The need for electricity generation to be clean and safe has never been more obvious.  Nor has it ever been as popularly supported. 

Environmental and health consequences of electricity generation are important issues, alongside the affordability of the power which is  produced.  

They are usually seen as external costs - those which are quantifiable but do not appear in the utility's accounts.  Hence they are not passed on to the consumer, but are borne by society at large.  They include particularly the effects of air pollution on human health, crop yields and buildings, as well as occupational disease and accidents. Though they are even harder to quantify and evaluate than the others, external costs include effects on ecosystems and the impact of global warming. 

Production of electricity from any form of primary energy has some environmental effect.  A balanced assessment of nuclear power requires comparison of its environmental effects with those of the principal alternative, coal-fired electricity generation, as well as with other options. 

Greenhouse gas emissions  

Greenhouse here refers to the effect of certain trace gases in the earth's atmosphere so that long-wave radiation such as heat from the earth's surface is trapped.  A build-up of greenhouse gases, notably CO2, appears to be causing a warming of the climate in many parts of the world, which will cause changes in weather patterns.  Much of the greenhouse effect is due to carbon dioxide[1]. 

While our understanding of relevant processes is improving, we do not know how much carbon dioxide the environment can absorb, nor how long-term global CO2 balance is maintained.  However, scientists are increasingly concerned about the steady worldwide build-up of CO2 levels in the atmosphere, and political initiatives reflect this concern.  The build-up is occurring as the world's carbon-based fossil fuels from the Earth's crust are being burned and rapidly converted to atmospheric CO2 e.g. in motor vehicles, domestic and industrial furnaces, and electric power generation.  Progressive clearing of the world's forests also contributes to the greenhouse effect by diminishing the removal of atmospheric CO2 by photosynthesis. 

As early as 1977 a USA National Academy of Sciences report concluded that "the primary limiting factor on energy production from fossil fuels over the next few centuries may turn out to be the climatic effects of the release of carbon dioxide".  Today this is conventional wisdom.  The inexorable increase of CO2 levels in the atmosphere, coupled with concern about their possible climate effect, is now a very significant factor in the comparison of coal and nuclear power for producing electricity. 

Worldwide emissions of CO2 from burning fossil fuels total about 25 billion tonnes per year.  About 38% of this is from coal and about 43% from oil.  Every 1000 MWe power station running on black coal produces CO2 emissions of about 7 million tonnes per year.  If brown coal is used, the amount is about 9 million tonnes.  Nuclear fission does not produce CO2, while emissions from other parts of the fuel cycle (e.g. uranium mining and enrichment) amount to about 2% of those from using coal and some audited figures show considerably less than this. 

Figure 1 

Figure 1 shows the relative levels of CO2 emission from generating one lilowatt-hour of electricity from different sources.  Every 22 tonnes of uranium (26 t U3O8) used[2] saves about one million tonnes of CO2 relative to coal. 

There is now widespread agreement that we need resource strategies and energy policies in every country which will minimize CO2 build-up.  In respect to base-load electricity generation, increased use of uranium as a fuel is the most obvious such strategy, utilizing proven technology on the scale required. 

Other environmental effects of electricity generation 

At a uranium mine ordinary operating procedures normally ensure that there is no significant water or air pollution.  The environmental effect of coal mining today is also small except that more extensive areas may require subsequent rehabilitation, and in certain areas acid mine drainage due to oxidation of sulfur can be a problem.  

Small amounts of radioactivity are released to the atmosphere from both coal-fired and nuclear power stations.  In the case of coal combustion small quantities of uranium, radium and thorium present in the coal cause the fly ash to be radioactive, the level varying considerably.  Nuclear power stations and reprocessing plants release small quantities of radioactive gases (e.g. krypton-85 and xenon-133) and iodine-131, which may be detectable in the environment with sophisticated monitoring and analytical equipment but are never at harmful levels.  Steps are being taken to reduce further emissions of both fly ash from coal-fired power stations and radionuclides from nuclear power stations and other plants.  At present neither constitutes a significant environmental problem. 

The solid high-level waste from nuclear power stations is hot and very radioactive, so is stored for 40-50 years while the radioactivity decays to less than one percent of its original level.  Then it will be finally disposed of deep underground and well away from the biosphere.  There has been no pollution or plausible hazard from such material and nor is any likely, either short- or very long-term. 

Intermediate-level waste is placed in underground repositories with little delay.  Low-level waste is generally buried more conventionally.  Radioactive fly ash from coal-fired power stations has in the past had a much greater environmental impact largely because it was not perceived as a problem and appropriate action was not taken.  Today most fly ash is removed from stack gases and buried where seepage and run-off can be controlled. 

Waste heat produced due to the intrinsic inefficiency of energy conversion, and hence as a by-product of power generation, is much the same whether coal or uranium is the primary fuel.  The thermal efficiency of coal-fired power stations ranges from about 20 percent to a possible 40 percent, with newer ones typically giving better than 33 percent.  That of nuclear stations mostly ranges from 29-37 percent with the common light water reactor today giving about 34 percent.  

There is no reason for preferring one fuel over the other on account of the amount of waste heat.  This is the case whether power station cooling is by water from a stream or estuary, or using evaporative cooling towers.  However, looking past this fact to what determines the location of power plant:  coal-fired plants are commonly built on coalfields inland, so usually evaporate a lot of fresh water for cooling.  Many nuclear power plants can easily be sited on the coast, and so use seawater for cooling (directly, not by evaporation).  This can save a lot of precious fresh water.  In any case the dumped heat need not always be "waste".  

In colder climates district heating and agricultural uses are increasingly found.   In France the waste heat from a nuclear plant is used for a crocodile farm.  With 6500 square metres of covered surface area this crocodile farm is the largest tropical greenhouse open to the public in France. Its more than 600 species of tropical plants will shortly earn it a listing in a French botanical gardens guide.  Helping to preserve endangered species, the farm is home to 500 crocodilians and 6 young gharials.  Some 200,000 visitors have already come through since the farm was opened, in 1994.  Any such use of waste heat decreases the extent to which local fogs result from its release to the environment. 

The main environmental matter relevant to power generation is the production of carbon dioxide (CO2) and sulphur dioxide (SO2) as a result of coal-fired electricity generation.  When coal of say 2.5 percent sulphur is used to produce the electricity for one person in an industrialized country for one year, then about 9 tonnes of CO2 and 120 kg of SO2 are produced.  

Sulphur dioxide emissions arise from the combustion of fossil fuels containing sulphur, as many of them do.  Released in large quantities to the atmosphere it can cause (sulfuric) "acid rains" in areas downwind.  In the northern hemisphere many millions of tonnes of SO2 are released annually from electricity generation, though such pollution has been dramatically reduced.  The acid rain (rainwater having a pH of 4 and lower) in north-eastern USA and Scandinavia causes ecological changes and economic loss.  In the UK and the USA electric power utilities at first sought to minimize this by increasing their use of natural gas, but costs now work against this. 

It is possible to remove a lot of the SO2 from coal stack gases using flue gas desulphurization equipment, but the cost is considerable.  Power utilities have spent many billions of dollars on this.  On the other hand, between 1980 and 1986 SO2 emissions in France were halved simply by replacing fossil fuel power stations with nuclear ones.  At the same time electricity production increased 40 percent and France became a significant exporter of electricity. 

Oxides of nitrogen (NOx) from fossil fuel power stations operating at high temperatures are also an environmental problem.  If high levels of hydrocarbons are present in the air, nitrogen oxides react with these to form photochemical smog.  Moreover, oxides of nitrogen have an adverse effect on the earth's ozone layer, increasing the amount of ultra-violet light reaching the Earth’s surface.  

Health and environmental effects of power generation  

Traditionally occupational health risks have been measured in terms of immediate accident, especially fatality, rates.  However, today, and particularly in relation to nuclear power, there is an increased emphasis on less obvious or delayed effects of exposure to cancer-inducing substances and radiation. 

Many occupational accident statistics have been generated over the last 50 years of civil nuclear power in North America and Europe.  These can be compared with those from coal-fired electricity generation.  All show that nuclear is distinctly the safer means of electric power generation in this respect.  Two simple sets of figures are quoted in Tables 1 & 2.  A major reason for coal showing up unfavourably is the huge amount of it which must be mined and transported to supply even a single large power station.  Mining and multiple handling of so much material of any kind involves hazards, and these are reflected in the statistics. 

Health risks in uranium mining are very minor today.  In the 1950s exposure of miners to radon gas led to a higher incidence of lung cancer.  For over forty years, however, exposure to high levels of radon has not been a feature of uranium (or other) mines.  Today,  the presence of some radon around a uranium mining operation and some dust bearing radioactive decay products - as well as the hazards of inhaled coal dust in a coal mine - are well understood.  In both cases, using the best current practice, the health hazards to miners are very small and certainly less than the risks of industrial accidents. 

(The radiation level one metre from a drum of freshly-processed U3O8 is about half that - from cosmic rays - on a commercial jet flight.)

In other parts of the nuclear fuel cycle, radiation hazards to workers are low, and industrial accidents are few.  Further comment on radiation is in the following section. 

Certainly nuclear power generation is not completely free of hazards in the occupational sense, but it does appear to be far safer than other forms of energy conversion.  Table 1 covers more than 20 years. 

Table 2 The Hazards of Using Energy: Some Energy-related Accidents since 1977 

LPG and oil accidents with less than 300 fatalities, and coal mine accidents with less than 100 fatalities are generally not shown unless recent.

Coal mining deaths range from 0.009 per million tonnes of coal mined in Australia through 0.034 in USA to 4 in China and 7 /Mt in Ukraine.  China’s total death toll from coal mining averages well over 5000 per year - official figures give 5300 in 2000, 5670 in 2001, 7200 in 2002, about 6400 in 2003, 6027 in 2004, about 6000 in 2005 and, 4746 in 2006.  These data omit the small illegal collieries.  However, the picture is improving: in the 1950s the annual death toll in coal mines was 70,000, in the 1980s it was 40,000 and 1990s it was 10,000.  Ukraine's coal mine death toll is over two hundred per year (eg. 1999: 274, 1998: 360, 1995: 339, 1992: 459).

Environmental (non-occupational) health effects are qualitatively similar to those affecting workers in the industry.  Popular concern about ionizing radiation initially grew out of the testing of nuclear weapons, not to mention the threat of their possible use.  Correspondingly, these tests provided the nuclear power industry with  a strong awareness of radiation hazards.  Fortunately radioactivity is readily measurable and its effects fairly well understood compared with those of other hazards with delayed effects – including virtually all chemical cancer-inducing substances.  Radiation is a weak carcinogen. 

The contrast between air quality effects from coal burning for electricity and increased  radiation from nuclear power is very marked: a person living next to a nuclear power plant receives less radiation from it than from a few hours flying each year (see Table 3).  On the other hand, anyone downwind of a coal-fired power plant can expect it to have some effect on the air quality.  

Radiation  

Table 3 shows some typical levels and sources of radiation exposure.  The contribution from the ground and buildings varies from place to place.  Exposure is measured in millisieverts (mSv).  In most parts of the world levels range up to 3 millisieverts per year (mSv/yr).  

Citizens of Cornwall, UK, receive an average of about 7mSv/yr.  Hundreds of thousands of people in India, Brazil and Sudan receive up to 40 mSv/yr.  Several places are known in Iran, India and Europe where natural background radiation gives an annual dose of more than 50 mSv and in Ramsar in Iran it can give up to 260 mSv .  Lifetime doses from natural radiation range up to several thousand millisievert.  However, there is no evidence of increased cancers or other health problems arising from these high natural levels. 

Cosmic radiation dose varies with altitude and latitude.  Aircrew can receive up to about 5 mSv/yr from their hours in the air, while frequent flyers can score a similar increment.  In contrast, UK citizens receive about 0.0003 mSv/yr from nuclear power generation and this would be typical of countries using on nuclear power. 

In practice, radiation protection is based on the understanding that small increases over natural levels of exposure are not likely to be harmful but should be kept to a minimum.  To put this into practice the International Commission for Radiological Protection (ICRP) has established recommended standards of protection based on three basic principles: 

• Justification.  No practice involving exposure to radiation should be adopted unless it produces a net benefit to those exposed or to society generally.
  
• Optimization.  Radiation doses and risks should be kept as low as reasonably achievable (ALARA), economic and social factors being taken into account.
  
• Limitation.  The exposure of individuals should be subject to dose or risk limits above which the radiation risk would be deemed unacceptable. 

These principles apply to the potential for accidental exposures as well as predictable normal exposures. 

Underlying these principles is the application of the "linear hypothesis" based on the idea that any level of radiation dose, no matter how low, involves the possibility of risk to human health.  This assumption enables "risk factors" derived from studies of high radiation dose to populations (e.g. from Japanese bomb survivors) to be used in determining the risk to an individual from low doses[3].  However the weight of scientific evidence does not indicate any cancer risk or immediate effects at doses below 50 millisievert (mSv) in a short time or at about 100 mSv per year.  At lower doses and dose rates (up to at least 10 mSv/yr) the evidence suggests that beneficial effects are at least as likely as harmful ones. 

Based on the three conservative principles, ICRP recommends that the additional dose above natural background and excluding medical exposure should be limited to prescribed levels.  These are: one millisievert per year for members of the public, and 20 mSv per year averaged over 5 years for radiation workers who are required to work under closely-monitored conditions (see Table 3). 

The actual level of individual risk at the ICRP recommended limit for general public exposure is very small (it is calculated to result in about 1 fatal cancer per year in a population of 20,000 people) and impossible to confirm directly.  In the Chernobyl accident a large number of people were subject to significantly increased radiation exposure, the actual doses being approximately known.  In due course this tragedy may result in a better understanding of the effects, if any, of exposure to various levels of radiation.  At present much of our knowledge about the effect of radiation on people is derived from the survivors of the Hiroshima and Nagasaki bombings in 1945, where the doses received were very brief and also difficult to estimate.  Certainly there was a clear increase in certain types of leukaemia and lymphoma and of solid cancers among the survivors.  Progressively there is more information based on exposure with low dose rate, where the body has time to repair damage. 

The body has defence mechanisms against damage induced by radiation as well as by chemical carcinogens.  These can be stimulated by low levels of exposure, or overwhelmed by very high levels[4].  

The occurrence of cancer is not uniform across the world population, and because of local differences it is not easy to see whether or not there is any association between low occupational radiation doses and excess cancers.  This question has been studied closely in a number of areas and work continues, but so far no conclusive evidence has emerged to indicate that cancers are more frequent in radiation workers (or those living near nuclear facilities) than in other people of similar ages in western countries.  

Plutonium is sometimes seen as a particular concern.  It is separated from spent fuel by reprocessing.  Plutonium has been called the most toxic element known to man and therefore represented as a hazard that we should do without.  However it is pertinent to compare its toxicity with that of other materials with which we live.  If swallowed, plutonium is much less toxic than cyanide or lead arsenate and about twice as toxic as the concentrate of caffeine from coffee.  Its main danger comes if inhaled as a fine dust and absorbed through the lungs.  This would increase the likelihood of cancer 15 or more years afterwards.  However, as a counterpoint to the folklore about plutonium is the fact that about seven tonnes of it were dispersed in the upper atmosphere by nuclear weapons testing over the 30 years following World War II without identifiable ill effects. 

The health effects of exposure both to radiation and to chemical cancer-inducing agents or toxins must be considered in relation to time.  We should be concerned not only about the effects on people presently living, but also about the cumulative effects of actions today over many generations.  Some radioactive materials which reach the environment decay to safe levels within days, weeks or a few years, while others continue their effect for a long time, as do some chemical cancer-inducing agents and toxins.  Certainly this is true of the chemical toxicity of heavy metals such as mercury, cadmium and lead, these of course being a natural part of the human environment anyway, like radiation, but maintaining their toxicity forever.  The essential task for those in government and industry is to prevent excessive amounts of such toxins harming people, now or in the future.  Standards are set in the light of research on environmental pathways by which people might ultimately be affected. 

About sixty years ago it was discovered that ionizing radiation could induce genetic mutations in fruit flies.  Intensive study since then has shown that radiation can similarly induce mutations in plants and test animals.  However evidence of genetic damage to humans from radiation, even as a result of the large doses received by atomic bomb survivors in Japan, has not shown any such effects. 

In a plant or animal cell the material (DNA) which carries genetic information necessary to cell development, maintenance and division is the critical target for radiation.  Much of the damage to DNA is repairable, but in a small proportion of cells the DNA is permanently altered.  This may result in death of the cell or development of a cancer, or in the case of cells forming gonad tissue, alterations which continue as genetic changes in subsequent generations.  Most such mutational changes are deleterious; very few can be expected to result in improvements. 

The relatively low levels of radiation allowed for members of the public and for workers in the nuclear industry are such that any increase in genetic effects due to nuclear power will be imperceptible and almost certainly non-existent.  Radiation exposure levels are set so as to prevent tissue damage and minimize the risk of cancer.  Experimental evidence indicates that cancers are more likely than genetic damage.  

Some 75,000 children born of parents who survived high radiation doses at Hiroshima and Nagasaki in 1945 have been the subject of intensive examination.  This study confirms that no increase in genetic abnormalities in human populations is likely as a result of even quite high doses of radiation.  Similarly, no genetic effects are evident as a result of the Chernobyl accident. 

Life on Earth commenced and developed when the environment was certainly subject to several times as much radioactivity as it is now, so radiation is not a new phenomenon.  If we ensure that there is no dramatic increase in people's general radiation exposure, it is most unlikely that health or genetic effects from radiation will ever become significant. 

Reactor safety  

There have been sophisticated statistical studies on reactor safety.  However, for most people actual performance is more convincing than probability statistics.  The situation to date is that in 13,000 reactor-years of civil operation there has been only one accident to a commercial reactor which was not substantially contained within the design and structure of the reactor.  To this experience one could add another 12,000 reactor-years of naval operation, which in the west has had an excellent safety record. 

Only the Chernobyl disaster in 1986 resulted in radiation doses to the public greater than those resulting from exposure to natural sources.  Other incidents (and one “accident”) have been completely confined to the plant.  The tragedy made it clear why such reactors have never been licensed in other parts of the world.  Apart from Chernobyl, no nuclear workers or members of the public have ever died as a result of exposure to radiation due to a commercial nuclear reactor incident.  This is remarkable for the first five decades of a complex new technology which is being used in 30 countries, some reactors now operating having been built over forty years ago.  

Most of the serious radiological injuries and deaths that occur each year (2-4 deaths and many more exposures above regulatory limits) are the result of large uncontrolled radiation sources, such as abandoned medical or industrial equipment.  These have nothing to do with nuclear power generation.  Others are due to military or research activities. 

Most accident scenarios involve primarily a loss of cooling.  This may lead to the fuel in the reactor core overheating, melting and releasing fission products.  Hence the provision of emergency core cooling systems on standby.  In case these should fail, further protective barriers come into play: the reactor core is normally enclosed in structures designed to prevent radioactive releases to the environment.  Regulatory requirements today are that the effects of any core-melt accident must be confined to the plant itself, without the need to evacuate nearby residents.  About one third of the capital cost of reactors is normally due to engineering designed to enhance the safety of people – both operators and neighbours, if and when things go wrong.  Table 4 shows the international scale for reporting nuclear accidents or incidents. 

The main safety concern has always been over the possibility of an uncontrolled release of radioactive material, leading to contamination and consequent radiation exposure to people nearby.  Earlier assumptions were that this would be likely in the event of a major loss of cooling accident which resulted in a core melt.  Experience has proved otherwise in any circumstances relevant to Western reactor designs.  In the light of better understanding of the physics and chemistry of material in a reactor core under extreme conditions it became evident that even a severe core melt coupled with breach of containment could not in fact create a major radiological disaster from any Western reactor design.  

Studies of the post-accident situation at Three Mile Island in 1979, where there was no breach of containment, supported this.  The total radioactivity release from this accident was small, and the maximum dose to individuals living near the power plant was well below accepted limits, even though the reactor was written off.  Nevertheless, this accident had a pronounced psychological impact, was a severe blow to the US nuclear industry and had an adverse effect on the growth of nuclear capacity in USA and beyond.  More positively it brought about profound changes in the way reactors are run, and in details of their engineering.  In retrospect it was a very valuable stimulus to improvements, and had much the same effect on reactor safety as the Comet airliner crashes of the 1950s did on the safety of pressurized jet aircraft – to everybody's benefit today. 

The 1986 accident at Chernobyl in Ukraine was very serious due to the design of the reactor and its burning graphite which dispersed radioactive contamination far and wide.  It cost the lives of 47 staff and firefighters, 28 of them from acute radiation exposure.  There have also been 1800 cases of thyroid cancer registered in children, most of which were curable, though about ten have been fatal.  No increase in leukaemia and other cancers had shown up in the first decades, but the World Health Organization (WHO) expects some increase in cancers in the future, and the death toll from delayed health effects may well climb beyond the ten or so thyroid cancer victims.  About 130,000 people received significant radiation doses (i.e. above ICRP limits), and are being closely monitored by WHO.  Radioactive pollution drifted across a wide area of Europe and Scandinavia, causing disruption to agricultural production and some exposure (small doses) to a large population. 

The accident drew public attention to the lack of an adequate containment structure such as is standard on Western reactors.  In addition, the RBMK design was such that coolant failure led to strong increase in power output from the fission process.  Under abnormal conditions all reactor types may experience power increases, which are controlled by the reactor shutdown system and by the design physics.  Light water reactors, in which the coolant serves as moderator, automatically reduce power when the coolant/moderator is lost, and can then be shut down using the control rods.  

It has long been asserted that nuclear reactor accidents are the epitome of low-probability but high-consequence risks.  However, the physics and chemistry of a reactor core, coupled with - but not wholly depending on - the engineering, mean that the consequences of an accident are likely in fact to be much less severe than those from other industrial and energy sources.  Experience bears this out. 

The Chernobyl accident was caused by a combination of design deficiencies and the violation of operating procedures resulting from an absence of a safety culture.  With assistance from the West, significant safety improvements have been made to the 12 RBMK reactors in operation in Russia and Lithuania and the one potentially under construction in Russia.  Russian reactor design has since been standardized on PWR types, with containment structures.  

Soon after the accident the destroyed Chernobyl 4 reactor was enclosed in a large concrete shell.  The other three units on the site initially resumed operation, though they have since shut down, the last at the end of 2000. 

An OECD expert report concluded that "the Chernobyl accident has not brought to light any new, previously unknown phenomena or safety issues that are not resolved or otherwise covered by current reactor safety programmes for commercial power reactors in OECD Member countries."  A very positive outcome of the accident was creation of the World Association of Nuclear Operators (WANO) which enables the sharing of expertise and experience across the world. 

See also: WNA information paper on Chernobyl Accident, & Three Mile Island Accident.
 

Table 4. The International Nuclear Event Scale

For prompt communication of safety significance 

Source: International Atomic Energy Agency  

There have been a number of accidents in experimental reactors and in one military plutonium-producing reactor, including a number of core melts, but none of these has resulted in loss of life outside the actual plant, or long-term environmental contamination.  Elsewhere we tabulate these, along with the most serious commercial plant accidents.  The list of ten probably corresponds to incidents rating 4 or higher on today’s International Nuclear Event Scale (Table 4).  All except Browns Ferry and Vandellos involved damage to or malfunction of the reactor core.  At Browns Ferry a fire damaged control cables and resulted in an 18-month shutdown for repairs; at Vandellos a turbine fire made the 17-year old plant uneconomic to repair.  

It should be emphasized that a commercial-type reactor simply cannot under any circumstances explode like a nuclear bomb.  (The well-publicized accident at Tokai Mura, Japan, in 1999 was at a fuel preparation plant for experimental reactors, and killed two workers from radiation exposure.  Many other such criticality accidents have occurred, some fatal, and practically all in military facilities prior to 1980.) 

In an uncontained reactor accident such as at Windscale (a military facility) in 1957 and at Chernobyl in 1986, the principal health hazard is from the spread of radioactive materials, notably volatile fission products such as iodine-131 and caesium-137.  These are biologically active, so that if consumed in food, they tend to stay in organs of the body.  I-131 has a half-life of 8 days, so is a hazard for around the first month, (and apparently gave rise to the thyroid cancers after the Chernobyl accident).  Caesium-137 has a half-life of 30 years,  and is therefore potentially a long-term contaminant of pastures and crops.  In addition to these, there is caesium-134 which has a half-life of about two years.  While measures can be taken to limit human uptake of I-131, (evacuation of area for several weeks, iodide tablets), radioactive caesium can preclude food production from affected land for a long time.  Other radioactive materials in a reactor core have been shown to be less of a problem because they are either not volatile (strontium, transuranic elements) or not biologically active (tellurium-132).

Despite the commercial nuclear power industry's impressive safety record and the thorough engineering of reactor structures and systems which make a catastrophic radioactive release from any Western reactor extremely unlikely, there are those who simply don't want to run any risk of this.  This fear must then be weighed against the benefits of nuclear power, in the same way that some people's fear of having aeroplanes crash on top of them must be balanced against the utility of air transport for the rest of the population.  Ultimately, balancing risks and benefits is not simply a scientific exercise. 

Terrorism  

Since the World Trade Centre attacks in New York in 2001 there has been concern about the consequences of a large aircraft being used to attack a nuclear facility with the purpose of releasing radioactive materials.  Various studies have looked at similar attacks on nuclear power plants.  They show that nuclear reactors would be more resistant to such attacks than virtually any other civil installations because of the large amounts of concrete used in the structures.  A thorough study was undertaken by the Electric Power Research Institute in 2002 using specialist consultants and partly funded by the US Dept. of Energy.  It concludes that US reactor structures "are robust and (would) protect the fuel from impacts of large commercial aircraft". 

The analyses used a fully-fuelled Boeing 767-400 of over 200 tonnes as the basis, at 560 km/h – the maximum speed for precision flying near the ground.  The wingspan of this aircraft is greater than the diameter of reactor containment buildings and the 4.3 tonne engines are 15 metres apart.  Hence analyses focused on single engine direct impact on the centreline and on the impact of the entire aircraft if the fuselage hit the centreline (in which case the engines would ricochet off the sides).  In each case no part of the aircraft or its fuel would penetrate the containment.  

Looking at spent fuel storage pools, similar analyses showed no breach.  Dry storage and transport casks retained their integrity.  "There would be no release of radionuclides to the environment". 

Switzerland's Nuclear Safety Inspectorate studied a similar scenario and reported in 2003 that the danger of any radiation release from such a crash would be low for the older plants and extremely low for the newer ones. 

Similarly, the massive structures mean that any terrorist attack even inside a plant (which are well defended) would not result in any significant radioactive releases. 

The conservative design criteria which caused most power reactors to be shrouded by massive containment structures has provided peace of mind in a suicide terrorist context.  Ironically and as noted earlier, with better understanding of what happens in a core melt accident inside, they are now seen to be not nearly as necessary in that internal accident mitigation role as was originally assumed. 

Updated in February 2008