Environmental impact of nuclear power

Technicians emplacing transuranic waste at the Waste Isolation Pilot Plant, near Carlsbad, New Mexico. Various mishaps at the plant in 2014 brought focus to the problem of what to do with a mounting stockpile of spent fuel, from commercial nuclear reactors, currently stored at individual reactor sites. In 2010, the USDOE mothballed plans to develop the Yucca Mountain nuclear waste repository in Nevada.[5]

The spent nuclear fuel from uranium-235 and plutonium-239 nuclear fission contains a wide variety of carcinogenic radionuclide isotopes such as strontium-90, iodine-131 and caesium-137, and includes some of the most long-lived transuranic elements such as americium-241 and isotopes of plutonium.[6] The most long-lived radioactive wastes, including spent nuclear fuel, are usually managed to be contained and isolated from the environment for a long period of time. Spent nuclear fuel storage is mostly a problem in the United States, following a 1977 President Jimmy Carter prohibition to nuclear fuel recycling. France, Great Britain and Japan, are some of the countries which rejected the repository solution. Spent nuclear fuel is a valuable asset, not simply waste.[7] Disposal of these wastes in engineered facilities, or repositories, located deep underground in suitable geologic formations is seen as the reference solution.[8] The International Panel on Fissile Materials has said:

It is widely accepted that spent nuclear fuel and high-level reprocessing and plutonium wastes require well-designed storage for long periods of time, to minimize releases of the contained radioactivity into the environment. Safeguards are also required to ensure that neither plutonium nor highly enriched uranium is diverted to weapon use. There is general agreement that placing spent nuclear fuel in repositories hundreds of meters below the surface would be safer than indefinite storage of spent fuel on the surface.[9]

Common elements of repositories include the radioactive waste, the containers enclosing the waste, other engineered barriers or seals around the containers, the tunnels housing the containers, and the geologic makeup of the surrounding area.[10]

The ability of natural geologic barriers to isolate radioactive waste is demonstrated by the natural nuclear fission reactors at Oklo, Africa. During their long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements were generated in the uranium ore body. This plutonium and the other transuranics remained immobile until the present day, a span of almost 2 billion years.[11] This is quite remarkable in view of the fact that ground water had ready access to the deposits and they were not in a chemically inert form, such as glass.

Despite a long-standing agreement among many experts that geological disposal can be safe, technologically feasible and environmentally sound, a large part of the general public in many countries remains skeptical.[12] One of the challenges facing the supporters of these efforts is to demonstrate confidently that a repository will contain wastes for so long that any releases that might take place in the future will pose no significant health or environmental risk.

Nuclear reprocessing does not eliminate the need for a repository, but reduces the volume, reduces the long term radiation hazard, and long term heat dissipation capacity needed. Reprocessing does not eliminate the political and community challenges to repository siting.[9]

The countries that have made the most progress towards a repository for high-level radioactive waste have typically started with public consultations and made voluntary siting a necessary condition. This consensus seeking approach is believed to have a greater chance of success than top-down modes of decision making, but the process is necessarily slow, and there is "inadequate experience around the world to know if it will succeed in all existing and aspiring nuclear nations".[13] Moreover, most communities do not want to host a nuclear waste repository as they are "concerned about their community becoming a de facto site for waste for thousands of years, the health and environmental consequences of an accident, and lower property values".[14]

In a 2010 Presidential Memorandum, U.S. President Obama established the "Blue Ribbon Commission on America’s Nuclear Future".[15] The Commission, composed of fifteen members, conducted an extensive two-year study of nuclear waste disposal.[15] During their research the Commission visited Finland, France, Japan, Russia, Sweden, and the UK, and in 2012, the Commission submitted its final report.[16] The Commission did not issue recommendations for a specific site but rather presented a comprehensive recommendation for disposal strategies.[17] In their final report the Commission put forth seven recommendations for developing a comprehensive strategy to pursue. A major recommendation was that "the United States should undertake an integrated nuclear waste management program that leads to the timely development of one or more permanent deep geological facilities for the safe disposal of spent fuel and high-level nuclear waste".[17]

Moderate amounts of low-level waste are through chemical and volume control system (CVCS). This includes gas, liquid, and solid waste produced through the process of purifying the water through evaporation. Liquid waste is reprocessed continuously, and gas waste is filtered, compressed, stored to allow decay, diluted, and then discharged. The rate at which this is allowed is regulated and studies must prove that such discharge does not violate dose limits to a member of the public (see radioactive effluent emissions).

Solid waste can be disposed of simply by placing it where it will not be disturbed for a few years. There are three low-level waste disposal sites in the United States in South Carolina, Utah, and Washington.[18] Solid waste from the CVCS is combined with solid radwaste that comes from handling materials before it is buried off-site.[19]

In the United States environmental groups have said that uranium mining companies are attempting to avoid cleanup costs at disused uranium mine sites. Environmental remediation is required by many states after a mine becomes inactive. Environmental groups have filed legal objections to prevent mining companies from avoiding compulsory cleanups. Uranium mining companies have skirted the cleanup laws by reactivating their mine sites briefly from time-to-time. Letting the mines sites stay contaminated over decades increases the potential risk of radioactive contamination leeching into the ground according to one environmental group, the Information Network for Responsible Mining, which started legal proceedings about March 2013. Among the corporations holding mining companies with such rarely used mines is General Atomics.[20]

The Grafenrheinfeld Nuclear Power Plant. The tallest structure is the chimney that releases effluent gases.

Most commercial nuclear power plants release gaseous and liquid radiological effluents into the environment as a byproduct of the Chemical Volume Control System, which are monitored in the US by the EPA and the NRC. Civilians living within 50 miles (80 km) of a nuclear power plant typically receive about 0.1 μSv per year.[21] For comparison, the average person living at or above sea level receives at least 260 μSv from cosmic radiation.[21]

All reactors in the United States are required by law to have a containment building. The walls of containment buildings are several feet thick and made of concrete and therefore can stop the release of any radiation emitted by the reactor into the environment. If a person is to worry about an energy source that releases large amounts of radiation into the environment, they should worry about coal-fired plants. "The waste produced by coal plants is actually more radioactive than that generated by their nuclear counterparts. In fact, the fly ash emitted by a [coal] power plant—a by-product from burning coal for electricity—carries into the surrounding environment 100 times more radiation than a nuclear power plant producing the same amount of energy." Coal-fired plants are much more hazardous to people's health than nuclear power plants as they release much more radioactive elements into the environment and subsequently expose people to greater levels of radiation than nuclear plants do. "Estimated radiation doses ingested by people living near the coal plants were equal to or higher than doses for people living around the nuclear facilities. At one extreme, the scientists estimated fly ash radiation in individuals' bones at around 18 millirems (thousandths of a rem, a unit for measuring doses of ionizing radiation) a year. Doses for the two nuclear plants, by contrast, ranged from between three and six millirems for the same period. And when all food was grown in the area, radiation doses were 50 to 200 percent higher around the coal plants."[22]

The total amount of radioactivity released through this method depends on the power plant, the regulatory requirements, and the plant's performance. Atmospheric dispersion models combined with pathway models are employed to accurately approximate the dose to a member of the public from the effluents emitted. Effluent monitoring is conducted continuously at the plant.

A leak of radioactive water at Vermont Yankee in 2010, along with similar incidents at more than 20 other US nuclear plants in recent years, has kindled doubts about the reliability, durability, and maintenance of aging nuclear installations in the United States.[23]

Tritium is a radioactive isotope of hydrogen that emits a low-energy beta particle and is usually measured in becquerels (i.e. atoms decaying per second) per liter (Bq/L). Tritium can be contained in water released from a nuclear plant. The primary concern for tritium release is the presence in drinking water, in addition to biological magnification leading to tritium in crops and animals consumed for food.[24]

Tritium,[25] the mass 3 isotope of hydrogen is deliberately created for thermonuclear weapons use, at government-owned reactors like Watts Bar, by irradiating lithium 6 with neutrons to fission i1. Light water reactors, the standard kind in the US, generate small quantities of deuterium by neutron capture in the water. This consumes enough neutrons that the natural uranium needs enrichment to raise its fissile U-235 content from 0.72% to 3.6% for Pressurised Water Reactors. Canada's CANDU design uses "heavy water", deuterium oxide, and can use un-enriched uranium because deuterium captures so very few of the neutrons. So the rate of production of tritium from the small amount of deuterium in US reactors must be quite low.

Legal concentration limits have differed greatly from place to place (see table right). For example, in June 2009 the Ontario Drinking Water Advisory Council recommended lowering the limit from 7,000 Bq/L to 20 Bq/L.[26] According to the NRC, tritium is the least dangerous radionuclide because it emits very weak radiation and leaves the body relatively quickly. The typical human body contains roughly 3,700 Bq of potassium-40. The amount released by any given nuclear plant also varies greatly; the total release for nuclear plants in the United States in 2003 was from nondetected up to 2,080 curies (77 TBq).

A drum of yellowcake

Rössing open pit uranium mine, Namibia

Uranium mining is the process of extraction of uranium ore from the ground. The worldwide production of uranium in 2009 amounted to 50,572 tonnes. Kazakhstan, Canada, and Australia are the top three producers and together account for 63% of world uranium production.[27] A prominent use of uranium from mining is as fuel for nuclear power plants. The mining and milling of uranium present significant dangers to the environment.[28]

"An average value for the thermal energy of coal is approximately 6150 kilowatt-hours(kWh)/ton. .... The thermal energy released in nuclear fission produces about 2 x 10E9 kWh/ton." [29]

It follows that, for the same amount of energy, much less uranium needs to be mined than coal, cutting the environmental impacts of uranium mining on nuclear energy generation.

In 2010, 41% of the world's uranium production was produced by in-situ leaching, which uses solutions to dissolve the uranium while leaving the rock in place.[30] The remainder was produced by conventional mining, in which the mined uranium ore is ground to a uniform particle size and then the uranium extracted by chemical leaching. The product is a powder of unenriched uranium, "yellowcake," which is sold on the uranium market as U₃O₈. Uranium mining can use large amounts of water — for example, the Roxby Downs Olympic Dam mine in South Australia uses 35,000 m³ of water each day and plans to increase this to 150,000 m³ per day.[31]

The Church Rock uranium mill spill occurred in New Mexico on July 16, 1979 when United Nuclear Corporation's Church Rock uranium mill tailings disposal pond breached its dam.[32][33] Over 1,000 tons of solid radioactive mill waste and 93 millions of gallons of acidic, radioactive tailings solution flowed into the Puerco River, and contaminants traveled 80 miles (130 km) downstream to Navajo County, Arizona and onto the Navajo Nation.[33] The accident released more radiation, although diluted by the 93 million gallons of mostly water and sulfuric acid, than the Three Mile Island accident that occurred four months earlier and was the largest release of radioactive material in U.S. history.[33][34][35][36] Groundwater near the spill was contaminated and the Puerco rendered unusable by local residents, who were not immediately aware of the toxic danger.[37]

Despite efforts made in cleaning up cold war nuclear arms race uranium sites, significant problems stemming from the legacy of uranium development still exist today on the Navajo Nation and in the states of Utah, Colorado, New Mexico, and Arizona. Hundreds of abandoned mines, primarily used for the US arms race and not nuclear energy production, have not been cleaned up and present environmental and health risks in many communities.[38] The Environmental Protection Agency estimates that there are 4000 mines with documented uranium production, and another 15,000 locations with uranium occurrences in 14 western states,[39] most found in the Four Corners area and Wyoming.[40] The Uranium Mill Tailings Radiation Control Act is a United States environmental law that amended the Atomic Energy Act of 1954 and gave the Environmental Protection Agency the authority to establish health and environmental standards for the stabilization, restoration, and disposal of uranium mill waste.[41]

Numerous studies have been done on possible effect of nuclear power in causing cancer. Such studies have looked for excess cancers in both plant workers and surrounding populations due to releases during normal operations of nuclear plants and other parts of the nuclear power industry, as well as excess cancers in workers and the public due to accidental releases. There is agreement that excess cancers in both plant workers and the surrounding public have been caused by accidental releases such as the Chernobyl accident.[42] There is also agreement that some workers in other parts of the nuclear fuel cycle, most notably uranium mining – at least in past decades – have had elevated rates of cancer.[43] However, numerous studies of possible cancers caused by nuclear power plants in normal operation have come to opposing conclusions, and the issue is a matter of scientific controversy and ongoing study.[44][45][46]

There have been several epidemiological studies that say there is an increased risk of various diseases, especially cancers, among people who live near nuclear facilities. A widely cited 2007 meta-analysis by Baker et al. of 17 research papers was published in the European Journal of Cancer Care.[47] It offered evidence of elevated leukemia rates among children living near 136 nuclear facilities in the United Kingdom, Canada, France, United States, Germany, Japan, and Spain. However this study has been criticized on several grounds – such as combining heterogeneous data (different age groups, sites that were not nuclear power plants, different zone definitions), arbitrary selection of 17 out of 37 individual studies, exclusion of sites with zero observed cases or deaths, etc.[48][49] Elevated leukemia rates among children were also found in a 2008 German study by Kaatsch et al. that examined residents living near 16 major nuclear power plants in Germany.[47] This study has also been criticised on several grounds.[49][50] These 2007 and 2008 results are not consistent with many other studies that have tended not to show such associations.[51][52][53][54][55] The British Committee on Medical Aspects of Radiation in the Environment issued a study in 2011 of children under five living near 13 nuclear power plants in the UK during the period 1969–2004. The committee found that children living near power plants in Britain are no more likely to develop leukemia than those living elsewhere[49] Similarly, a 1991 study for the National Cancer Institute found no excess cancer mortalities in 107 US counties close to nuclear power plants.[56] However, in view of the ongoing controversy, the US Nuclear Regulatory Commission has requested the National Academy of Sciences to oversee a state-of-the-art study of cancer risk in populations near NRC-licensed facilities.[44]

A subculture of frequently undocumented nuclear workers do the dirty, difficult, and potentially dangerous work shunned by regular employees. The World Nuclear Association states that the transient workforce of "nuclear gypsies" – casual workers employed by subcontractors has been "part of the nuclear scene for at least four decades."[57] Existent labor laws protecting worker's health rights are not properly enforced.[58] A 15-country collaborative cohort study of cancer risks due to exposure to low-dose ionizing radiation, involving 407,391 nuclear industry workers showed significant increase in cancer mortality. The study evaluated 31 types of cancers, primary and secondary.[59]

Nuclear power reactor accidents can result in a variety of radioisotopes being released into the environment. The health impact of each radioisotope depends on a variety of factors. Iodine-131 is potentially an important source of morbidity in accidental discharges because of its prevalence and because it settles on the ground. When iodine-131 is released, it can be inhaled or consumed after it enters the food chain, primarily through contaminated fruits, vegetables, milk, and groundwater. Iodine-131 in the body rapidly accumulates in the thyroid gland, becoming a source of beta radiation.[60]

The 2011 Fukushima Daiichi nuclear disaster, the world's worst nuclear accident since 1986, displaced 50,000 households after radiation leaked into the air, soil and sea.[61] Radiation checks led to bans of some shipments of vegetables and fish.[62]

Production of nuclear power relies on the nuclear fuel cycle, which includes uranium mining and milling. Uranium workers are routinely exposed to low levels of radon decay products and gamma radiation. Risks of leukemia from acute and high doses of gamma radiation are well-known, but there is a debate about risks from lower doses. The risks of other hematological cancers in uranium workers have been examined in very few studies.[63]

In terms of net radioactive release, the National Council on Radiation Protection and Measurements (NCRP) estimated the average radioactivity per short ton of coal is 17,100 millicuries/4,000,000 tons. With 154 coal plants in the United States, this amounts to emissions of 0.6319 TBq per year for a single plant.

In terms of dose to a human living nearby, it is sometimes cited that coal plants release 100 times the radioactivity of nuclear plants. This comes from NCRP Reports No. 92 and No. 95 which estimated the dose to the population from 1000 MWe coal and nuclear plants at 4.9 man-Sv/year and 0.048 man-Sv/year respectively (a typical Chest x-ray gives a dose of about 0.06 mSv for comparison).[64] The Environmental Protection Agency estimates an added dose of 0.3 µSv per year for living within 50 miles (80 km) of a coal plant and 0.009 milli-rem for a nuclear plant for yearly radiation dose estimation.[65] Nuclear power plants in normal operation emit less radioactivity than coal power plants.[64][65]

Unlike coal-fired or oil-fired generation, nuclear power generation does not directly produce any sulfur dioxide, nitrogen oxides, or mercury (pollution from fossil fuels is blamed for 24,000 early deaths each year in the U.S. alone[66]). However, as with all energy sources, there is some pollution associated with support activities such as mining, manufacturing and transportation.

A major European Union-funded research study known as ExternE, or Externalities of Energy, undertaken over the period of 1995 to 2005 found that the environmental and health costs of nuclear power, per unit of energy delivered, was €0.0019/kWh. This is lower than that of many renewable sources including the environmental impact caused by biomass use and the manufacture of photovoltaic solar panels, and was over thirty times lower than coals impact of €0.06/kWh, or 6 cents/kWh. However, the energy source of the lowest external costs associated with it was found to be wind power at €0.0009/kWh, which is an environmental and health impact just under half the price of Nuclear power.[67]

Proponents argue that the problems of nuclear waste "do not come anywhere close" to approaching the problems of fossil fuel waste.[68][69] A 2004 article from the BBC states: "The World Health Organization (WHO) says 3 million people are killed worldwide by outdoor air pollution annually from vehicles and industrial emissions, and 1.6 million indoors through using solid fuel."[70] In the U.S. alone, fossil fuel waste kills 20,000 people each year.[71] A coal power plant releases 100 times as much radiation as a nuclear power plant of the same wattage.[72] It is estimated that during 1982, US coal burning released 155 times as much radioactivity into the atmosphere as the Three Mile Island accident.[73] The World Nuclear Association provides a comparison of deaths due to accidents among different forms of energy production. In their life-cycle comparison, deaths per TW-yr of electricity produced from 1970 to 1992 are quoted as 885 for hydropower, 342 for coal, 85 for natural gas, and 8 for nuclear.[74] The figures include uranium mining, which can be a hazardous industry, with many accidents and fatalities.[75]

The North Anna plant uses direct exchange cooling into an artificial lake.

As with all thermoelectric plants, nuclear power plants need cooling systems. The most common systems for thermal power plants, including nuclear, are:

• Once-through cooling, in which water is drawn from a large body, passes through the cooling system, and then flows back into the water body.
• Cooling pond, in which water is drawn from a pond dedicated to the purpose, passes through the cooling system, then returns to the pond. Examples include the South Texas Nuclear Generating Station. The North Anna Nuclear Generating Station uses a cooling pond or artificial lake, which at the plant discharge canal is often about 30 °F warmer than in the other parts of the lake or in normal lakes (this is cited as an attraction of the area by some residents).[76] The environmental effects on the artificial lakes are often weighted in arguments against construction of new plants, and during droughts have drawn media attention.[77] The Turkey Point Nuclear Generating Station is credited with helping the conservation status of the American Crocodile, largely an effect of the waste heat produced.[78]
• Cooling towers, in which water recirculates through the cooling system until it evaporates from the tower. Examples include the Shearon Harris Nuclear Power Plant.

A 2011 study by the National Renewable Energy Laboratory determined that the median nuclear plant with cooling towers consumed 672 gallons of water per megawatt-hour, less than the median consumption of concentrating solar power (865 gal/MWhr for trough type, and 786 gal/MWhr for power tower type), slightly less than coal (687 gal/MWhr), but more than that for natural gas (198 gal/MWhr). Once-through cooling systems use more water, but less water is lost to evaporation. In the median US nuclear plant with once-through cooling, 44,350 gal/MWhr passes through the cooling system, but only 269 gal/MWhr (less than 1 percent) is consumed by evaporation.[79]

Nuclear plants exchange 60 to 70% of their thermal energy by cycling with a body of water or by evaporating water through a cooling tower. This thermal efficiency is somewhat lower than that of coal-fired power plants,[80] thus creating more waste heat.

It is possible to use waste heat in cogeneration applications such as district heating. The principles of cogeneration and district heating with nuclear power are the same as any other form of thermal power production. One use of nuclear heat generation was with the Ågesta Nuclear Power Plant in Sweden. In Switzerland, the Beznau Nuclear Power Plant provides heat to about 20,000 people.[81] However, district heating with nuclear power plants is less common than with other modes of waste heat generation: because of either siting regulations and/or the NIMBY effect, nuclear stations are generally not built in densely populated areas. Waste heat is more commonly used in industrial applications.[82]

During Europe's 2003 and 2006 heat waves, French, Spanish and German utilities had to secure exemptions from regulations in order to discharge overheated water into the environment. Some nuclear reactors shut down.[83][84]

With Climate change causing weather extremes such as heat waves, reduced precipitation levels and droughts can have a significant impact on all thermal power station infrastructure, including large biomass-electric and fission-electric stations alike, if cooling in these power stations, namely in the steam condenser is provided by certain freshwater sources.[85] A number of thermal stations use indirect seawater cooling or cooling towers that in comparison use little to no freshwater, while during heat waves, those that were designed to heat exchange with rivers and lakes, are under regulations to reduce output or cease operations to protect water levels and aquatic life.

This presently infrequent problem common among all thermal power stations may become increasingly significant over time.[85] If global warming continues, disruption of electricity may occur if station operators do not have other means of cooling, like cooling towers available, these in the decades before newer squat mechanical draft designs, were frequently large structures and therefore sometimes unpopular with the public.

Following the 2011 Japanese Fukushima nuclear disaster, authorities shut down the nation's 54 nuclear power plants. As of 2013, the Fukushima site remains highly radioactive, with some 160,000 evacuees still living in temporary housing, and some land will be unfarmable for centuries. The difficult cleanup job will take 40 or more years, and cost tens of billions of dollars.[102][103]

Japan towns, villages, and cities around the Fukushima Daiichi nuclear plant. The 20km and 30km areas had evacuation and sheltering orders, and additional administrative districts that had an evacuation order are highlighted.

In March 2011 an earthquake and tsunami caused damage that led to explosions and partial meltdowns at the Fukushima I Nuclear Power Plant in Japan.

Radiation levels at the stricken Fukushima I power plant have varied spiking up to 1,000 mSv/h (millisievert per hour),[104] which is a level that can cause radiation sickness to occur at a later time following a one-hour exposure.[105] Significant release in emissions of radioactive particles took place following hydrogen explosions at three reactors, as technicians tried to pump in seawater to keep the uranium fuel rods cool, and bled radioactive gas from the reactors in order to make room for the seawater.[106]

Concerns about the possibility of a large-scale release of radioactivity resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. Later, the UK, France and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading nuclear contamination.[107] New Scientist has reported that emissions of radioactive iodine and cesium from the crippled Fukushima I nuclear plant have approached levels evident after the Chernobyl disaster in 1986.[108] On March 24, 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures". Officials said also that the fallout from the Dai-ichi plant is "hindering search efforts for victims from the March 11 earthquake and tsunami".[109]

According to the Federation of Electric Power Companies of Japan, "by April 27 approximately 55 percent of the fuel in reactor unit 1 had melted, along with 35 percent of the fuel in unit 2, and 30 percent of the fuel in unit 3; and overheated spent fuels in the storage pools of units 3 and 4 probably were also damaged".[110] As of April 2011, water is still being poured into the damaged reactors to cool melting fuel rods.[111] The accident has surpassed the 1979 Three Mile Island accident in seriousness, and is comparable to the 1986 Chernobyl disaster.[110] The Economist reports that the Fukushima disaster is "a bit like three Three Mile Islands in a row, with added damage in the spent-fuel stores",[112] and that there will be ongoing impacts:

Years of clean-up will drag into decades. A permanent exclusion zone could end up stretching beyond the plant’s perimeter. Seriously exposed workers may be at increased risk of cancers for the rest of their lives...[112]

John Price, a former member of the Safety Policy Unit at the UK's National Nuclear Corporation, has said that it "might be 100 years before melting fuel rods can be safely removed from Japan's Fukushima nuclear plant".[111]

In the second half of August 2011, Japanese lawmakers announced that Prime Minister Naoto Kan would likely visit the Fukushima Prefecture to announce that the large contaminated area around the destroyed reactors would be declared uninhabitable, perhaps for decades. Some of the areas in the temporary 12 miles (19 km) radius evacuation zone around Fukushima were found to be heavily contaminated with radionuclides according to a new survey released by the Japanese Ministry of Science and Education. The town of Okuma was reported as being over 25 times above the safe limit of 20 millisieverts per year.[113]

Instead, 5 years later, the government expects to gradually lift the designation of some “difficult-to-return- zones”, a total 337 square kilometres (130 sq mi) area, from around 2021. Rain, wind and natural dissipation have removed radioactive contaminants, lowering levels, like at the central district of Okuma town, to 9 mSv/year, one-fifth the level of five years ago.[114]

Map showing Caesium-137 contamination in the Chernobyl area in 1996

As of 2013 the 1986 Chernobyl disaster in the Ukraine was and remains the world's worst nuclear power plant disaster. Estimates of its death toll are controversial and range from 62 to 25,000, with the high projections including deaths that have yet to happen. Peer reviewed publications have generally supported a projected total figure in the low tens of thousands; for example an estimate of 16,000 excess cancer deaths are predicted to occur due to the Chernobyl accident out to the year 2065, whereas, in the same period, several hundred million cancer cases are expected from other causes (from International Agency for Research on Cancer published in the International Journal of Cancer in 2006).[115] The IARC also released a press release stating "To put it in perspective, tobacco smoking will cause several thousand times more cancers in the same population", but also, referring to the numbers of different types of cancers, "The exception is thyroid cancer, which, over ten years ago, was already shown to be increased in the most contaminated regions around the site of the accident".[116] The full version of the World Health Organization health effects report adopted by the United Nations, also published in 2006, included the prediction of, in total, no more of 4,000 deaths from cancer.[117] A paper which the Union of concerned scientists took issue with the report, and they have, following the disputed linear no-threshold model (LNT) model of cancer susceptibility,[118] instead estimated, for the broader population, that the legacy of Chernobyl would be a total of 25,000 excess cancer deaths worldwide.[119] That places the total Chernobyl death toll below that of the worst dam failure accident in history, the Banqiao Dam disaster of 1975 in China.

Large amounts of radioactive contamination were spread across Europe due to the Chernobyl disaster, and cesium and strontium contaminated many agricultural products, livestock and soil. The accident necessitated the evacuation of the entire city of Pripyat and of 300,000 people from Kiev, rendering an area of land unusable to humans for an indeterminate period.[120]

As radioactive materials decay, they release particles that can damage the body and lead to cancer, particularly cesium-137 and iodine-131. In the Chernobyl disaster, releases of cesium-137 contaminated land. Some communities, including the entire city of Pripyat, were abandoned permanently. One news source reported that thousands of people who drank milk contaminated with radioactive iodine developed thyroid cancer.[121] The exclusion zone (approx. 30 km radius around Chernobyl) may have significantly elevated levels of radiation, which is now predominantly due to the decay of cesium-137, for around 10 half-lives of that isotope, which is approximately for 300 years.[122]

Due to the bioaccumulation of cesium-137, some mushrooms as well as wild animals which eat them, e.g. wild boars hunted in Germany and deer in Austria, may have levels which are not considered safe for human consumption.[123] Mandatory radiation testing of sheep in parts of the UK that graze on lands with contaminated peat was lifted in 2012.[124]

In 2007 The Ukrainian government declared much of the Chernobyl Exclusion Zone, almost 490 square kilometres (190 sq mi), a zoological animal reserve.[125] With many species of animals experiencing a population increase since human influence has largely left the region, including an increase in moose, bison and wolf numbers.[126] However other species such as barn swallows and many invertebrates, e.g. spider numbers are below what is suspected.[127] With much controversy amongst biologists over the question of, if in fact Chernobyl is now a wildlife reserve.[128]

This image of the SL-1 core served as a sober reminder of the damage that a nuclear meltdown can cause.

The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor which underwent a steam explosion and meltdown on January 3, 1961, killing its three operators; John Byrnes, Richard McKinley, and Richard Legg.[129] The direct cause was the improper manual withdrawal of the central control rod, responsible for absorbing neutrons in the reactor core. This caused the reactor power to surge to about 20,000MW and in turn, an explosion occurred. The event is the only known fatal reactor accident in the United States and the first to occur in the world.[130][129] The accident released about 80 curies (3.0 TBq) of iodine-131,[131] which was not considered significant due to its location in a remote desert of Idaho. About 1,100 curies (41 TBq) of fission products were released into the atmosphere.[132]

Radiation exposure limits prior to the accident were 100 röntgens to save a life and 25 to save valuable property. During the response to the accident, 22 people received doses of 3 to 27 Röntgens full-body exposure.[133] Removal of radioactive waste and disposal of the three bodies eventually exposed 790 people to harmful levels of radiation.[134] The hands of the initial victims were buried separately from their bodies as a necessary measure in response to their radiation levels.[129]

Nuclear power plants, uranium enrichment plants, fuel fabrication plants, and even potentially uranium mines are vulnerable to attacks which could lead to widespread radioactive contamination. The attack threat is of several general types: commando-like ground-based attacks on equipment which if disabled could lead to a reactor core meltdown or widespread dispersal of radioactivity; and external attacks such as an aircraft crash into a reactor complex, or cyber attacks.[135] Terrorists could target nuclear power plants in an attempt to release radioactive contamination into the environment and community.

Nuclear reactors become preferred targets during military conflict and have been repeatedly attacked by military air strikes:[136]

• In September 1980, Iran bombed the incomplete Osirak reactor complex in Iraq.
• In June 1981, an Israeli air strike completely destroyed Iraq's Osirak reactor.
• Between 1984 and 1987, Iraq bombed Iran's incomplete Bushehr nuclear plant six times.
• In Iraq in 1991, the U.S. bombed three nuclear reactors and an enrichment pilot facility.

The United States 9/11 Commission has said that nuclear power plants were potential targets originally considered for the September 11, 2001 attacks. If terrorist groups could sufficiently damage safety systems to cause a core meltdown at a nuclear power plant, and/or sufficiently damage spent fuel pools, such an attack could lead to a widespread radioactive contamination. According to a 2004 report by the U.S. Congressional Budget Office, "The human, environmental, and economic costs from a successful attack on a nuclear power plant that results in the release of substantial quantities of radioactive material to the environment could be great."[137] An attack on a reactor's spent fuel pool could also be serious, as these pools are less protected than the reactor core. The release of radioactivity could lead to thousands of near-term deaths and greater numbers of long-term fatalities.[135]

Insider sabotage occurs because insiders can observe and work around security measures. In a study of insider crimes, the authors repeatedly said that successful insider crimes depended on the perpetrators’ observation and knowledge of security vulnerabilities. Since the atomic age began, the U.S. Department of Energy’s nuclear laboratories have been known for widespread violations of security rules. A better understanding of the reality of the insider threat will help to overcome complacency and is critical to getting countries to take stronger preventative measures.[138]

Researchers have emphasized the need to make nuclear facilities extremely safe from sabotage and attacks that could release massive quantities of radioactivity into the environment and community. New reactor designs have features of passive safety, such as the flooding of the reactor core without active intervention by reactor operators. But these safety measures have generally been developed and studied with respect to accidents, not to the deliberate reactor attack by a terrorist group. However, the US Nuclear Regulatory Commission does now requires new reactor license applications to consider security during the design stage.[135]