Transcript
Page 1: Problems of safety of nuclear power plants

REVIEWS

P R O B L E M S OF S A F E T Y OF N U C L E A R P O W E R P L A N T S *

V. A. S i d o r e n k o UDC 621.039.51

P rob l ems of safety of nuclear power plants occupied one of the most prominent places among the sub- jects d i scussed at the IV Geneva Conference.

Of 505 r epo r t s read at the Conference , 78 dealt with safety. In accordance with the topics they d i s - cussed, these r epor t s can be a r b i t r a r i l y divided into five groups:

1. Discuss ion of the general scientif ic and engineer ing aspec ts of nuclear power plant safe ty including a study of the fundamental approach to safe ty control (so-cal led "philosophy of safety") (24 repor t s ) .

2. Legis la t ion concerning safety, no rms , s tandards , andlegal p rob lems assoc ia ted with the division of respons ib i l i t i es , etc. , (nine repor t s ) .

3. Effect of nuclear power on the environment including a discuss ion of the actual conditions p r e v a i l - ing at nuclear power plant s i tes (23 repor t s ) .

4. Scientific and engineer ing p rob lems assoc ia ted with the remova l of radioact ive waste and i ts bur ia l (eight repor t s ) .

5. Effects of radiat ion on living o r g a n i s m s , radiat ion protect ion and shielding (14 repor t s ) .

Approximate ly the same attention has been devoted to two aspec ts of nuclear safety: the effect on en- v i ronment and handling of nuclear waste (3rd and 4th groups, 31 repor t s ) and the scientif ic and engineer ing pr inc ip les of nuc lear plant safe ty and set t ing up no rms for safe ty control ( ls t and 2nd groups, 33 repor t s ) .

The ma te r i a l s p resen ted at the Conference re f lec t cons iderable advances in nuclear safe ty control: the prob lem is now much be t t e r understood, technical and organizat ional m e a s u r e s of safe ty control have improved , and a re l iable bas i s has been provided for nuclear safe ty taking into account the expected growth of nuclear energy.

The expected growth of nuclear power focussed attention on the effect on the environment of nuclear engineer ing in general and of specif ic power plants whose operat ion proved the adequacy of the safety m e a - su res provided. One conclusion that follows f rom the discuss ion is that the radioact iv i ty level in the vicini ty of nuclear power plants and fuel p rocess ing plants is ve ry low and that the amount of radioact ive waste is cons iderably l ess than allowed by national supe rv i so ry and legis la t ive organs for eve ry specif ic plant. De- tai led information on the environmental effects of a tomic instal lat ions was p resen ted in the Amer ican Report No. 087~, English Report No. 512, West German Repor t No. 399, etc. F o r example , the annual waste of radioact ive ma te r i a l s of c o m m e r c i a l nuclear power plants in 1970 in the USA amounted to 0.14-25% and 0.002-6.5~c of the allowed level of liquid and gaseous waste (Report No. 087). Even now it is poss ib le to design nuclear r e a c t o r s with a radioact ive waste level as low as des i red . Br i t i sh exper ience indicates that fuel p rocess ing plants produce the g rea t e s t amount of radioact ive waste . Many countr ies have undertaken specia l studies whose aim is the reduction of radioact ive d ischarge f rom future high-output fuel p rocess ing plants.

* Review of pape r s p resen ted at the IV Internat ional Conference on Peaceful Uses of Atomic Energy , Geneva, 1971. "~ Lis ts of r epo r t s p resen ted at the Geneva Conference were published in the October i ssues of Atomnaya t~nergiya (Soviet Reports) and Atomnaya Tekhnika za Rubezhom (foreign repor t s ) in 1971.

Trans la t ed f rom Atomnaya ]~nergiya, Vol. 32, No. 4, pp. 269-272, Apri l , 1972. Original a r t ic le submit ted J anua r y 13, 1972.

C 1972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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Page 2: Problems of safety of nuclear power plants

By consider ing the possible ways of penetra t ion of radioact ive products into the surroundings , and by compar ing environmenta l pollution due to nuclear and foss i l fuels, the authors of the Soviet Repor t No. 684 a r r i v e d at the conclusion that nuclear energy allows the conserva t ion of a reasonably clean environment; what is more , r ep lacemen t of foss i l e - fue l power by nuclear power should lead to a cons iderably lower level of contaminat ion of the surroundings by toxic m a t e r i a l s and so improve the environment .

Despi te the opt imis t ic p rospec t s of the p re sen t and future s ta tes of nuclear safe ty , all r epor t s call for stil l more s t r ingent m e a s u r e s to ensure the safety of nuclear power plants with r e spec t to both "actual" and "potential" radiat ion dangers (reduction of allowed rad ioac t ive -d i scha rge leve ls and improvemen t of the re l iabi l i ty of safe ty devices and radioact iv i ty containment devices) . This t rend is assoc ia ted with the eve r inc reas ing number of nuclear power plants and their location in densely populated a r ea s .

Per fec t ion of safe ty s y s t e m s , including containment devices , p roceeds in the di rect ion of higher de- vice eff ic iency and reduced size and cost . This is quite p re s s ing as the r i se in the cost of nuclear power plants is due l a rge ly t ~ the additional safe ty m e a s u r e s n e c e s s a r y to meet the more s t r ingent r equ i rement s .

Another fea ture cha r ac t e r i s t i c of modern t rends is the increas ingly important role of equipment r e - l iabi l i ty in secur ing actual plant safe ty and in reducing the probabi l i ty of accidents . Although these aspec ts of the p rob lem of safe ty a re not ent i re ly new, they have become recent ly of p r i m a r y impor tance together with safe ty m e a s u r e s that can be t e r m e d "obvious." Of independent s ignif icance is the superv is ion of plant equipment at all s tages , f rom its manufacture to uti l ization. The accumulated operat ional exper ience helped to improve methods of continuous superv is ion and per iodic inspection of nuclear plants (ul trasonic flaw de- tect ion, noise monitor ing, etc.) .

As follows f rom the r e p o r t s , pa r t i cu la r attention is now devoted to specif ic solutions to key technolo- gical safe ty p rob l ems that a re revea led in studies of the possible developments of dangerous p r o c e s s e s in nuclear plants espec ia l ly under e m e r g e n c y conditions.

The bas ic t rends in nuclear plant safe ty were d iscussed in USA repo r t s (Nos. 038, 040) f rom the point of view of "protect ion in depth" which includes the following s teps in the provis ion of nuclear safety: s e c u r - ing equipment re l iabi l i ty , provis ion of technological and c i rcui t imposs ib i l i ty of dangerous consequences of any single fa i lure or damage, l imi ta t ion of the consequences of any possible emergency case .

The fact that p rob lems of nuclear plants safety a re complex and many-s ided and have no single uni- ve r s a l solution has been s t r e s s e d in many repor t s . In pa r t i cu la r , one cannot expect that secur i ty can be provided only by high-qual i ty equipment (much be t t e r than used in conventional power plants) or only by en- sur ing containment of the effects of poss ib le emergenc i e s (such as isolat ion of the plant). The p rob lem mus t be cons idered f rom all i ts a spec t s .

The implementa t ion of the sa fe ty - in -dep th pr inciple can be seen in the different approaches to nuclear plant safety.

In one of the possible va r i an t s all s y s t e m s and equipment of the plant are divided into three functional par t s : the r e ac to r p rope r , equipment and s y s t e m s that ensure i ts normal operat ion; "ex terna l" protect ion s y s t e m s that reduce the possibi l i ty of hazardous deviation of plant p a r a m e t e r s f rom their design values and p ro tec t the plant in case of fa i lure of normal opera t ing devices; sy s t ems whose task is to reduce as much as poss ib le the consequences of any potential accident. The safety of nuclear power plants is ensured by independent and re l iable pe r fo rmance of all these three functional pa r t s .

In a different approach, sa fe ty - in -dep th is ensured by different independent and re l iable " b a r r i e r s , ' that p revent penet ra t ion of f i ss ion f r agments to inhabited a reas : f rom the fuel to coolant, f rom the coolant to the r e a c t o r locat ion, f rom the power plant locat ion to the surroundings , and finally f rom the su r round- ings by var ious means to the population.

The development of specif ic concepts and c r i t e r i a of safe ty in s eve ra l countr ies m e r i t s special a t ten- tion.

The des i rab i l i ty of a numer ica l and probabi l i s t ic approach to the evaluation and s tandardizat ion of nuclear safe ty has been recent ly f requent ly s t r e s s e d together with the fact that the amount of s ta t i s t ica l data on the pe r fo rmance of nuclear power plant equipment is st i l l insufficient. The expens iveness of putting this approach into p rac t i ce and the impor tance of international cooperat ion for the solution of this p rob lem has been pointed out in the F rench Repor t No. 579. The Conference proved beyond doubt that the necess i ty of a

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numerical approach to safety analysis is now general ly accepted, but that, at the same time, the feasibility of a probabil ist ic approach to safety standardization is still t reated with rese rve . Numerical methods of est imating equipment reliability and accident probabili ty are quite advanced and should be widely used in design pract ice of nuclear power plants. These methods make it possible to compare various approaches and to select optimum solutions in the design of safety equipment and devices. However, there is apparent- ly no sufficient basis for a numerical t reatment of the standardization of nuclear plant safety in the immedi - ate future. In the West German Report No. 364 attention is called to the fact that throughout a normal life span all existing nuclear power plants are unable to provide sufficient stat ist ical data for very grave acc i - dents whose probability is est imated a s 10 -7 per year ; in such cases the numerical approach becomes mean- ingless.

The most frequently used concept in safety control is still the concept of the "basic design accident." Perfect ion of this concept is part ial ly evident in the fact that attempts are made to apply probabilist ic meth- ods in selecting the basic design accident. One of the most important advances in this field is the use of not only "the maximum probable accident" but of a full spectrum of possible accidents of which the basic design accident is one. This is done in order to protect the plant not only against major but little probable potential dangers but also against real dangers presented by much more probable equipment fai lures. An effective and sys temat ic approach to the analysis of the spectrum of emergency situations is the so-cal led "failure t ree ." This method reveals all situations that are liable to resul t f rom any specific failure or damage. The failure tree makes it possible to demonstra te and evaluate numerical ly various combinations of ser ious damages that can lead to an emergency situation.

In aqueous reac tors the basic design accident is still assumed to be the total disruption of the main pipeline of the circulat ing loop. The possibili ty of reac tor vessel rupture is also considered. Some designs (e.g., in West Germany) even consider the probability of an accident involving vessel damage. The most probable location of vessel damage is considered to be the region where the circulat ing loop pipes are con- nected to the vessel . An analysis of the probability of crack development in the vessel makes it possible to take into account in the design defects in the vessel that can cause leakages much smal le r than result ing f rom a burs t in the main pipeline (West German Report No. 364).

A modification of the basic design accident concept was descr ibed in the Canadian Report No. 150. The plant safety is evaluated by quantitatively analyzing the frequency of occur rence of probable hazardous p ro - cesses in the system, but the use of specif ic maximum radiation exposure of the population is based on two principal schemes of accident occurrence: a single failure in standard technological equipment with the p r e - servat ion of full capability of accident prevention and containment devices or the coincidence of failure of both the standard operating system and the accident prevention system. In the f i rs t case maximum radiation exposure is that acceptable for normal operation; the second case involves the use of special maximum radi - ation exposure rates.

The available design and operational experience made it possible to find many specific solutions in various safety control systems (USA Report No. 040). For example, methods have been developed for the construct ion of equipment and buildings resis tant to earthquakes (Japanese Report No. 226, Swiss Report No. 672), hurr icanes , floods, and other natural d i sas te rs . There is also the experience of building a nu- c lear power plant near an airfield where the danger of collision or fire caused by an airplane accident must be taken into account. Inst ruments and a p rogram have been developed for monitoring the spread of radio- activity in the locality surrounding nuclear power plants.

Devices have been developed for aqueous reac to r s which monitor and if necessa ry suppress effects associated with xenon power fluctuations (such as, for example, absorbing rods of partial length); fixed in- termitent absorbers are used for canceling the positive temperature coefficient of the modera tor reactivity. Comprehensive p rograms have been developed for analyzing the vibrations of intravessel devices in the course of s ta r t -up tests for the detection and elimination of weak points. Specific solutions aimed at im- proving the construct ion reliabili ty, provision of continuous and periodic supervision, and ensurance of operating efficiency under emergency conditions are incorporated in the design of reac to r cooling sys tems, of buildings and containment installations, of safety control sys tems , monitoring and measur ing apparatus, and other sys tems.

Among problems that require further r e sea rch are:

development of flaws in steel s t ruc tures of circulation loops;

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t he rma l in teract ion of fuel and coolant, in pa r t i cu la r the heat exchange c r i s i s (Bri t ish Repor t No. 477);

embr i t t l emen t of thick s teel s am p l e s (including the rma l shock in case of emergency cooling of the r e - ac tor core) ;

p e r f o r m a n c e of safe ty s y s t e m s under e m e r g e n c y conditions;

c r i t i ca l p a r a m e t e r s and power (this can be said to be a "perennia l" problem);

the probabi l i ty of natural phenomena which must be allowed for in the design of nuclear power plants;

i m p r o v e m e n t of the safe ty s y s t e m s of core cool ing (USA Repor ts Nos. 040 and 039);

conditions of heat r emova l in t ime of and af ter e m e r g e n c i e s involving the loss of coolant;

hydrodynamic effects in the r e a c t o r ves se l and in the cooling loop in case of l a rge leakages ;

i m p r o v e m e n t of r emote moni tor ing methods of the equipment s tate in the course of r e a c t o r operat ion and perfec t ion of u l t rasonic methods;

mel t ing of the r e ac t o r core (West Ge rman Repor t s Nos. 365 and 364).

Special attention has been devoted to the safe ty of fas t -neu t ron r e a c t o r s . In sod ium-cooled fas t r e a c - to r s cha r ac t e r i s t i c hazardous events in which a single fa i lure or damage is l iable to cause grave conse - quences a re damages in the p r i m a r y loop, sodium ignition, chain damage of fuel e lements , the passage of l a rge gas bubbles through the co re , etc. Accordingly, in the analys is of var ious design accidents (fast r e - act ivi ty buildup, s toppage of coolant c i rcula t ion, etc.) , and in the development of p ro tec t ive m e a s u r e s p a r t i - cu lar attention was given to the study of such phenomena as the fo rmat ion of voids in sodium, in teract ion of the coolant with molten fuel, the mechan i sm of the sp read of fuel e lement damage, the Doppler effect , the fo rmat ion and sp read of ae roso l s in connection with sodium ignition (USA Repor t No. 041).

In conclusion, one should s t r e s s once more the genera l ly accepted impor tance of the c rea t ion of a s y s t e m of no rms and ru les for all s tages of the design, equipment manufac ture , construct ion, operat ion, and maintenance of nuclear power plants . The reac t ion of such a sy s t em of no rms and ru les is a continuous p r o c e s s . Bes ides this work, which is conducted by the Atomic Energy Commiss ion for the development of general ru les , c r i t e r i a , speci f ica t ions , p rocedures , e tc . , with the par t ic ipat ion of 1200 r ep re sen ta t i ves of 400 organiza t ions , work is going on in the USA on the crea t ion of a sy s t em of 78 mos t impor tan t nuclear s tandards . The development is now being concluded of the f i r s t ten s tandards to which belong:

secondary c r i t e r i a for p r e s s u r i z e d - w a t e r r eac to r s ;

secondary c r i t e r i a for boil ing wa te r r e a c t o r s ;

c r i t e r i a for taking into account s e i smic effects in the location and design of power r eac to r s ;

qualif ication and t ra in ing of nuclear power plant personnel ;

specif icat ions of per iodic tes t in nuclear power plants;

speci f ica t ions on p r e s t a r t and s ta r tup t e s t s of nuclear power plants;

c r i t e r i a and prac t ica l m e a s u r e s for secur ing quality pe r fo rmance of nuclear power plants , etc.

One thousand and five hundred nuclear s tandards a re to be developed in the next decade. Exper ience indicates that nuc lear power plants can be and are designed to opera te re l iably and safely. The development and introduction into p rac t i ca l use of a s y s t e m of no rms and s tandards should consolidate the p re sen t level of technology and extend it success fu l ly to other f ields.

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