Regulatory Guide 1.115

Protection Against Low-Trajectory Turbine Missiles.

ML13350A282
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Issue date:	03/31/1976
From:	NRC/OSD
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RG-1.115
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U.S. NUCLEAR REGULATORY COMMISSIONMarch 1976REGULATORY GUIDEOFFICE OF STANDARDS DEVELOPMENTREGULATORY GUIDE 1.115PROTECTION AGAINST LOW-TRAJECTORY TURBINE MISSILESA. INTRODUCTIONGeneral Design Criterion 4, "Environmental and speed, at which point the low-pressure wheels or rotorMissile Design Bases," of Appendix A, "General Design will undergo ductile failure. The kinetic energy ofCriteria 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 missilesrequires. 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 missilesthat might result from equipment failures; This guidedescribes a method acceptable to the NRC staff for Missiles from a turbie if'lhurd cani be divided into twoprotecting such components against low-trajectory mis- groups: "high-traj~iory" missiles, which are ejectedsiles resulting from turbine failure by appropriate orien- uppward through theturb'ine casing and may causetation and placement of the turbine. Another guide is damage by filliig back:d6Wvn on an essential system (seeunder preparation with regard to protection against regulatory jýillbn C'l) and "low-trajectory" orhigh-trajcctory (lob shot) missiles resulting from turbine is es, w0ich are ejected from the turbinefailures. cing 'd: I an essential system. This guideoutlites 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 relevanttion of safety-related portions of nuclear poiA plant for those plant systems and components necessary tofrom turbine missiles is an appropriatee ik-ty sdc .. i ft shut down a plant safely. The potential consequences ofation, although there is little informiion avaiab_'lI'on turbine nmssiles include direct effects (e.g., damage tofailures of large turbines. The two biad cat ries of the spent fuel storage pool), as well as indirect effectsturbine failures are usually referred tolfý design over- (e.g., impairment of vital control room functions). Inspeed failures and destructive overspeed failures. Missiles either case, it is necessary to show that the risk fromresulting from design ovefpcLed failures are the result of turbine missiles is acceptably small, either because designbrittle fracture of tw6'ine. *Iade wheels, or portions of features are provided to prevent damage or because thethe turbine rotor itse ( .-IM " or this type can occur probability of a strike by a turbine missile is sufficientlyduring startu wOirmi operation. Usually they are low. Turbine orientation and placement, shielding,*characte as occurring at about 120 quality assurance in design and fabrication, inspectionto 130, e Cf no ,__al speed. Missiles resulting from and testing programs, and overspeed protection systemsdestru~ ;failures would be generated if the are the principal means of safeguarding against turbineoversptcr a system malfunctions and the tur- missiles. The first of these, turbine orientation andbine speefl~iocreases to about 180 percent of normal placement, provides a high degree of confidence thatlow-trajectory missiles resulting from turbine failuresBush S. [i.. "Probability of Damnage to Nuclear Components," will not cause damage that would prevent a safeNuclear 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. NuclearRegulatory Guides ate ised to describe and make avilabl to the public Regulatory Commissionr, Washington. D.C 205M. Attention Doclketing andmatthods 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 divisionssting specitrc oroblems or postulated accidents. oat to provilde guidance to appliCatns. Regulatory Guides sae not Subslitutes for regulation$. and compliance 1, Power Reactors 6 ProductswIth them is not required Methods and solutions different from those set Out in 2. Research and Test Reactors 1 T1tSpornotionthe guides will be acceptable of they opovide a basis for the findings requisite to 3 Fuels and Materials Facilities 9. Occupational Healththe issuance or continuance of a permit or license by the Commission 4 Enviaonmenleand Siting 9 Antitrust ReviewComments and suggestion% for improvements in these guides ate encouraged S Matetials and Plant Protection 10. Generalat all times, dand guides wilt he revised, as apotootirate. to accommodate cornmeans and to reflect new atrmafio tr etpetlence However, comments on Copies of published guides may be obtained by written request indicatinq thethis 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 dataon conventional units indicate that an incidence rate ofabout 10-4 per turbine year is appropriate for materialfailures at speeds up to design overspeed (120% to 130%of turbine operating speed). Although turbine manufac-turers feel that improved technology will reduce failurerates below those historically observed, the staff believesthat these improvements are offset by factors associatedwiLh increased turbine power output and the increasednumber of wheels (as many as 42 on some maclines) incurrent turbines. Careful attention to turbine disk androtor properties is therefore warranted to ensure thatfailure rates do not increase above historical levels.A more difficult protection problem is pteseited byrunawvay turbine failures that may result in turbinespeeds of 180% to 190% prior to destructive failure ofthe turbine wheels or shaft. Again, historical failure ratesindicate that destructive overspeed failures could occurat the rate of about 10,4 per turbine year. The staffsview is, however, that significant reduction in the rate ofdestructive overspeed failures may be obtained by theapplication of improved overspeed protection systems,redundant turbine stcam valving, improved valve design,and frequent valve testing. The degree of credit forimproved systems and procedures appears to be limitedprimarily by the reliability of turbine steam valving.Many of the destructive overspeed failures of recentyears were caused by the failure of turbine steam valvesto close and stop the flow of steam even though a tripsignal was generated. A definitive study of turbine valvefailure modes is not available in the published literature.Past experience with turbine failures, as well as thelaws of mechanics, indicates that turbine missiles areejected primarily in a direction perpendicular to theturbine axis, i.e., within the plane of rotation of thefailed turbine wheel. Thus targets aligned with theturbine shaft have a much reduced probability of beingstruck directly by turbine missiles. On the basis ofpresent information, the staff concludes that, in futurenuclear power plants, all essential systems should belocated outside the area most likely to sustain direct hitsin the event of a turbine failure at destructive overspeed.Evidence currently available2 indicates that low-trajectory turbhie missile strikes will be concentratedwithin an area bounded by lines inclined at 25 degrees tothe turbine wheel planes and nassing through the. endwheels of the low.pressure :!:iges (see Figure I). This2Ibid.I Turbine AxisFigure 1 Low-Trajectory Turbine Missile Strike Zone1.115-2 applies to the low-pressure stage shrunk-on wheels of the1800-rpm turbines generally used with light-water-cooled reactors. Essential systems within this area andclose to the turbine axis arc most vulnerable. Those.urther removed Irom the turbine axis are less likely tobe hit by a missile.For essential systems within the low-trajectory missilestrike zone, an acceptable basis for determining adequatesafety against low-trajectory turbine missiles is that thesystem is either small enough or far enough removedfrom the turbine that its probability of being struck by aturbine missile is less than 10-3. This criterion is aconservative way to ensure that the hazard rate due toloW-trajectory turbine missiles is less than 10-7 per year.Computational methods reflecting the reduction in riskprovided by intermediate barriers and the fact that amissile strike will not always lead to an event withradiological consequences are still in the developmentstage.This guide addresses only large missiles that might beejected in the event of a turbine failure. The inherentprotection provided in most plants (generally 1-1/2 to 2feet of reinforced concrete) ensures that minor missiles,which could be ejected in significant numbers and inwidely scattered directions once the casing is breached,would not result in damage to essential systems. Someattention should be directed to this problem, however,when turbine buildings themselves are relied on asbarriers 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 ofpresent and, to the extent possible, future units on thesite. It should be recognized that the placement ofcurrently proposed plants may affect the future place-ment of additional units.C. REGULATORY POSITION1. Essential systems of a nuclear power plant shouldbe protected against low-trajectory turbine missiles. Foithe purposes of this guide, essential systems are definedas all plant structures and equipment for which damageby turbine missiles could lead to significant radiologicalconsequences either by the direct release of radioactivityfrom the damaged system itself, e.g., spent fuel pools, orby failing in a manner that could lead to unacceptableconditions for other systems, e.g., emergency dieselgenerators. The control room should be included as anessential system.2. Each essential system and its location should beidentified, and a physical 'description should be pro-vided. Dimensioned plan and elevation layout drawingsand wall thicknesses and materials of pertinent stijc-tures should be included.3. Protection of essential systems or structuresagainst direct strikes by low-trajectory turbine missilescan be provided by appropriate placement and orien-tation of the turbine units. The protection of anessential system is acceptable if the system is locatedoutside the low-trajectory missile strike zones, which aredefined hy +/-25-degree lines emanating from the centersof the first and last low-pressure turbine wheels asmeasured from the plane of the wheels (see Figure 1).The strike zones associated with the turbines of allpresent and future nuclear and nonnuclear units at thesite should be considered.4. The protection of an essential system locatedwithin the low-trajectory missile strike zone is accept-able if, in the ev.nt of a turbine failure, the probabilityof its being hit by such a missile is less than 10"35. 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 strikezone,D. IMPLEMENTATIONThe purpose of this section is to provide informationto applicants regarding the NRC staff's plans for usingthis regulatory guide.Except in those cases in which the applicant proposesan alternative method for complying with specifiedportions of the Commission's regulations, the methoddescribed herein will be used in the evaluation ofsubmittals for construction permit applications docketedafter November 15, 1976.If an applicant wishes to use this regulatory guide indeveloping submittals for applications docketed on orprior to November 15, 1976, the pertinent portions ofthe application will be evaluated on the basis of thisguide.1.115-3