Accident risks in nuclear-power plants

Accident risks in nuclear-power plants

A. Strupczewski*

Institute of Atomic Energy 05-400 Swierk, Poland

Accepted 28 January 2003

Abstract

The paper presents the results of estimates of nuclear-power plant safety based on prob-

abilistic safety analyses and discusses the means used to decrease core damage factors, largerelease frequency and cancer deaths due to nuclear accidents. Estimates made for new nuclearpower plants show that these risks are negligibly small. The radiological effects of the Cher-

nobyl accident are discussed and compared with the negligibly small effects of the TMI-2accident in the USA in 1978, the only accident with core damage that occurred in pressurizedor boiling-water reactors. The accident risks in present and future nuclear-power plants are

compared with the accident risks due to other energy sources. Considering the whole fuelcycle, from mineral extraction to waste management, the risks due to nuclear-power accidentsare much smaller than the accident risks due to oil, gas or coal fuel cycles. This conclusion is

obtained from historical data, taking into account the accidents that have really occurredover the last 40 years, and is confirmed by probabilistic safety analyses for various powerindustries.# 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Nuclear power; Accident risks; Safety goals; Core damage factors

1. Safety goals for nuclear power

The general safety objective for nuclear-power plants (NPPs) is to protect theindividual, society and the environment by establishing and maintaining in NPPseffective measures against radiological hazards. To reach this objective, safety goalsfor nuclear power were established from the very beginning of its development, andmade more demanding as the technology matured.The initial qualitative targets were that no individual should bear a significant

additional risk due to nuclear-power plant operation and the societal risks from

Applied Energy 75 (2003) 79–86

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power-plant operation should not be a significant addition to other societal risks [1].They were followed by quantitative requirements, which according to US rules setthe design targets so that the calculated plant core-damage frequency (CDF) shouldbe less than 10�4 events per reactor year (R–Y) [2], and the calculated large releasefrequency (LRF) less than 10�6/R–Y for sequences resulting in a greater than 0.25Sv whole-body dose over 24 h at one-half mile from the reactor.These requirements for NPP design corresponded to the cancer risk to the people

in the critical population group equal to 10�10/R–Y [3]. Presently the safety objec-tives developed by the US and European utilities for the new generation of NPPsinclude a maximum permissible CDF equal to 10�5/R–Y [4]. It must also bedemonstrated that early containment failure is avoided for all risk-significant sce-narios. The cumulative LRF must be less than 10�6/R–Y.In parallel with the development of these targets, the nuclear industry and reg-

ulators in the countries leading in nuclear safety have developed the contemporarynuclear safety philosophy, which resulted in reducing risks in NPPs far below thoserisks typical for other power-industry branches. It places the principle ‘safety first’ asits cornerstone and includes several principles that are today the basis of NPP designand operation in all western countries.

2. Nuclear-power plant safety indicators

The progress in the safety level of NPPs is reflected in the probabilistic safetyanalyses (PSAs), initiated in the US in 1975 by the Rasmussen Study and system-atically developed to become standard tools used for safety analysis of every NPP.The importance of PSA in the evaluation of NPP safety is due to the fact that therehas been only one severe core damage accident in water-moderated reactors, namelythe Three Mile Island accident in the USA in 1978, so there are no historical statis-tical data as for coal-mine accidents, oil-transport accidents, gas explosions or dambreaks. Minor incidents that do happen in NPPs, although they are eagerly pub-licized by the media, usually are far below the level at which any hazard to the plantor the public would be involved. Moreover, in view of fast improvements of NPPtechnology, the analysis of the safety of the plants to be built cannot be based onhistorical experience with the plants put into operation 20 or even 10 years ago, butmust reflect the actual safety features of the upgraded new designs. PSA makes itpossible to study the new design features and evaluate which of the safety improve-ments will bring the required safety upgrading.The main condition for preventing massive releases of radioactivity is to maintain

the reactor containment integrity, first of all in the early stage of the accident, thenin the later stages when the releases of radioactivity would be less but still significant.In the middle of the 1990s, several mechanisms were considered as possible con-tributors to an early containment failure.Over the last decade, the intensive research and development of the technical

means of coping with severe accidents have resulted in our being able to treat theseissues as resolved.

80 A. Strupczewski / Applied Energy 75 (2003) 79–86

The results of several reactor-safety studies performed in Western countries showthat the safety of the modern NPPs is very high. For example the German risk-studyphase B [5] indicated that the frequency of core melt in Biblis B NPP was 10�4/(R–Y) and that of large radioactive releases 2.6�10�5/(R–Y). After taking into accountoperator actions preventing the reactor’s pressure-vessel melt-through under highpressure, the frequency of the core melt frequency was reduced to 2.6�10�6/(R–Y).Subsequent analyses performed for KONVOI plants [6] gave similar results, withabsolute numbers lower due to improvements in the KONVOI type plants as com-pared to the Biblis B. Core-damage frequency without bleed and feed in KONVOIplants was 1.4�10�6/R–Y, and after considering the effects of operator actions inthose plants, the CDF was reduced to 3.5�10�7/R–Y. These results can be con-sidered as typical for modern PWRs.The project for the European Pressurized-Water Reactor (EPR) assumes that the

design will limit the maximum possible releases so that the following safety objec-tives will be reached:

1. No need for short-term (about 24 h) off-site countermeasures
2. No need for population evacuation beyond 2–3 km 3. For long-term countermeasures, limited restriction of the consumption of
agricultural products for a limited period (about 1 year) in a limited area isacceptable [7].

This is the level of safety of NPPs expected as a reference base in the future. Spe-cific designs, which have been already licensed for construction, include reactorswith passive safety-features AP 600 or Advanced BWR [8], for which the CDF isbelow 2�10�7/R–Y. The releases of radioactivity are at least ten times smaller andthe health risks are negligible.

3. Radiological effects of nuclear-power plant accidents

The level of safety of modern NPPs is surprisingly far from the mass-media pic-ture of consequences of a nuclear accident. Actually, the only accidents with radio-active releases in NPPs were those in TMI and in Chernobyl. In TMI there was areactor-core melt, but the integrity of the remaining barriers (reactor pressure vesseland containment) was maintained and the releases were so limited that the averageeffective dose to the public was 0.015 mSv [9]. The corresponding cancer risk was below10�6 per lifetime, less than the risk due to NORMAL yearly emissions from a coal-firedpower plant at that time [10], and no health effects have ever been identified.In Chernobyl, the quantities of released fission products were significant, from

100% of noble gases down to about 4% of solid fission-products. The doses in theearly phase after the accident were high. In the rescue team, 28 men died in con-sequence of exposure to radiation and several more of those who were treated forradiation sickness died from illnesses that may have been associated with theirexposure. However, as confirmed in the UNSCEAR report of 2000, there has been

A. Strupczewski / Applied Energy 75 (2003) 79–86 81

no statistically significant increase in the incidence of leukaemia or any other formof cancer among workers or the public (except for child thyroid cancer), nor ofdeformities of babies born to members of the public [11]. An increase in the inci-dence of occult thyroid cancer was predicted to occur after 10 years, but actually itwas found already in the first year after the accident [11]. This shows that thescreening effect can be largely responsible for this observed increase. Generally theoccult thyroid cancer is not fatal and can be successfully treated. Although some2000 cases of thyroid cancer are attributed to the accident, less than 10 fatal caseshave been observed.Much greater damage to health has been caused by well meaning but misguided

attempts to protect and help people living near Chernobyl at the time of the acci-dent. The evacuation of hundreds of thousands of them is now seen as an overreaction, which in many cases did more harm than good. The first reaction was tomove people out. Only later, was it realized that many of them had not needed to bemoved. The relocation of people destroyed communities, broke up families, and ledto unemployment, depression, hypochondria and stress-related illnesses. Among therelocated populations, there has been a massive increase in stress-related illnesses,such as heart disease and obesity, unrelated to radiation.A major factor causing distress has been uncertainty about risks and in particular

belief that all radiation doses can lead to cancer, as stated in the Linear NoThreshold hypothesis presently used for the purpose of radiological protection. Therecent report of UNPD and UNICEF [12] confirms the above statements andacknowledges that the people living in the contaminated areas receive low doses ofradiation, being less than those occurring naturally in many other parts of theworld. This is illustrated in Fig. 1 taken from [13] comparing lifetime doses to peoplearound Chernobyl with the doses in European countries including Finland andSweden, in which the population enjoys very good health and low cancer rates inspite of the high radiation background.According to Russian sources, medical monitoring of the clean-up staff has shown

no increase of cancer rate and no relationship between the dose and the mortality.The overall mortality rate among the clean-up staff was statistically lower than themortality rate of the control group from the public [14]. The UNSCEAR report alsoconfirms that no radiation illnesses (with the exception of child thyroid diseases)have been found in the exposed population [11]. Thus, although it should beacknowledged that the effects of the Chernobyl accident are important, it should bealso stressed that most of them are due to excessive fear motivated and politicallyexpedient decisions, not to the radiation doses themselves.The NPPs planned to be built are completely different from RBMKs. The nega-

tive temperature reactivity coefficient ensures that, in accident conditions, theirpower will decrease, not increase as in Chernobyl, the containment (which did notexist in Chernobyl) would remain intact even after severe accidents and the accident-management procedures and safety-upgrading measures implemented in the NPPswould prevent such large releases of radioactivity as was the case in Chernobyl.Thus, the radiological results of Chernobyl cannot be treated as representative ofnuclear accidents in NPPs. The estimates of probable releases are made for each


NPP separately within PSA studies and generally show that the hazards are muchsmaller than for other energy sources.

4. Comparison of nuclear-power risks with accident risks due to other energy

sources

The risks of electricity generation should be evaluated considering the whole cycle,from fuel mining to plant construction, to waste management and site recultivation.While in the case of the nuclear-fuel cycle, the accident risks are mostly connectedwith the power plant, in other fuel cycles the dominant contribution can be made byother fuel stages.For example, in the case of coal mining, the fatality ratio in the US is about 0.1

death/million tons or 3.5 death/GW(e).a [15]. In very large regions of the world, thesituation can be much worse. In China, the average value for the country was about4.6 deaths per MT in 1997 [16] and the number of mining fatalities per unit of energyproduced from coal was 17 deaths/GW(e).a. In addition to that, the accident deathrate in coal-fired power plants was about 2 deaths/GW(e).a [17] and in coal trans-port sector 8.5 deaths/GW(e).a [17]. These numbers add up to the accidental mor-tality in China coal power system being equal 27.5 deaths/GW(e).a.The number of fatalities due to severe accidents (involving more than 5 fatalities

each) for the coal chain in OECD countries is 0.13 per GW(e) [19]. In non-OECD

Fig. 1. Lifetime radiation dose in various regions of Europe [13].


countries, it is much higher. The everyday occupational hazards for the coal chainwill be taken as 1.27 fatalities/GW(e).a according to [18], that is for Europeancountries. It is seen, that the small accidents involve more fatalities than the largeones, so both numbers must be taken into account.The differences of the safety of hydropower in OECD and non-OECD countries are

most pronounced. While the fatality ratio for OECD countries is only 0.004, it is 2.187for non-OECD countries [15]. The data on dam safety show that differences in technol-ogy and safety practices influence very much the risk of power generation from a givenfacility. These differences are taken into account while discussing risks of the conven-tional power industry and nobody discussing the safety of a dam to be erected in thetwenty-first century would base its safety indicators on accidents of dams built in say1920. In a recent ExternE report on hydropower, the authors do not include any risk dueto dam failures in the overall health risks due to hydropower [18], because they maintainthat the dams built in Norway provide ‘‘negligibly small risk’’. Similarly, the progress incoal-mining safety is taken into account while estimating the number of fatalities perGW(e).a.Of course this is a correct approach. However, if we take into account the progress

in dam construction before and after 1930, then the differences in NPP technologyexisting between RBMK reactors and LWR NPPs should be also considered. Simi-larly, if introducing strict regulations requiring qualified engineering supervision hada strong effect on dam safety, it is evident that the whole concept of safety cultureimplemented in Western NPPs has also a significant influence on nuclear-reactorsafety. As the differences in design between modern PWRs and the ChernobylRBMK are much more significant that any differences in dams erected in Norwayversus those built in the USA, Italy, France etc., then following the logic accepted byEC ExternE study, the hazards due to Chernobyl should not be considered as thebasis for evaluating the safety of future NPPs.If nuclear power is to be compared on an equal footing with other energy systems,

the hazard indicators for RBMK reactors should be calculated for RBMK reactorsconsidering the years of operation of RBMK units and the results of the Chernobylaccident, and hazard indicators for LWRs should be calculated on the basis ofLWRs experience and either historical experience of Three Mile Island accident, orPSA results for the reactor design under consideration.Such a comparison is presented in Fig. 2, showing fatalities due to severe accidents

that have occurred historically in various branches of the power industry. The datafor fossil-fuel plants in OECD countries and for hydro-power plants in OECD andnon-OECD countries have been taken from Ref. [15]. The indicators for NPPs werecalculated separately for RBMKs and for all other reactor types and integrated untilMay 2002. It is seen that the highest accident risks are associated with hydro-power innon-OECD countries and with LPG, then come the risks for oil, coal and natural gasin OECD countries, while the historically-based risks for hydro-power in OECDcountries in the reviewed period (since 1976) are an order of magnitude smaller thanthose of natural gas. The risks for nuclear-power plants in OECD countries and gen-erally for all NPPs with the exception of RBMKs are simply zero. The risks of earlyfatalities for RBMKs are on a level similar to that of coal mining, but the catastrophic


consequences of inappropriate decisions and the disruption of social life in the regionaround Chernobyl put these reactors in a separate class of hazardous systems.In order to have a full picture of the hazards due to electricity production from

various energy sources, the health hazards due to low-level emissions in the full fuel

Fig. 2. Early deaths due to severe accidents in various energy-systems [14,15].

Fig. 3. Loss of expected life due to electricity production in various fuel cycles [19].


cycle should be also compared. The results of such a comparison in terms of expec-ted years-of-life lost due to various energy sources is presented in Fig. 3 on the basisof Ref. [19]. Both Figs. 2 and 3 show that nuclear power is one of the most man-friendly energy sources and the level of safety reached in present practice can betreated as an example to other branches of the power industry.The review of the safety level of nuclear power reached in actual international

practice shows that the original goal for nuclear power, namely that the risks asso-ciated with NPPs be much less than those due any other energy sources, has beensuccessfully reached.

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