention.


Assurance of low failure rates can be enhanced by an inservice inspection program.
Diecltor.


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.
Office of Standatds Development.


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
t',:
Evidence currently available'
The overall probability of damage by low-trajectory.
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;
turbine missiles is large enough to warrant design-precautions in future plants. The historical failure data on conventional units indicate that an incidence rate of about 10-4 per turbine year is appropriate for material failures at speeds up to design overspeed
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.
(120% to 130%of turbine operating speed). Although turbine manufac-turers feel that improved technology will reduce failure rates below those historically observed, the staff believes that these improvements are offset by factors associated wiLh increased turbine power output and the increased number of wheels (as many as 42 on some maclines)
in current turbines.


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.
Careful attention to turbine disk and rotor properties is therefore warranted to ensure that failure rates do not increase above historical levels.A more difficult protection problem is pteseited by runawvay turbine failures that may result in turbine speeds of 180% to 190% prior to destructive failure of the turbine wheels or shaft. Again, historical failure rates indicate that destructive overspeed failures could occur at the rate of about 10,4 per turbine year. The staffs view is, however, that significant reduction in the rate of destructive overspeed failures may be obtained by the application of improved overspeed protection systems, redundant turbine stcam 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.


However, plants with less favorable tur bine orientation have been found acceptable.
A definitive study of turbine valve failure modes is not available in the published literature.


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.
Past experience with turbine failures, as well as the laws of mechanics, indicates that turbine missiles are ejected primarily in a direction perpendicular to the turbine axis, i.e., within the plane of rotation of the failed turbine wheel. Thus targets aligned with the turbine shaft have a much reduced probability of being struck directly by turbine missiles.


Some degree of protection against low-trajectory turbine missiles may be provided by barriers.
On the basis of present information, the staff concludes that, in future nuclear power plants, all essential systems should be located outside the area most likely to sustain direct hits in the event of a turbine failure at destructive overspeed.


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.
Evidence currently available 2 indicates that low-trajectory turbhie missile strikes will be concentrated within an area bounded by lines inclined at 25 degrees to the turbine wheel planes and nassing through the. end wheels of the lo


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 data 3 were motivated by protec tion against tornado missiles.
====w. pressure ====
:!:iges (see Figure I). This 2 Ibid.I Turbine Axis Figure 1 Low-Trajectory Turbine Missile Strike Zone 1.115-2 applies to the low-pressure stage shrunk-on wheels of the 1800-rpm turbines generally used with light-water- cooled reactors.


For cases in which the impact was normal and the impact area known, there was good correlation
Essential systems within this area and close to the turbine axis arc most vulnerable.
3 between actual penetration dis tances into reinforced concrete and those predicted
2 Ibid.  " 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) formula 5 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:
Those.urther removed Irom the turbine axis are less likely to be hit by a missile.For essential systems within the low-trajectory missile strike zone, an acceptable basis for determining adequate safety against low-trajectory turbine missiles is that the system is either small enough or far enough removed from the turbine that its probability of being struck by a turbine missile is less than 10-3. This criterion is a conservative way to ensure that the hazard rate due to loW-trajectory turbine missiles is less than 10-7 per year.Computational methods reflecting the reduction in risk provided by intermediate barriers and the fact that a missile strike will not always lead to an event with radiological consequences are still in the development stage.This guide addresses only large missiles that might be ejected in the event of a turbine failure. The inherent protection provided in most plants (generally  
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)  
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 essential systems. Some attention should be directed to this problem, however, when turbine buildings themselves are relied on as barriers to missilvs (e.g., for control room areas).Since turbine missile hazards may arise from nonnu-clear as well as other nuclear units on the site, consideration should be given to the placement of present 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
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.
POSITION 1. Essential systems of a nuclear power plant should be protected against low-trajectory turbine missiles.


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
Foi the purposes of this guide, essential systems are defined as all plant structures and equipment for which damage by turbine missiles could lead to significant radiological consequences either by the direct release of radioactivity from the damaged system itself, e.g., spent fuel pools, or by failing in a manner that could lead to unacceptable conditions for other systems, e.g., emergency diesel generators.
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.
The control room should be included as an essential system.2. Each essential system and its location should be identified, and a physical 'description should be pro-vided. Dimensioned plan and elevation layout drawings and wall thicknesses and materials of pertinent stijc-tures should be included.3. Protection of essential systems or structures against direct strikes by low-trajectory turbine missiles can be provided by appropriate placement and orien-tation of the turbine units. The protection of an essential system is acceptable if the system is located outside the low-trajectory missile strike zones, which are defined hy +/-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.


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
4. The protection of an essential system located within the low-trajectory missile strike zone is accept-able if, in the ev.nt of a turbine failure, the probability of its being hit by such a missile is less than 10"3 5. Turbine designs significantly different from cur-rent 1800-rpm machines will be reviewed on a case-by-ease basis to determine the applicability of the strike zone,
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==
==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.
The purpose of this section is to provide information to applicants regarding the NRC staff's plans for using this regulatory guide.Except in those cases in which the applicant proposes an alternative method for complying with specified portions of the Commission's regulations, the method described herein will be used in the evaluation of submittals for construction permit applications docketed after November 15, 1976.If an applicant wishes to use this regulatory guide in developing submittals for applications docketed on or prior to November 15, 1976, the pertinent portions of the application will be evaluated on the basis of this guide.1.115-3}}
 
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}}


{{RG-Nav}}
{{RG-Nav}}

Revision as of 12:57, 17 September 2018

Protection Against Low-Trajectory Turbine Missiles.
ML13350A282
Person / Time
Issue date: 03/31/1976
From:
NRC/OSD
To:
References
RG-1.115
Download: ML13350A282 (3)
v • d • e


U.S. NUCLEAR REGULATORY

COMMISSION

March 1976 REGULATORY

GUIDE OFFICE OF STANDARDS

DEVELOPMENT

REGULATORY

GUIDE 1.115 PROTECTION

AGAINST LOW-TRAJECTORY

TURBINE MISSILES

A. INTRODUCTION

General Design Criterion 4, "Environmental and speed, at which point the low-pressure wheels or rotor Missile Design Bases," of Appendix A, "General Design will undergo ductile failure. The kinetic energy of Criteria for Nuclear Power Plants," to 10 CFR Part 50, ejected missiles can be sufficient to cause penetration of"Licensing of Production and Utilization Facilities," several feet of reinforced concrete.

ThitS, turhine missiles requires.

in part. that structures.

systems, and compo- have the potential for damaging safety-related structures, nents of nuclear power plants important to safety be systems, and components of the Olant."ý:aC.

appropriately protected against the effects of missiles that might result from equipment failures;

This guide describes a method acceptable to the NRC staff for Missiles from a turbie if'lhurd cani be divided into two protecting such components against low-trajectory mis- groups: "high-traj~iory" missiles, which are ejected siles resulting from turbine failure by appropriate orien- uppward through theturb'ine casing and may cause tation and placement of the turbine. Another guide is damage by filliig back:d6Wvn on an essential system (see under preparation with regard to protection against regulatory jýillbn C'l) and "low-trajectory" or high-trajcctory (lob shot) missiles resulting from turbine is es, w0ich are ejected from the turbine failures.

cing 'd: I

an essential system. This guide outlites a'eptable methods of protection against low-

B. DISCUSSION

..rtia ctoy turbine missiles.Cumulative failure data based on turbine operti" .history for conventional plants' indicate that tlheprotec Consideration of turbine missile protection is relevant tion of safety-related portions of nuclear poiA plant for those plant systems and components necessary to from turbine missiles is an appropriatee ik-ty sdc .. i ft shut down a plant safely. The potential consequences of ation, although there is little informiion avaiab_'lI'on turbine nmssiles include direct effects (e.g., damage to failures of large turbines.

The two biad cat ries of the spent fuel storage pool), as well as indirect effects turbine failures are usually referred tolfý design over- (e.g., impairment of vital control room functions).

In speed failures and destructive overspeed failures.

Missiles either case, it is necessary to show that the risk from resulting from design ovefpcLed failures are the result of turbine missiles is acceptably small, either because design brittle fracture of tw 6'ine. *Iade wheels, or portions of features are provided to prevent damage or because the the turbine rotor itse ( .-IM " or this type can occur probability of a strike by a turbine missile is sufficiently during startu wOirmi operation.

Usually they are low. Turbine orientation and placement, shielding,*characte as occurring at about 120 quality assurance in design and fabrication, inspection to 130, e Cf no ,__al speed. Missiles resulting from and testing programs, and overspeed protection systems destru~

failures would be generated if the are the principal means of safeguarding against turbine oversptcr a

system malfunctions and the tur- missiles.

The first of these, turbine orientation and bine speefl~iocreases to about 180 percent of normal placement, provides a high degree of confidence that low-trajectory missiles resulting from turbine failures Bush S. [i.. "Probability of Damnage to Nuclear Components," will not cause damage that would prevent a safe Nuclear Safety. Vol. 14, No. 3, May-June 1973. shutdown of the reactors on a sit

e. USNRC REGULATORY

GUIDES Ctimr,,ents should he sent to the Secrtairy of the Commission.

U.S. Nuclear Regulatory Guides ate ised to describe and make avilabl to the public Regulatory Commissionr, Washington.

D.C 205M. Attention Doclketing and matthods acceptable to the NRC stall Ot irrmplementing specific paris of the S rVicO Section.Commitsron,'

regu a ions, tO delineate techniques used by the iteft in evalu The guides are issued in the following ten broad divisions sting specitrc oroblems or postulated accidents.

oat to provilde guidance to appli Catns. Regulatory Guides sae not Subslitutes for regulation$.

and compliance

1, Power Reactors 6 Products wIth them is not required Methods and solutions different from those set Out in 2. Research and Test Reactors 1 T1tSpornotion the guides will be acceptable of they opovide a basis for the findings requisite to 3 Fuels and Materials Facilities

9. Occupational Health the issuance or continuance of a permit or license by the Commission

4 Enviaonmenleand Siting 9 Antitrust Review Comments and suggestion%

for improvements in these guides ate encouraged S Matetials and Plant Protection

10. General at all times, dand guides wilt he revised, as apotootirate.

to accommodate corn means and to reflect new atrmafio tr etpetlence However, comments on Copies of published guides may be obtained by written request indicatinq the this guide, it received within abioul two months abter its issuance.

wil be poa divisions desired to the U S Nucleat Regulatory Commission.

Washington.

0 C.ticultaty useful m evaluating the noeed tot in early tevision 20566. Attention.

Diecltor.

Office of Standatds Development.

t',:

The overall probability of damage by low-trajectory.

turbine missiles is large enough to warrant design-precautions in future plants. The historical failure data on conventional units indicate that an incidence rate of about 10-4 per turbine year is appropriate for material failures at speeds up to design overspeed

(120% to 130%of turbine operating speed). Although turbine manufac-turers feel that improved technology will reduce failure rates below those historically observed, the staff believes that these improvements are offset by factors associated wiLh increased turbine power output and the increased number of wheels (as many as 42 on some maclines)

in current turbines.

Careful attention to turbine disk and rotor properties is therefore warranted to ensure that failure rates do not increase above historical levels.A more difficult protection problem is pteseited by runawvay turbine failures that may result in turbine speeds of 180% to 190% prior to destructive failure of the turbine wheels or shaft. Again, historical failure rates indicate that destructive overspeed failures could occur at the rate of about 10,4 per turbine year. The staffs view is, however, that significant reduction in the rate of destructive overspeed failures may be obtained by the application of improved overspeed protection systems, redundant turbine stcam 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.

Past experience with turbine failures, as well as the laws of mechanics, indicates that turbine missiles are ejected primarily in a direction perpendicular to the turbine axis, i.e., within the plane of rotation of the failed turbine wheel. Thus targets aligned with the turbine shaft have a much reduced probability of being struck directly by turbine missiles.

On the basis of present information, the staff concludes that, in future nuclear power plants, all essential systems should be located outside the area most likely to sustain direct hits in the event of a turbine failure at destructive overspeed.

Evidence currently available 2 indicates that low-trajectory turbhie missile strikes will be concentrated within an area bounded by lines inclined at 25 degrees to the turbine wheel planes and nassing through the. end wheels of the lo

w. pressure

!:iges (see Figure I). This 2 Ibid.I Turbine Axis Figure 1 Low-Trajectory Turbine Missile Strike Zone 1.115-2 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 arc most vulnerable.

Those.urther removed Irom the turbine axis are less likely to be hit by a missile.For essential systems within the low-trajectory missile strike zone, an acceptable basis for determining adequate safety against low-trajectory turbine missiles is that the system is either small enough or far enough removed from the turbine that its probability of being struck by a turbine missile is less than 10-3. This criterion is a conservative way to ensure that the hazard rate due to loW-trajectory turbine missiles is less than 10-7 per year.Computational methods reflecting the reduction in risk provided by intermediate barriers and the fact that a missile strike will not always lead to an event with radiological consequences are still in the development stage.This guide addresses only large missiles that might be ejected in the event of a turbine failure. The inherent 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 essential systems. Some attention should be directed to this problem, however, when turbine buildings themselves are relied on as barriers to missilvs (e.g., for control room areas).Since turbine missile hazards may arise from nonnu-clear as well as other nuclear units on the site, consideration should be given to the placement of present 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.

Foi the purposes of this guide, essential systems are defined as all plant structures and equipment for which damage by turbine missiles could lead to significant radiological consequences either by the direct release of radioactivity from the damaged system itself, e.g., spent fuel pools, or by failing in a manner that could lead to unacceptable conditions for other systems, e.g., emergency diesel generators.

The control room should be included as an essential system.2. Each essential system and its location should be identified, and a physical 'description should be pro-vided. Dimensioned plan and elevation layout drawings and wall thicknesses and materials of pertinent stijc-tures should be included.3. Protection of essential systems or structures against direct strikes by low-trajectory turbine missiles can be provided by appropriate placement and orien-tation of the turbine units. The protection of an essential system is acceptable if the system is located outside the low-trajectory missile strike zones, which are defined hy +/-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.

4. The protection of an essential system located within the low-trajectory missile strike zone is accept-able if, in the ev.nt of a turbine failure, the probability of its being hit by such a missile is less than 10"3 5. Turbine designs significantly different from cur-rent 1800-rpm machines will be reviewed on a case-by-ease basis to determine the applicability of the strike zone,

D. IMPLEMENTATION

The purpose of this section is to provide information to applicants regarding the NRC staff's plans for using this regulatory guide.Except in those cases in which the applicant proposes an alternative method for complying with specified portions of the Commission's regulations, the method described herein will be used in the evaluation of submittals for construction permit applications docketed after November 15, 1976.If an applicant wishes to use this regulatory guide in developing submittals for applications docketed on or prior to November 15, 1976, the pertinent portions of the application will be evaluated on the basis of this guide.1.115-3


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