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 =  
Line 10: Line 10:
| 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:Revisom 1 U.S. NUCLEAR REGULATORY COMMISSION                                                                             July 1977 x REGULATORY GUIDE
{{#Wiki_filter:U.S. NUCLEAR REGULATORY COMMISSION
                                        OFFICE OF STANDARDS DEVELOPMENT
March 1976 REGULATORY GUIDE
                                                                REGULATORY GUIDE 1.115 PROTECTION AGAINST LOW-TRAJECTORY TURBINE MISSILES
OFFICE OF STANDARDS DEVELOPMENT
REGULATORY GUIDE 1.115 PROTECTION AGAINST LOW-TRAJECTORY TURBINE MISSILES


==A. INTRODUCTION==
==A. INTRODUCTION==
*                  mal operation. Missiles resulting from destructive
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,
                                                                                      -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.
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.


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.
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
"direcet,* is es, w0ich are ejected from the turbine failures.


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.
cing 'd:  
I
c.il*toward an essential system. This guide outlites a'eptable methods of protection against low-  


regulatory position.
==B. DISCUSSION==
..
rtia ctoy turbine missiles.


Consideration of turbine missile protection is rele
Cumulative failure data based on turbine operti"
 
.
==B. DISCUSSION==
history for conventional plants' indicate that tlheprotec
vant for essential systems, i.e., those structures, systems, and components necessary to ensure:
*- Consideration of turbine missile protection is relevant tion of safety-related portions of nuclear poiA
    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
plant for those plant systems and components necessary to from turbine missiles is an appropriatee ik-ty sdc
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
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
**Linesindicate substantive changes from previous issue.                               storage pool) as well as indirect effects (e.g., impair
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,
'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.
*characte ze*.0nseýtiel, 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~ ve.*ive-* ;failures would be generated if the are the principal means of safeguarding against turbine oversptcr a
*otection 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.


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:
shutdown of the reactors on a site.
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===
USNRC REGULATORY GUIDES
                                                                                      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.
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.


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.
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.


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.
ticultaty useful m evaluating the noeed tot in early tevision
20566. Attention. Diecltor. Office of Standatds Development.


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.
t',:


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
The overall probability of damage by low-trajectory.


====g. The degree of====
turbine missiles is large enough to warrant design
(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
-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 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.
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.


I~
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.
                            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.
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.


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:
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.
    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
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 low.pressure :!:iges (see Figure I). This
2Ibid.


====e. The in====
I
"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.
Turbine Axis Figure 1 Low-Trajectory Turbine Missile Strike Zone
1.115-2


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.
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 vulnerabl


turbine missiles may be provided by barriers. There is no body of experimental evidence on the impact ef                       
====e. Those====
.urther removed Irom the turbine axis are less likely to be hit by a missile.


==C. REGULATORY POSITION==
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.
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-.
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.
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
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).
                                                            "Kennedy, R. P., "A Review of Procedures for Analysis and
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.
2                                                            Design of Concrete Structures to Resist Missile Impact Effects,"
  Ibid.                                                     Nuclear Engineering and Design, 1976.


3
==C. REGULATORY POSITION==
""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. 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.


1.115-3
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.


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
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).
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
The strike zones associated with the turbines of all present and future nuclear and nonnuclear units at the site should be considered.
:+/-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                 
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==
==D. IMPLEMENTATION==
be considered.
The purpose of this section is to provide information to applicants regarding the NRC staff's plans for using this regulatory guide.
 
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.
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.


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
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.
    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
1.115-3}}
            UNITED STATES
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Revision as of 00:17, 11 January 2025

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

"direcet,* is es, w0ich are ejected from the turbine failures.

cing 'd:

I

c.il*toward 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 ze*.0nseýtiel, 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~ ve.*ive-* ;failures would be generated if the are the principal means of safeguarding against turbine oversptcr a

  • otection 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 site.

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 low.pressure :!:iges (see Figure I). This

2Ibid.

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 vulnerabl

e. 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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