ML19323B924

Nuclear Power Plant Safety

ML19323B924
Person / Time
Site:	Crane
Issue date:	01/22/1979
From:	NRC COMMISSION (OCM)
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ML19323B925	List: ML19323B924
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TASK-TF, TASK-TMR NUDOCS 8005140425
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{{#Wiki_filter:i L,-* = b.=i 8005140CSh$p C O, [ m.. iirii .,e 7. .i.. ..= NUCLEAR POWER PLANT SAFETY G '^ ?.. ~..... M.. :- 1.~

SUMMARY

[:;,'-- =.:. A primary responsibility of the NRC is the safety of nuclear power plants. G.k7.;;;7 This responsibility is implemented through a licensing process that provides T-? r_. for the issuance of licenses for construction and operation of these plants l.l: g;; g only after a thorough and multi-level agency review that includes public E participation and input at its key stages. The licenses issued in accordance with this process specify the framework and necessary details of actions that builders and operators of nuclear power plants must follow in order to prea?.n. t public health and safety. Compliance with these licenses conditions is enforced by NRC inspectors during both plant construction and cperation. b Licenses are issued only for those nuclear power plants which, based on {.. e.. careful and inde' ndent reviews by the NRC, can meet the stringent safetu

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standards and criteria required by our regulations within the bounds of I' ,_1 F -we.

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f',r h' G: conservative engineering practice. These safety standards include [.. . {.m requirements for considerable cargins between design and operating d k.7 - conditions and for redundancy in primary and backup equipment in order to M; b compensate for the fact that no body of knowledge can ever be complete Y~ t\\ enough to reduce uncertainties and risks to zero. Thus, although the . T' c. '.*]~ operation of nuclear power plants is not completely risk-free, the safety f t .nr objsctive of the NRC, as imp 1.r.%ed through this licensing process, is to [f .:g -~ w.= -g F ?. ~s

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n 1 ' Ehmi==E kg - g;7===7 Um m (J3.1[? "j ~ el "a re A E.3 m.... m.~r. m: f2,Qh :i.+=: L"ti= = 7 '-(;icii iig 24 :< require plant builders and operators to take all those actions considered

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~~' -tCa g9: = necessary to assure that the risk to public -! :_' G is and continues to 2 = k ikk.. d l_5 ~ [ be very small. pc_'. tr 21 ...'ny!!. fi= -[- y::- :.= . en xr: ~ One of the primary methods for achievina this safety objective is the use jf =1 of a defense-in-depth concept for protecting public health and safety. This gg: concept, which is required by NRC regulations, results in the provision i* i... =?: = of multiple physical barriers in nuclear power plants between the radioactivity ~~ t. Ee contained in the reactor fuel and the plant environment. These regulatory 9 requirements are supplemented by a comprehensive quality assurance program L, that is designed to assure their proper implementation. Further assurance [: t -... y i. -. of achieving our safety objective is provided by the comprehensive safety t.Q'.;"; reviews carried out by the NRC Staff during plant design, construction, and J.-f - & t - W -:+:+ = operation, and by the independent safety reviews conducted by the Commission's d-jl.fE55 ~ fC~ii .= t ,Q._; =j Advisory Committee on Reactor Safeguards and Atomic Safety and Licensing {I.._.IE ~ [- 1-lis Boards. c _.^

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g., M V W.7,#.i. k.f.t":M wu Operating experience provides another important contribution to the assurance t?. .r........ W e. E '....... of nuclear power plant safety. Design improvements, based on this '; ~ " ~~[ @jr_d Eh!i experience', can be and are being incorporated into new plants, and mistakes \\ e. '...b:::

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".~.8 p.' .n1 I..--... in dssign and construction of operating plants can be and are being .i1 W, c. corrected in order to further. minimize public risk. The results to dat;e L.'.:. [.:: .:= . s= ... - - rn:n -- - D=[f5 Zu ". %W ~ $%pi

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==: tr-u E".. =. conservatists in order to compensate for the existing lack of complete Y.W ( "~ t*:. 22..' t[i, knowledge. Operating results'have also provides a basis in experience for fL*)::

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~- improvements in the design and operation of features important to continued [. '. J. ;..;,. ~ P ant safety.

== l - ::= t-Based on these considerations and without prejudice to the conclusions we .y t might reach in any individual licensing pmceeding, we believe that nuclear power plants that have been designed, constructed and licensed to operate ~ in accordance with.these NRC r_egulations and practices are safe; that is, , ~ ' _ they present no undue risk to ^the health and safety of the public. q. h-::.. p.'~ ' It would be nice to be able to also say that there are absolutely no problems . hhk,;. C '-~:: - with respect to the safety of nuclear power plants, that perfection has f' ___ y, ;__ ;

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= been achieved, and that all risks have been eliminated. This is not the case. Y g.-- - E" ^ !i R Nevertheless, we believe that nuclear power plants are safe and that the l'?-l, E.. t;: M.'-m "... :D risk to th'e public health and safety fwm their operation is very small. >-R. !f.-

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r. MEE =1 Ths Commission's intention is to assure that this risk remains at or below Ca.lll... _. J..K.:. E ~~: : e thic level so that nuclear power can continue to represent a suitable and L =,,s. t.. g: i~;7!?i s saf@ alternative for satisfying the nation's energy needs. h_ p}. $^ ' 5J.. I I" ~ j=. & -r EE . ~. :. tx p.g ._W WN.i.!- - 4a - - _e .a ,,1u.g..

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== .~ : hb = $1Di.YJ-- NUCLEAR POWER PLANT SAFETY 2[.- L. [_ :--- 7 _,_ e...s..:. ; :::== =- t:-,-:::. ' =.:==:. ii- - = = ..e -- A DEFENSE-IN-DEPTH Eh%. - .") rd: The defense-in-depth concept is a principal means of assuring a small risk k m.. m( E= I-? t r :-In - i=..... ~ to public health and safety from the operation of nuclear,cwer plants. il. * [ :' ' :.:( This concept calls for the incorporation of three levels of safety in f ~

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The three levels concern different design f~,_, [-, 2:= considerations, but they necessarily intennesh and overlap, so that certain [f-f.- i Q,q.~_:.. design features can 'be assigned to avre than one of these levels. }: .,.5:: _y [?..;-:.i" The Firs't Level of Safety a=== A simplified statement of the first level philosophy is to design and build J*.., _... : r ~

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a nucicar power plant so it will, with a high degree of reliability,

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r.:p.a - [i.T_-it.,. 6-.. operate without failures that could lead to accidents. The first level of =~'T =--

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safety involves designing a plant to conservative standards so that it will ^?" V,.. G -.?- g.a. TQ:: " = _ be safe in all phases of operation and has a maximum tolerance for errors, M.-f._._.^: . :-: :r: :. off-normal operation and component malfunction. ~~'~f.]., -Z r. : :- :. t::. '.us?: ..!N, &. 'M 5. E u.. _. ::N. 2 .Gw.,n -.-[= The Second Level Of Safety .- =% n TCT"' y.9:g.( :...... r..-.. Despita the care that is taken in the first level of safety to avoid j== S.. ': E equipment failures or operating errors that could lead to safety problems, Q.g-j!! .8 the NRC believes it is prudent to anticipate that some incidents will occur K..'.Y:.Q N p,-,. during the service life of a nuclear' power plant and to provide measures CQ[.f M,- M.,.,~ ~..-.. .. ~. - ~ y.'-__.. 9MWys As'C-D*.' ?..........

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- t: [h$_" HE5 E - f" EE~ LuE to cope with them. This second level of protection for the reactor P.- operating staff and the public is provided by reliable protection devices } ~.,}__ __

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and systems designei to assure that expected occurrences and off-normal 5:~. Ei: b5 isr conditions will be detected and either arres'ted or accommodated safely. y;l The requirements for these protection systems are based on a consideration of a spectrum of events that could lead to off-normal operations which 7;=====

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the plant design must safely accommodate. In addition, extensive testing -iEf i

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programs are carried out to verify that the protection systems will

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.E; function as designed. l. ^ "L The Third Level 0f Safety _.r.:.: The third level of safety supplements the first two through features that h ~ provide additional margins in the plant design to protect the public even Q.... h II -3 iEE in the event of the occurrences of very unlikely accidents. These margins Ly:p WE 2,+ f,3= 21 are assessed primarily by evaluating the response of the plant to a number g m,- of assumed accidents, involving in some instances the assumption of an ?:f L-independent failure of a redundant protective system simultaneously with f.'

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g,, the occurrence of the accident it is intended to control. From analyses (5..}. t.= - =: : ~: of these postulated accidents, a number of sequences called design basis ( ~ i==..

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b. Eccidents are selected as a basis for the design of additional plant i- .bhu E=?? features and equipment that are provided to further protect public health ?_ -===- hit......... and safety. One of the third-level systems provided on all modern nuclear p;.=='== a: !:0. @ g ;::

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e.. b5hk plants is the reactor containment. Another third-level system is the cmergency coro cooling syste'ms that is designed to cool the core in ):{:.. 6 . =n the event of a major instantaneous rupture of all normal plant cooling i L. systems. i. I 556 h:: f. 4:. '= NRC SAFETY REVIEWS The safety reviews and inspections conducted by the NRC are designed to l) 5 j assure the proper and conservative application of the Commission's siting, design, construction, and operation regulations which implement the defense-in-depth concept for nuclear power plants. The purpose, scope and effect )" + of these reviews in minimizing public risk can best be shown by relating __ 3 F.' them to the siting, design, construction and operation phases of nuclear i t power plants. !i nhn ~P ~ G :. n.n:. q....... '.;,:." 2..... Siting Reviews r t The principal NRC requirements for the siting of nuclear plants are found V.

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p in 10 CFR part 100 and its Appendix A, Geologic and Seismic Siting r.. V.E Criteria for Nuclear Power Plants. The siting reviews carried out by the f.? i f.= "z t staff in impicmenting this regulation play an important role in assuring

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that the likelihood of severe reactor accidents due to siting considerations _l is very low. For example, the requirements of this regulation, supported 7-((i; li.ES.:... by the independent evaluations of seismic and geol _ogic conditions at and [ Qi;== F0 r0: 55.. ... y::. G,R

) i ..F . dga h:!= ?.'{Naiisi n.g- ~" --@~';.l, &.).~ f p N: h51 ~ P EW - %. b,%%g 4.i.. .n E near reactor site by the NRC staff and itt consultants, provide the basis ?. Q _ _ ... -;: =g;-::

== for establishing the likelihood of severe accidents from seismic events. [' ~

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The objective in the developnent of these requirements was to specify (:? :'.... investigations and procedures such that the likelihood of occurrence of pQ. ~= fib. an carthquake more severe than required to be used for design purposes L. - would be low and the possibility of a severe accident resulting from L. 7" such an carthquake even lower. Similarly, NRC regulations require that V. other environmental considerations at or near a reactor site that have the !. _..: = = : t-L potential to cause a severe reactor accident, such as flooding, tornadoes, } g-V... and overflying aircraft, be evaluated and to the extent necessary designed l.:." against a pursuant to the requirements of Criterion 2 of the NRC Gereral f.* y : :- ; Design Criteria, to assure that the likelihood of accidents from these if~ causes is comparable to the likelihood of accidents caused by earthquakes. (.l.~. ' ' .:= g-{~ -; F ~:.:.= .~ ~:- Design Paviews t ? p eu-, There are many NRC regulations that require the implementation of the It f ..aK defense-in-depth concey in the design of nuclear power plants. These Nl em .. c include the majority of the present 64 General Design criteria, other f!;{ h$ Appendices of 10 CFR' Part 50, and Section 50.bSa, of Part 50, Codes and f lk. _ N.J. ;;; ~ Standards for Nuclear Power Plants. A large fraction of the effort involved

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g, 1.$ E in NRC design reviews is for the purpose of determining whether these [:!.} s..- : ::._ requirements are being properly ar}d conservatively implemented. These r[p WlTYll h.=p-@.g 1k d:talled reviews of proposed reactor delsgns are relied upon to a &7 f** *

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ide':... considerable degree in helping form a considerable portion of the basis fr?. ~=- %-- 5-E..... f, ;.

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for achieving our safety objective of assurihg that the risk to the g ;s....... public from nuclear power plants is very small. WEi" iin bi:;. i": ~ @mer.E -- j m,." =: 'QQ:.. "' In addition to the reviews of design adequacy conducted by the NRC staff, f(.T - ~ ~ E ~ EE.:: AEC regulations require license applicants to perform analyses of various t:: -- postulated equipment, system and personnel failures. Independent evaluations ll-====:. k of these events on a selective basis are then performed by the NRC to / *. -==WW+ Cll @.@M assure that equipment and personnel performance under the assumed conditions f ,.c. are properly described and the accident consequences conservatively calculated. ,,, -~:: These independent accident analyses provide further assurance of the design ?- ~

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(c;.- 1 -i adequacy of licensed nuclear power plants. = =... [ _.. L =.f.

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==^ g_ p. Appendix B to 10 CFR Part 50 of the NRC regulations establishes quality ([F g.g g- - - -g assurance criteria required for all phases of nuclear power plant design, 2l*.b " S$$. construction and operation. These criteria, as implemented by licensees

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.n w.. v: w.- : - quality assurance programs and enforced by field reviews and inspections Q'\\ dal;; EE i c.e. - ,&.,. GE b by the NRC staff, provide assurance that the as-built plant structures, _. p.._... ,~ ,.. ;.. ~ t... systems and components important to safety reflect those features required hW1[g bh*?:!"^=.;:.=_ y as a result of NRC design reviews and accident analyses. In addition, QQ; . :.7 l===.

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NRC inspections during operation provide further assurance that plant w

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....=.... ,..,;. 2*J-- operations are conducted in accordance with license conditions. These _C.yff== =.=,.L=.

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a r :: {(%@ license condittons are conservatively estan11shed so as to maintain the { very low risk to public health and. safety from plant opere lon. 6==.___: ,

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i g-s, ~ " ~ ((- fii OPERATING EXPERIENCE (b.l.1; b _ The siting, design, construction and operating reviews previously described have been designed to' provi'e the necessary and sufficiant conditions L. needed to assure, in conjunction with the NRC regulations, that nuc1 car L.- power plants present no undue risk to public health and safety. The over 300 reactor years of successful nuclear power plant operation in this country provide added support for such a judgment. Of course, this V L.: - F amount of experience does not provide an adequate statistical basis for [ I. : definitively quantifying the probability of reactor accidents or the (' l-' }f.. _.. risk to public health and rafety. Within, these limitations, however, y

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[NS of nuclear power plant safety. Design improvements, based on experience, h:. l:.. can be and are being incorporated into new plant designs, and mistakes in T.b b...?: iS? ~=? lil._...... design and construction of operating plants can be and are being rectified N.* e. -~ , '. '~ E.... = to further improve plant safety. The results to date of this operating r. l

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cxperience have shown in the most practical way the wisdom of implementing [f {Q.l"."G.. u,. the defense-in-depth concept for nuclear plants with ample 'conservatisms iT_ {?? I =- - b.! p_. p.m: u-r e=w ~' ~ "E...... . r a I-

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