Nuclear Power, electrical power produced from energy released by controlled fission or fusion of atomic nuclei in a nuclear reaction. Mass is converted into energy, and the amount of released energy greatly exceeds that from chemical processes such as combustion.
The first experimental nuclear reactor was constructed in 1942 amid tight wartime secrecy in Chicago, Illinois, in the United States. A prototype reactor was demonstrated at Oak Ridge, Tennessee, in 1943, and by 1945 three full-scale reactors were in operation at Hanford, in Washington State. These were dedicated to plutonium production for nuclear weapons; however, the first large-scale commercial reactor generating electrical power was started up in 1956 at Calder Hall, United Kingdom.
Nuclear power is now a well-established source of electricity worldwide. The most common types of reactor are light water reactors, mostly pressurized water reactors (PWRs) together with boiling water reactors (BWRs). Gas-cooled and heavy water reactors make up the rest. Worldwide there are currently about 430 reactors operating in 25 countries providing about 17 percent of the world’s electricity. Nuclear reactors are also used for propulsion of submarines and ships, and there are a number of prototype and experimental reactors around the world. At present, only a few experimental fusion reactors exist, none of which produce usable amounts of electrical power.
Few nuclear power stations are under construction at present, and some have been cancelled when partly built. This is mainly because of long-term resistance from the environmental movement (in particular since the Chernobyl disaster of 1986), but nuclear power stations are also not competitive with natural gas- and coal-fired power stations at present. It is uncertain whether nuclear power generation will increase or decrease worldwide over the next 50 years. However, the very low carbon dioxide emissions from nuclear power stations compared with coal-, gas-, or oil-fired units mean that there is potentially a future expansion in nuclear power driven by the need to control climate change.
More than 40 million kilowatt-hours (kWh) of electricity can generally be produced from one tonne of natural uranium. Over 16,000 tonnes of coal or 80,000 barrels of oil would need to be burned to make the same amount of electricity. Moreover, the amount of carbon dioxide produced in generating one kWh of electricity would be 1 kg for coal, 0.5 kg for gas, and only 10 grams for nuclear power.
Other than economic factors, the main issues limiting the expansion of nuclear power are the disposal of radioactive waste (including waste left over from decommissioning of old facilities), radioactivity in liquid effluent and gaseous discharges, security concerns over stockpiled plutonium, and the historical connection with nuclear weapons. Availability of nuclear fuel is unlikely to limit nuclear power production in the foreseeable future.
II THE BASICS OF NUCLEAR POWER
Nuclear power plants generate electricity from fission, usually of uranium-235 (U-235), the nucleus of which has 92 protons and 143 neutrons. When it absorbs an extra neutron, the nucleus becomes unstable and splits into smaller pieces (“fission products”) and more neutrons. The fission products and neutrons have a smaller total mass than the U-235 and the first neutron; the mass difference has been converted into energy, mostly in the form of heat, which produces steam and in turn drives a turbine generator to produce electricity.
Natural uranium is a mixture of two isotopes, fissionable U-235 (0.7 per cent) and non-fissionable U-238. However, U-238 can absorb neutrons to form plutonium-239 (P-239), which is fissionable, and up to half the energy produced by a reactor can, in fact, come from fission of P-239. Some types of reactor require the amount of U-235 to be increased above the natural level, which is called enrichment. Pressurized water reactors (PWRs), the most common type of reactor, require fuel enriched to about 3 percent U-235.
Reactor fuel is made up of fuel pellets or pins enclosed in a tubular cladding of steel, zircaloy, or aluminium. Several of these fuel rods make up each fuel assembly. The fast neutrons released in the fission reaction need to be slowed down before they will induce further fissions and give a sustained chain reaction. This is done by a moderator, usually water or graphite, which surrounds the fuel in the reactor. However, in “fast reactors” there is no moderator and the fast neutrons sustain the fission reaction.
A coolant is circulated through the reactor to remove heat from the fuel. Ordinary water (which is usually also the moderator) is most commonly used but heavy water (deuterium oxide), air, carbon dioxide, helium, liquid sodium, liquid sodium-potassium alloy, molten salts, or hydrocarbon liquids may be used in different types of reactor.
The chain reaction is controlled by using neutron absorbers such as boron, either by moving boron-containing control rods in and out of the reactor core or by varying the boron concentration in the cooling water. These can also be used to shut down the reactor. The power level of the reactor is monitored by temperature, flow, and radiation instruments and used to determine control settings so that the chain reaction is just self-sustaining.
The main components of a nuclear reactor are the pressure vessel (containing the core); the fuel rods, moderator, and primary cooling system (making up the core); the control system; and the containment building. This last element is required in the event of an accident, to prevent any radioactive material being released to the environment, and is usually cylindrical with a hemispherical dome on top.
During operation, and also after it is shut down, a nuclear reactor will contain a very large amount of radioactive material. The radiation emitted by this material is absorbed in thick concrete shields surrounding the reactor core and primary cooling system. An important safety feature is the emergency core cooling system, which will prevent overheating and “meltdown” of the reactor core if the primary cooling system fails. See also Nuclear Fission.
III HISTORICAL OVERVIEW
Radioactivity was discovered by Antoine Henri Becquerel in 1896, although not called this until two years later when Pierre and Marie Curie discovered the radioactive elements polonium and radium, which occur naturally with uranium. In 1932 the neutron was discovered by British scientist James Chadwick. Enrico Fermi and colleagues in Italy then discovered that bombarding uranium with neutrons slowed by means of paraffin produced at least four different radioactive products. Six years later, German scientists Otto Hahn and Fritz Strassman demonstrated that the uranium atom was actually being split. The Austrian-born Swedish physicist Lise Meitner continued the work with her nephew Otto Frisch and defined nuclear fission for the first time.
In 1939, Fermi travelled to the United States to escape the Fascist regime in Italy and was followed by physicist Niels Bohr, who fled the German occupation of Denmark. Collaborating at Columbia University, they developed the concept of a chain reaction as a source of power. With the outbreak of World War II, concerns arose among refugee European physicists in France, the United Kingdom, and the United States that Nazi Germany might develop an atomic bomb. The focus of research then changed to military applications.
The Manhattan Project began in the United States in 1940, with the aim to develop nuclear weapons. In 1942, Fermi constructed the first experimental nuclear reactor at the University of Chicago. One year later, a prototype plutonium production reactor was demonstrated at Oak Ridge and by 1945 three full-scale reactors were in operation at Hanford. The first nuclear bomb was tested at Alamogordo Air Base in New Mexico in July 1945. Two bombs were then dropped on Japan in August, the first at Hiroshima and the second at Nagasaki.
With the end of World War II in 1945, the Cold War and the East-West arms race took over. The Union of Soviet Socialist Republics (USSR) mounted a crash development programme and soon began plutonium production. The United States continued with plutonium production and also developed different types of reactor, as did the USSR, United Kingdom, France, and Canada. Both sides developed a range of technologies that was also applicable to nuclear power generation. Reliable energy supplies were important to national recovery, and nuclear power was seen as an essential element of national power programmes.
The first purpose-built reactor for electrical power generation was started up in 1954 at Obninsk, near Moscow, in the USSR. In 1956 the first large-scale commercial reactor generating electrical power (as well as producing plutonium) began operating at Calder Hall, England. In the United States, three types of the reactor were being developed for commercial use, namely the pressurized water reactor (PWR), boiling water reactor (BWR), and the fast breeder reactor (FBR). In 1957 the first commercial power unit, a BWR, was started up in the United States.
There have been some major incidents in nuclear power plants. In 1957 a plutonium production reactor caught fire at Windscale (modern-day Sellafield) in Cumbria, England, spreading large amounts of radioactivity across Britain and northern Europe. It was the worst nuclear accident in the history of the UK. In 1979, in the worst nuclear accident in US history, a core meltdown occurred at Three Mile Island power plant near Harrisburg, Pennsylvania. The worst nuclear accident to date occurred in 1986, when a runaway nuclear reaction at Chernobyl power plant near Kiev, USSR (modern-day CIS), led to a series of explosions that dispersed massive amounts of radioactive material throughout the Northern hemisphere. In 1999 a “criticality incident” occurred at the Tokai-Mura plant in Japan, causing the worst nuclear damage in that country. (See also section on Nuclear Accidents.)
The number of nuclear reactors in the world has grown steadily. By 1964 there were 14 reactors connected to electricity distribution systems worldwide. In 1970 there were 81; this number grew to 167 by 1975, to 365 by 1985, to 435 by 1995, and then decreased to 428 by 1999.
IV TYPES OF REACTOR
Most of the world’s reactors are located in nuclear power plants, the rest are research reactors or reactors used for propulsion of submarines and ships. Some designs can be re-fuelled while in operation, others need to be shut down to refuel. Several advanced reactor designs, which are simpler, more efficient, and inherently safer, are also under development.
There are two basic types of fission reactors: thermal reactors and fast reactors. In thermal reactors, the neutrons created in the fission reaction lose energy by colliding with the light atoms of the moderator until they can sustain the fission reaction. In fast reactors, “fast” neutrons sustain the fission reaction and a moderator is not needed. They require enriched fuel, but the fast neutrons can be used to convert U-238 into fissile material (plutonium), creating more nuclear fuel than the amount consumed. They can also be used to “burn” plutonium as a means of reducing the amount that is stockpiled.
For the purpose of electricity generation, there are five main categories of reactors, each comprising one or more types. Light Water Reactors include Pressurized Water Reactors (PWRs), together with the Russian VVER design, and Boiling Water Reactors (BWRs). Gas Cooled Reactors comprise Magnox reactors and Advanced Gas-Cooled Reactors (AGR), developed in the United Kingdom, as well as High-Temperature Gas-Cooled Reactors (HTGR). Pressurized Heavy Water Reactors include the CANDU reactor developed in Canada. Light Water Graphite Reactors comprise the RBMK reactors, developed in the USSR. Lastly, Fast Breeder Reactors include Liquid Metal Fast Breeder Reactors (LMFBR).
In the early 1950s, enriched uranium was only available in the United States and the USSR. For this reason, reactor development in the United Kingdom (Magnox), Canada (CANDU), and France was based on natural uranium fuel. The Russian RBMK design also used natural uranium fuel.
In natural uranium reactors, ordinary water cannot be used as the moderator, because it absorbs too many neutrons. In the successful CANDU design, this was overcome by using heavy water (deuterium oxide) for the moderator and coolant. Nearly all reactors in the United Kingdom have used a graphite moderator and carbon dioxide as the coolant.
In the United Kingdom, the Magnox reactors of the 1960s were followed by the AGRs, which used enriched fuel and were able to operate at higher temperatures and with greater efficiency. The Steam Generating Heavy Water Reactor (SGHWR) design was intended as the next technological step but this policy was changed in favour of the more established PWR design, of which many were already in operation. However, only one PWR was subsequently constructed in the United Kingdom, at Sizewell. Nuclear power generates about 25 per cent of the country’s electricity.
French researchers abandoned the design they had initially developed and embarked in the early 1970s on a nuclear power programme based totally on PWRs when French-produced enriched uranium became available. These now supply almost 80 percent of France’s electricity.
Worldwide 56 per cent of power reactors are PWRs, 22 percent are BWRs, 6 percent are pressurized heavy water reactors (mostly CANDUs), 3 percent are AGRs, and 23 percent are other types. Eighty-eight percent are fuelled by enriched uranium oxide, the rest by natural uranium, with a few light water reactors, also using mixed oxide fuel (MOX), which contains plutonium as well as uranium. Light water is the coolant/moderator for 80 per cent to 85 per cent of all reactors.
The most important factors to be considered for any type of nuclear reactor are safety; cost per kilowatt of generating the capacity to construct; cost per kilowatt delivered (to include fuel, operation, and downtime costs); operating lifetime; and decommissioning costs.
A Pressurized Water Reactor (PWR)
PWRs are normally fuelled with uranium oxide pellets in a zirconium cladding, although in recent years some mixed oxide fuel (MOX), which contains plutonium, has been used. The fuel is enriched to 3 percent U-235. The moderator is the ordinary water coolant, which is kept pressurized at about 150 bars to stop it boiling. It is pumped through the reactor core, where it is heated to about 325° C (about 620° F). The superheated water is pumped through a steam generator, where, through heat exchangers, a secondary loop of water is heated and converted to steam. This steam drives one or more turbine generators, is condensed, and pumped back to the steam generator. The secondary loop is isolated from the reactor core water and is therefore not radioactive. The third stream of water from a lake, river, the sea, or cooling tower is used to condense the steam. A typical reactor pressure vessel is 15 m (49 ft) high and 5 m (16 ft) in diameter, with walls 25 cm (10 in) thick. The core contains about 90 tonnes of fuel.
The PWR was originally designed by Westinghouse Bettis Atomic Power Laboratory for military ship applications, then by Westinghouse Nuclear Power Division for commercial applications. The Soviet-designed VVER (Veda-Vodyanoi Energetichesky Reaktor) design is similar to Western PWRs but has different steam generators and safety features.
B Boiling Water Reactor (BWR)
The BWR is simpler than the PWR but less efficient in its fuel use and has a lower power density. Like the PWR, it is fuelled by uranium oxide pellets in a zirconium cladding, but slightly less enriched. The moderator is the ordinary water coolant, which is kept at lower pressure (70 bars) so that it boils within the core at about 300° C. The steam produced in the reactor pressure vessel is piped directly to the turbine generator, condensed, and then pumped back to the reactor. Although the steam is radioactive, there is no intermediate heat exchanger between the reactor and turbine to decrease efficiency. As in the PWR, the condenser cooling water has a separate source, such as a lake or river.
The BWR was originally designed by Allis-Chambers and General Electric (GE) of the United States. The GE design has survived, and other versions are available from ASEA-Atom, Kraftwerk Union, and Hitachi.
C Gas-Cooled Reactors
Magnox reactors take their name from the magnesium-based alloy used as cladding for the natural uranium metal fuel. The moderator is graphite and the carbon dioxide coolant is circulated through the core at a pressure of about 27 bars, exiting at about 360° C. The heat is transferred to the secondary water loop, in which steam is raised to drive the turbine generators. Early units had a steel pressure vessel with the steam generators outside the containment. Later versions had a concrete pressure vessel containing the core and the steam generators. Magnox reactors are a British design but were also built in Tokai-Mura (Japan) and Latina (Italy).
The Advanced Gas-Cooled Reactor (AGR) is a development of the Magnox design using uranium oxide fuel enriched to 2-3 percent U-235 and clad in stainless steel or zircaloy. The moderator is graphite and the carbon dioxide coolant circulates at about 40 bars, exiting the core at 640° C. The heat is transferred to the secondary water loop, in which steam is raised to drive the turbine generators. A concrete pressure vessel is used, with walls about 6 m (20 ft) thick. AGRs are unique to the UK.
High-Temperature Gas-Cooled Reactors (HTGRs) are largely experimental. The fuel elements are spheres made from a mixture of graphite and nuclear fuel. The German version has the fuel loaded in a silo, the US version loads the fuel into hexagonal graphite prisms. The coolant is helium, pressurized to about 100 bars, circulated through the interstices between the spheres or through holes in the graphite prisms. An example of this type is described in the Advanced Reactors section later in this article.
D Pressurized Heavy Water Reactor
The most widely used reactor of this type is the Canadian CANDU (Canadian Deuterium Uranium Reactor). The moderator and coolant are heavy water (deuterium oxide) and the fuel consists of natural uranium oxide pellets in zircaloy tubes. These are contained in pressure tubes mounted horizontally through a tank of heavy water called the “calandria”, which acts as the moderator. This feature avoids the need for a pressure vessel and facilitates on-load refuelling. The heavy water coolant is pumped through the pressure tubes at 110 bar and exits at about 320° C. The heat is transferred to the secondary water loop, in which steam is raised to drive the turbine generators.
The CANDU was designed by Atomic Energy of Canada Limited (AECL) to make the best use of Canada’s natural resources of uranium without needing enrichment technology, although requiring heavy water production facilities. In total, 21 CANDUs have been built, 5 of them outside of Canada.
E Light Water Graphite Reactor
The Soviet-designed Reaktor Bolshoi Moshchnosty Kanalyny (RBMK) is a pressurized water reactor with individual fuel channels. The moderator is graphite, the coolant—ordinary water, and the fuel—enriched uranium oxide. The fuel tubes and coolant tubes pass vertically through a massive graphite moderator block. This is contained in a pressure vessel and filled with helium-nitrogen mixture to improve heat transfer and prevent oxidation of the graphite. The coolant is maintained at 75 bar and exits at up to 350° C. The water is permitted to boil and the steam, after removal of water, is fed to the turbine generators.
Following the 1986 Chernobyl disaster, the design weaknesses of the RBMK were recognized and modifications made to help overcome them. The last operating reactor at the Chernobyl site was closed down in December 2000, and others will eventually be phased out.
F Fast Breeder Reactor
The Liquid Metal Fast Breeder (LMFBR) uses molten sodium as the coolant and runs on fuel enriched with U-235. Instead of a moderator being employed, the core is surrounded by a reflector, which bounces neutrons back into the core to help sustain the chain reaction. A blanket of “fertile” material (U-238) is included above and below the fuel, to be converted into fissile plutonium by capture of fast neutrons. The core is compact, with a high power density. The molten sodium primary coolant transfers its heat to a secondary sodium loop, which heats water in a third loop to raise steam and drive the turbine generators.
Development of fast reactors proceeds only in France, India, Japan, and Russia. The only commercial power reactors of this type are in Kazakhstan and Russia. The British fast reactor, which generated 240 megawatts, was closed down in the 1990s and is being decommissioned.
G Propulsion Reactors
Propulsion reactors are used to propel military submarines and large naval ships such as the aircraft carrier USS Nimitz. The US, UK, Russia, and France all have nuclear powered submarines in their fleets. The basic technology of the propulsion reactor was first developed in the US naval reactor programme directed by Admiral Hyman George Rickover. Submarine reactors are generally small, with compact cores and highly enriched uranium fuel.
The former USSR built the first successful nuclear-powered icebreaker Lenin for use in clearing the Arctic sea lanes. Three experimental nuclear powered cargo ships were operated for limited periods by the US, Germany, and Japan. Although technically successful, economic conditions and restrictive port regulations brought an end to these projects.
H Research Reactors
A variety of small nuclear reactors has been built in many countries for use in education, training, research, and production of radioactive isotopes for medical and industrial use. These reactors generally operate at power levels near 1 MW and are more easily started up and shut down than large power reactors.
A widely used type is the swimming-pool reactor. The core consists of partially or fully enriched U-235 contained in aluminium alloy plates immersed in a large tank of water that serves as both coolant and moderator. Materials to be irradiated with neutrons may be placed directly in or near the reactor core. This process is used to make radioactive isotopes for medical, industrial, and research use (see also Isotopic Tracer). Neutrons may also be extracted from the reactor core and directed along beam tubes for use in experiments.
I Advanced Reactors
Several new designs are under development around the world which are simpler, more efficient in their utilization of fuel, cheaper to build and operate, and inherently safer. They typically include passive safety features that avoid relying on pumps and valves, along with increased time for operators to respond to abnormal situations.
Some have evolved from established designs, taking into account the lessons learned from operating experience over the years and advances in fuel design for increased “burnup”. Others represent greater departures from established designs and would require a demonstration unit to be constructed before being used commercially. The cost and technical demands of these projects mean that national or international collaboration is usually necessary.
Projects are currently under way in Canada, France, Germany, Japan, Russia, the US, and South Africa. They fall into the three categories of water-cooled reactors, fast reactors, and gas-cooled reactors. Capacities cover all ranges—small, medium, and large (1,000 MW and above). The large capacity Advanced Boiling Water Reactor (ABWR) design is already in commercial operation in Japan. Others are under construction or on hold, awaiting favourable economic circumstances; the rest are still on the drawing board.
As an example of a design not based on existing commercial units, the South African Pebble Bed Modular Reactor (PMBR) is due to begin construction in 2001 and should be in commercial operation by 2005. It is a High Temperature Gas-Cooled Reactor (HTGR) of 110 MW capacity per module and is fuelled by several hundred thousand graphite-uranium oxide pebbles, each the size of a tennis ball. The helium coolant passes through a gas turbine to drive electrical generation with high efficiency and returns to the reactor in a closed loop. Each pebble passes through the reactor about ten times before needing to be replaced, which is carried out continuously without shutting the reactor down. Four modules would fit inside a football stadium and the design lifetime is 40 years.
Reactors have to be approved and certificated by the national safety regulatory authority before they can be used in a nuclear power station. International certification of reactors, as with new aircraft, is some way in the future.
J Fusion Reactors
Nuclear fusion is the process that powers the Sun, and for several decades people have looked at it as the answer to energy problems on Earth. However, the technological problems are complex, and a fusion power plant has not yet been built. (In 2005 agreement was reached between China, the European Union (EU), Japan, South Korea, Russia, and the US on the building of the world’s first nuclear fusion power plant at Cadarache, in southern France. The International Thermonuclear Experimental Reactor (ITER), as it is to be known, is scheduled to be in operation by 2016.) Fundamentally, any useful fusion reactor needs to confine plasma at a high enough density for sufficient time to generate more energy than the energy which was put in to create and confine the plasma. This occurs when the product of the confinement time and the density of the plasma, known as the Lawson number, is 1014 or above.
Numerous schemes for magnetic confinement of plasma have been tried since 1950. Thermonuclear reactions have been observed but the Lawson number has rarely exceeded 1012. The Tokamak device, originally suggested in the USSR by Igor Tamm and Andrei Sakharov, began to give encouraging results in the 1960s.
The confinement chamber of a Tokamak has the shape of a torus (doughnut), with a minor diameter of about 1 m (3 ft 4 in) and a major diameter of about 3 m (9 ft 9 in). A toroidal magnetic field of about 5 tesla is established inside this chamber by large electromagnets. This is about 100,000 times the Earth’s magnetic field at the planet’s surface. A longitudinal current of several million amperes is induced in the plasma by the transformer coils that link the torus. The resulting magnetic field lines are spirals in the torus and confine the plasma.
Following the successful operation of small Tokamaks at several laboratories, two large devices were built in the early 1980s, one at Princeton University in the US and one in the USSR. In the Tokamak, high plasma temperature naturally results from resistive heating by the very large toroidal current, and additional heating by neutral beam injection in the new large machines should result in ignition conditions.
Another possible route to fusion energy is that of inertial confinement. In this technique, the fuel (tritium or deuterium) is contained within a tiny pellet that is bombarded on several sides by a pulsed laser beam. This causes an implosion of the pellet, setting off a thermonuclear reaction that ignites the fuel. Several laboratories in the US and elsewhere are currently pursuing this possibility.
A significant milestone was achieved in 1991 when the Joint European Torus (JET) in the UK produced for the first time a significant amount of power (about 1.7 million watts) from controlled nuclear fusion. And in 1993 researchers at Princeton University in the US used the Tokamak Fusion Test Reactor (TFTR) to produce 5.6 million watts. However, both JET and TFTR consumed more energy than they produced in these tests.
There has been promising progress in fusion research around the world for several decades; however, it will take decades more to develop a practical fusion power plant. It has been estimated that an investment of US $50-100 billion is needed to achieve this, but each year only US $1.5 billion is being spent worldwide. The main areas where work is needed include superconducting magnets; vacuum systems; cryogenic systems; plasma purity, heating, and diagnostic systems; sustainment of plasma current; and safety issues.
The JET project has achieved “breakeven” operation, where the fusion power generated exceeds the input power, but only by injecting tritium that has made the structure radioactive. ITER is scheduled to begin by 2016. A demonstration fusion power plant would be built about 15 years later and, if successful, commercial fusion power plants could be operating by about 2050. This timescale could be significantly delayed or accelerated by the rate of progress in understanding plasma behaviour and by the rate of funding.
If fusion energy does become practicable it would offer the following advantages: (1) an effectively limitless source of fuel—deuterium from the ocean; (2) inherent safety, since the fusion reaction would not “run away” and the amount of radioactive material present is low; and (3) waste products that are less radioactive and simpler to handle than those from fission systems. However, the structure will become radioactive due to absorption of neutrons, so decommissioning will be a serious undertaking.
V THE NUCLEAR FUEL CYCLE
Nuclear power is based on uranium, a slightly radioactive metal that is relatively abundant (about as common as tin and 500 times as abundant as gold). Thorium is also usable as a nuclear fuel, but there is no economic incentive to exploit it at present. Economically extractable reserves at current low world prices amount to just 4.4 million tonnes, from the richer ores. At the current world usage rate of 50,000 tonnes per annum this would last only another 80 years. But if prices were to rise significantly, the usable reserves would increase to the order of 100 million tonnes. And if prices were to rise to several hundred dollars per kilogram, it may become economic to extract uranium from seawater, in which it is present at about 3 mg per tonne. This would be a sufficient supply for a greatly enlarged industry for several centuries.
A Uranium Production
The world’s uranium reserves are mostly located in Australia (35 percent), countries of the former USSR (29 percent), Canada (13 percent), Africa (8 percent), and South America (8 percent). In terms of production, Canada (33 percent) is followed by Australia (15 per cent) and Nigeria (10 per cent). Other producers are Kazakhstan, Namibia, Russia, South Africa, the US, and Uzbekistan.
Uranium ore contains about 1 percent uranium. It is mined either by open-pit or deep-mining techniques and milled (crushed and ground) to release the uranium minerals from the surrounding rock. The uranium is then dissolved, extracted, precipitated, filtered, and dewatered to produce a uranium ore concentrate called “yellowcake” which contains about 60 percent uranium. This has a much smaller volume than the ore and hence is less expensive to transport. It is either shipped to the fuel enrichment plant or, alternatively, to the fuel fabrication plant if it is not to be enriched.
The yellowcake is converted to uranium hexafluoride (UF6), which is a gas above 50° C and is used as the feedstock for the enrichment process. Because most reactors require more than the 0.7 percent natural concentration of U-235, some of the U-238 needs to be removed to give a concentration of 3 percent U-235 or thereabouts.
Enrichment is carried out using either the gaseous diffusion process or the newer gas centrifuge process. A laser process is also under development. The gas centrifuge process requires only 5 percent of the energy to separate the same amount of U-235 as the diffusion process, although diffusion plants are still dominant worldwide.
C Fuel Fabrication
The enriched UF6 is converted to uranium dioxide in the form of a ceramic powder. This is pressed and then sintered in a furnace to produce a dense ceramic pellet. Pellets are welded into fuel rods and combined into fuel assemblies, which are then transported to the nuclear power station for loading into the reactor.
Plutonium oxide may also be mixed with the uranium oxide to make mixed oxide fuel (MOX), as a means of reducing the amount of stockpiled plutonium (although not the total amount in circulation) and avoiding the need to enrich the uranium. MOX fuel is manufactured at the reprocessing plant where the plutonium is held and is increasingly being used in light water reactors, up to a maximum of about 30 percent of the fuel in a PWR. Because spent MOX fuel is highly radioactive, the plutonium is unlikely to be illegally diverted into the manufacture of nuclear weapons.
D Power Generation
The fuel assemblies are loaded into the reactor in a planned cycle to “burn” the fuel most efficiently. The “burnup” is expressed as gigawatt-days per tonne (GWd/te) of uranium. The early Magnox stations achieved 5 GWd/te but by the late 1980s, PWRs and BWRs were achieving 33 GWd/te. Figures of 50 GWd/te are now being achieved, and this is forecast to increase.
E Spent Fuel
The fuel elements are removed from the reactor when they have reached the design burnup level, typically after four years. At this point, they are intensely radioactive and generate a lot of heat, so the spent fuel is placed in a cooling pond adjacent to the reactor. The water (which is dosed with boric acid to absorb neutrons and prevent a chain reaction) acts as a radiation shield and coolant. The fuel elements remain there for at least five months until the radioactivity has decayed enough to permit them to be transported.
Where the fuel is to be reprocessed, it is transported in shielded flasks by rail or road to the reprocessing plant. Where this is not the case, it will remain in the cooling pond. Older ponds were designed to accommodate up to ten years’ worth of spent fuel but may be able to accommodate more by removing older fuel into dry storage facilities. But ultimately the spent fuel will need to be sent for permanent disposal if it is not to be reprocessed.
The spent fuel is typically made up of non-fissile U-238 (about 95 percent), fissile U-235 (about 0.9 percent), various highly radioactive fission products, and a mixture of plutonium isotopes (more than half of which are fissile). Reprocessing separates the uranium and plutonium from the waste and was historically carried out to recover plutonium for manufacture of nuclear weapons. In the UK this was also carried out to deal with the magnesium alloy Magnox fuel casings, which are eventually corroded by the water in the cooling pond and are not suitable for dry storage. The recovered U-235 is used for the manufacture of new fuel, and the plutonium can be used for the manufacture of MOX fuel (see Fuel Fabrication above), although the majority is stockpiled at present.
The spent fuel received from the nuclear power station is stored in a cooling pond and then mechanically cut up. In the commonly used Purex process, the fuel is dissolved in nitric acid and then the uranium, plutonium, and fission products are separated by solvent extraction using a mixture of tributyl phosphate and kerosene. The uranium goes to fuel fabrication and the plutonium is either stored or used for MOX fuel production. The fission products are separated into a liquid stream, which is processed with glass-making materials into a vitrified high-level waste (HLW) product. Other liquids and solid waste streams are also generated, and these are discussed in the section on radioactive waste management later in the article.
Reprocessing in the civil nuclear industry is a contentious and complex issue. Between 1976 and 1981 it was not carried out in the United States due to concerns that plutonium could be illegally diverted into the manufacture of nuclear weapons (although now permitted, it has not been resumed). Instead, a “once through” policy for nuclear fuel is followed, with spent fuel regarded as waste destined for permanent disposal. The UK, France, Japan, and Russia have reprocessing plants and all are busy reducing their stock of nuclear weapons (apart from Japan, which has none), so the amounts of stored plutonium are increasing. Options for handling plutonium include “burning” it in a fast reactor, or using it up as MOX fuel followed by disposal of the spent fuel. As well as the plutonium issue, decision-making factors include the economics of the process and national perceptions of future energy needs.
Uranium concentrate, new nuclear fuel, spent fuel, and radioactive waste are transported by rail, road, ship, and air in packages designed to prevent the release of radioactive material under all foreseeable accident scenarios. The most radioactive items such as spent fuel or vitrified high level waste are transported in extremely rugged “flasks” or “casks”, which will typically have undergone high-speed impact tests and fire tests to demonstrate their integrity.
VI RADIOACTIVE WASTE MANAGEMENT
Nuclear power stations, reprocessing plants, fuel fabrication plants, uranium mines, and all other nuclear facilities produce solid and liquid wastes of varying characteristics and amounts. These are internationally classified as high level waste (HLW), intermediate level waste (ILW), and low level waste (LLW).
A typical 1000 MW nuclear power station produces about 300 cu m of LLW and ILW waste each year, of which 95 percent would be classified as LLW. It also produces about 30 tonnes of spent fuel, classified as HLW. In comparison, a coal-fired power station of the same capacity would produce 300,000 tonnes of ash per year, containing a very large amount of radioactivity and toxic heavy metals, which would be dispersed into landfill sites and the atmosphere. Worldwide, about 200,000 cu m of low and intermediate waste are produced from nuclear power stations each year, together with 10,000 cu m of HLW (primarily spent fuel).
Wastes of lower activity are also produced, including very low level waste from most nuclear facilities which can be disposed of in normal municipal waste disposal sites without special precautions. Uranium mines and mills produce large volumes of waste containing low concentrations of radioactive and toxic materials, which are handled by normal mining techniques such as tailings dams. The enrichment process produces depleted uranium, primarily consisting of U-238, which is slightly radioactive and requires some precautions for safe disposal.
A High Level Waste
This is highly radioactive, heat generating, long-lived material, which will remain biologically hazardous for thousands of years. The spent fuel from nuclear power plants, destined for permanent disposal, is classified as HLW, as is the concentrated liquid waste generated by reprocessing. The 30 tonnes of spent fuel produced each year by a typical power station will, after ten years, still produce a power of several hundred kilowatts and cooling will be necessary for about 50 years overall. For final disposal of spent fuel, the fuel rods would be removed from their assemblies and repacked in a dense lattice within a corrosion-resistant steel canister. A cover would be welded on and the canister covered with an overpack. However, this is not yet carried out (see section on Disposal below), and some countries (notably Russia) are reluctant to dispose of spent fuel because, if reprocessed, it is an energy resource.
Reprocessing one tonne of spent fuel produces about 0.1 cu m of radioactive liquid, containing about 99 percent of the fission product radioactivity. The liquid is stored in tanks with multiple cooling systems designed to remove the heat produced by the radioactive decay, and after several tens of years it can be processed for final disposal. For example, the vitrification process operated at the Sellafield reprocessing plant in England converts the liquid to a stable, solid form by turning it into a borosilicate glass (referred to as vitrified high level waste, VHLW) in a stainless steel container suitable for long-term storage and final disposal. Processes based on other immobilization technologies are in development elsewhere, such as the SYNROC process in Australia.
B Intermediate Level Waste
This consists of solid and liquid materials such as fuel cladding, contaminated equipment, sludges, evaporator concentrates, and spent ion-exchange resin. This material is not sufficiently radioactive to require cooling. Reprocessing one tonne of spent fuel produces about 1 cu m of ILW, containing about 1 per cent of the radioactivity in the fuel.
Various processes for retrieval, volume reduction, incineration, conditioning, and immobilization of ILW to convert it to stable, solid forms (usually based on cement, but also polymers and bitumen) are operated at power stations and reprocessing plants. The final product is typically contained in a drum, suitable for long-term storage or final disposal.
C Low Level Waste
This consists of trace-contaminated used protective clothing, gloves, contaminated rags, filters, and the like, and also larger items of lightly contaminated equipment. Brazil nuts and coffee beans contain as much natural radioactivity as typical LLW. Reprocessing one tonne of spent fuel produces about 4 cu m of LLW, containing about 0.001 percent of the radioactivity in the fuel. Size reduction techniques include shearing, shredding, and compaction. The waste is grouted into containers called “overpacks” to produce a stable waste form suitable for final disposal.
About 40 near-surface disposal sites for LLW have been in operation for over 30 years in countries with nuclear power industries, and another 30 are expected to come into operation in the next 15 years. They typically have concrete-lined trenches, an impervious cap, and systems for collecting water from the base of the trenches.
The intention in all countries with nuclear power industries is eventually to dispose of ILW and HLW in deep underground repositories, where the long-lived radioactive isotopes will be segregated for more than 100,000 years by a combination of engineered and natural barriers. Development and selection of final disposal sites is under way in all countries where they will be needed, although the rate of progress is generally slow due to the need to obtain public acceptance and address the main issues, which have been identified as transport of radioactivity in groundwater; migration of radioactivity in gas generated by the waste; natural disruptive events and inadvertent human intrusion; and the question of whether or not the waste should be retrievable. Meanwhile, ILW and HLW are stored at the sites where they are produced, which can generally be continued for 50 years or more.
As an alternative to constructing a series of national repositories (where geological conditions may be less than ideal), it has been proposed that waste should be transported to disposal sites in sparsely populated and more geologically suitable areas of the world such as Western Australia or the Gobi Desert. This, however, remains contentious.
A number of countries used to dispose of radioactive waste by dumping at sea. This practice is now discontinued following the London Convention of 1983; however, disposal of deep-sea sediments several hundred metres below the bottom of the sea in water depths of at least 4,000 m (13,123 ft) is a potentially attractive option where it is not envisaged that the waste would ever need to be retrieved. This option would require a large international collaborative effort to develop.
E Return of Wastes
The policy of some countries with small-scale nuclear power industries is to return spent fuel to the foreign supplier of the fuel. And the policy of European reprocessing plants is eventually to return the wastes arising from large-scale reprocessing of spent fuel to the country where the spent fuel came from. For example, vitrified HLW is shipped from the reprocessing plant at Cap la Hague in France back to Japan.
F Liquid Discharges
The liquid effluents generated by nuclear power stations, reprocessing plants, and other nuclear facilities are treated by a variety of efficient processes to remove radioactivity. Stringent limits are set for each site, radiation levels in discharge streams are monitored, and efforts are made to improve year on year. Any residual radioactivity in the effluent will generally end up in the sea where its uptake by “critical groups” (those most likely to receive the radiation) can be estimated. At the 1998 Oslo-Paris Commission (OSPAR) meeting in Portugal, the EU member states committed themselves to reduce discharges of radioactivity to the point where additional concentrations above background levels are close to zero.
Reprocessing effluents presents the greatest challenge. Techniques such as sand bed filtration, ion exchange, neutralization of acidic effluents to precipitate solids, removal of solids by hydrocyclone or ultrafiltration, and alkaline hydrolysis of organic solvent are used to clean the effluents until they can be discharged to sea. The separated radioactive material is processed as ILW, as described above.
G Aerial Discharges
The radioactive gases that are discharged are subject to similar limits and monitoring as liquid effluents to ensure the minimum uptake by “critical groups”.
VII NUCLEAR SAFETY
Before discussing the safety issues surrounding nuclear power it is necessary to understand the basics of radiation.
A Introduction to Radiation
Heat and light are types of radiation that people can feel or see, but we cannot detect ionizing radiation in this way (although it can be measured very accurately by various types of instrument). Ionizing radiation passes through matter and causes atoms to become electrically charged (ionized), which can adversely affect the biological processes in living tissue.
Alpha radiation consists of positively charged particles made up of two protons and two neutrons. It is stopped completely by a sheet of paper or the thin surface layer of the skin; however, if alpha-emitters are ingested by breathing, eating, or drinking they can expose internal tissues directly and may lead to cancer.
Beta radiation consists of electrons, which are negatively charged and more penetrating than alpha particles. They will pass through 1 or 2 centimetres of water but are stopped by a sheet of aluminium a few millimetres thick.
X-rays are electromagnetic radiation of the same type as light, but of much shorter wavelength. They will pass through the human body but are stopped by lead shielding.
Gamma rays are electromagnetic radiation of shorter wavelength than X-rays. Depending on their energy, they can pass through the human body but are stopped by thick walls of concrete or lead.
Neutrons are uncharged particles and do not produce ionization directly. However, their interaction with the nuclei of atoms can give rise to alpha, beta, gamma, or X-rays, which produce ionization. Neutrons are penetrating and can be stopped only by large thicknesses of concrete, water, or paraffin.
Radiation exposure is a complex issue. We are constantly exposed to naturally occurring ionizing radiation from radioactive material in the rocks making up the Earth, the floors and walls of the buildings we use, the air we breathe, the food we eat or drink, and in our own bodies. We also receive radiation from outer space in the form of cosmic rays.
We are also exposed to artificial radiation from historic nuclear weapons tests, the Chernobyl disaster, emissions from coal-fired power stations, nuclear power plants, nuclear reprocessing plants, medical X-rays, and from radiation used to diagnose diseases and treat cancer. The annual exposure from artificial sources is far lower than from natural sources. The dose profile for an “average” member of the UK population is shown in the table above, although there will be differences between individuals depending on where they live and what they do (for example, airline pilots would have a higher dose from cosmic rays and radiation workers would have a higher occupational dose).
B Radiation Effects and Dose Limits
Large doses of ionizing radiation in short periods of time can damage human tissues, leading to death or injury within a few days. Moderate doses can lead to cancer after some years. And it is generally accepted that low doses will still cause some damage, despite the difficulty in detecting it (although there is a body of opinion that there exists a “threshold” below which there is no significant damage). There is still no definite conclusion as to whether exposure to the natural level of background radiation is harmful, although damaging effects have been demonstrated at levels a few times higher.
Absorbed radiation dose is measured in sieverts (Sv), although doses are usually expressed in millisieverts (mSv). One chest X-ray gives a dose of about 0.2 mSv. The natural background radiation dose in the UK is about 2.5 mSv per annum, although it doubles in some areas, and in certain parts of the world, it may reach several hundred mSv. A dose of 5 Sv (that is, 5,000 mSv) is likely to be fatal.
Basic principles and recommendations on radiation protection are issued by the International Commission on Radiological Protection (ICRP) and used to develop international standards and national regulations to protect radiation workers and the general public. The basic approach is consistent all over the world. Over and above the natural background level, the dose limit for a radiation worker is set at 100 mSv per year averaged over five years, and 1mSv per year over five years for a member of the general public. Doses should always be kept as low as reasonably achievable, and the limits should not be exceeded.
In the UK the recommended maximum annual dose for a radiation worker is set at 20 mSv (although higher limits may apply elsewhere in the world) and the typical annual dose for a radiation worker would be controlled to less than 1.5 mSv. However, some may receive more than 10 mSv, and a few may approach the annual limit.
C Ensuring Nuclear Safety
In common with all hazardous industrial activities, the risk of major nuclear accidents is minimized at power stations and reprocessing plants by means of multiple levels of protection. In order of importance, engineered systems are provided for prevention, detection, and control of any release of radioactive material. Escape and evacuation of people on site and nearby are available as the last resort. Sophisticated analysis is carried out to evaluate the effect of the protective systems in all foreseeable accident scenarios and to demonstrate that the risk of failure is sufficiently low.
For example, for a major release of radioactivity from a modern nuclear power station there would have to be a whole series of failures. The primary cooling system would have to fail, followed by the emergency cooling system, then the control rods, then the pressure vessel, and finally the malfunction of the containment building before significant amounts of radioactivity could be released.
The safety record of the nuclear industry worldwide over the last 45 years has been generally good, with the exception of the Windscale (Sellafield) fire in 1957 (which actually happened with a military plutonium production reactor rather than a power reactor), the Three Mile Island accident in 1979, the Chernobyl disaster of 1986, and the most recent accident, at Tokai-Mura in Japan in 1999, all of which are discussed in more detail in the next section. However, there have also been a number of incidents at nuclear power stations and reprocessing plants over the years, which resulted in severe damage and/or had the potential to escalate into major accidents, and should, therefore, be classified as “near misses”.
Lessons have been learned. While safety relies on the design (that is, engineered safety systems), just as importantly it depends on how the reactor is operated. Improvements have been made to reactor control systems in response to Three Mile Island and Chernobyl but without trained and competent operators following valid procedures there remains the possibility of a major nuclear accident. And there are 12 RBMK reactors of the same type as the Chernobyl device still in operation, which have an inherently less safe design than any other type of nuclear power reactor.
D Nuclear Accidents
There have been four particularly severe nuclear accidents in the past 45 years, which released, or almost released, large amounts of radioactivity.
The 1957 Windscale fire was the worst nuclear accident in UK history. It happened when an early plutonium production reactor (with few safety systems) caught fire and is not representative of modern nuclear power reactors.
The 1979 core meltdown at the Three Mile Island PWR was the worst nuclear accident in US history. The disaster was largely contained but happened because of deficiencies in the control system and incorrect responses by the operators when abnormal circumstances arose initially, which then escalated into a far worse situation.
The 1986 Chernobyl disaster was the worst nuclear accident in history. It was caused by the operators carrying out an unauthorized and previously untried procedure on an RBMK reactor that involved them disabling a number of safety devices. This led to the reactor becoming unstable and eventually exploding. In the years following the accident over 30 people (mainly firefighters) died from radiation exposure. A further 300 workers and firefighters suffered radiation sickness (those who were sent in to clean up the plant following the explosion were later found to have been at a significantly increased risk of lung cancer) and almost 2,000 people in the surrounding area who were children at the time have developed thyroid cancer (which is, fortunately, treatable, and so few have died), with more cases expected. Massive amounts of radioactive material were dispersed throughout the Northern hemisphere.
In recent years confidence in Japan’s nuclear industry has been shaken by a number of serious accidents. In 1999 a “criticality incident” occurred at the Tokai-Mura nuclear plant. There was a sustained burst of neutrons caused by a chain reaction that was triggered when operators carried out a prohibited procedure while manufacturing highly enriched fuel (15 per cent to 20 per cent U-235) for an experimental fast reactor. There were a small number of fatalities (those closest to the incident, which is typical of criticality accidents). People living in the surrounding area were irradiated with neutrons for some hours. In 2004, four people were killed and seven injured at a plant in Mihama, Western Japan, when a corroded pipe exploded, covering workers with scalding water that caused severe burns. Although officials insisted that there had been no radiation leak from the plant and that there was no danger to the surrounding area, the casualties were the highest in Japan’s history of nuclear power.
VIII NUCLEAR POWER TODAY AND TOMORROW
Worldwide, there are about 430 power reactors operating in 25 countries, providing about 17 percent of the world’s electricity. Of these 56 percent are PWRs, 22 percent are BWRs, 6 percent are pressurized heavy water reactors (mostly CANDUs), 3 percent are AGRs, and 23 percent are other types. In all, 88 percent are fuelled by enriched uranium oxide, the rest by natural uranium. A few light water reactors also use mixed oxide fuel (MOX) and this is likely to increase, partly as a way to dispose of the growing stocks of military plutonium. The number of fast breeder reactors (FBRs) has reduced with the closure of FBR programmes in several countries.
Nuclear power and hydro-electric power together provide 36 per cent of world electricity: neither put carbon dioxide into the atmosphere. In both cases the technology is mature. The new renewable technologies hardly appear in the statistics but, with financial support, they are starting to make their presence felt. It is unlikely, however, that renewables will ever provide more than 20 percent of world energy. In a world greedy for energy, where oil is already beginning to be supply-constrained and gas will follow by 2010, concerns about the security of energy supply are now being voiced. In the US, where power blackouts are prevalent in some states, life extension of nuclear stations is being implemented urgently to ensure supply despite pressures from the environmental movement to phase out nuclear power.
Life extension of nuclear reactors in the UK has been very successful but current policy appears to be to retire stations at the end of their useful lives and replace them with gas-fired stations. However, replacement by gas brings a huge carbon dioxide penalty which will derail the UK’s Kyoto Protocol’s obligations. The much more stringent post-2010 requirements that have been called for by the Royal Commission on Environmental Pollution require a 60 percent carbon dioxide reduction by 2050 but have virtually no chance of success without a nuclear input. Sweden and Germany are following a similar nuclear closure route.
Nuclear power construction is on the plateau or in decline in some developed countries and consequently, the teams of experienced nuclear engineers have been dispersed (despite the 38 new nuclear power plants currently under construction). Some countries, such as the UK, have lost the capacity to build a nuclear power station. University departments teaching nuclear technology have all but disappeared, which could be a limiting factor on new nuclear construction and will take the time to change.
Nuclear power is generally not discussed by politicians with the exception of France, some Far Eastern countries, China, and Russia. The EU energy commissioner Loyola de Palacio said, on November 7, 2000: “From the environmental point of view nuclear energy cannot be rejected if you want to maintain our Kyoto commitments.” She went on to imply she wanted nuclear power to be part of the Kyoto Protocol’s Clean Development Mechanism.
B The Short-Term Future
Attention is beginning to move towards building new nuclear power stations. Looking ahead to a doubling of energy demand by 2050, and with the world now trying to reduce its use of fossil fuels in order to contain carbon dioxide emissions, it is difficult to see how this can be achieved without a substantial increase in nuclear power. Nevertheless, the public perception of the industry in many countries is that it is more dangerous than other forms of energy and the problems of storing nuclear waste have not been fully solved (though there is considerable evidence to counter this argument).
A new generation of advanced reactors is being developed, which are more fuel efficient and inherently safer, with passive safety systems. The new designs are based on accumulated experience derived from operating PWRs and BWRs. Advanced boiling water and pressurized water reactors are already operating and the smaller AP 600 Westinghouse design has been certificated (as already mentioned, global certification, as with new aircraft, will be essential to get new designs into production). The European pressurized water reactor is available for construction, a number of liquid and gas cooled fast reactor systems have been designed, and some prototypes constructed. These new designs will produce electricity more cheaply than coal-fired stations and than gas-generated electricity (if gas prices continued to increase), and probably also more cheaply than renewable electricity (and with better availability).
Interest in high-temperature gas-cooled reactors (HTGRs) using helium at 950o C has been revived, particularly in Japan and China, and a Pebble Bed Modular Reactor (PBMR) with direct cycle gas turbine generator is being developed in South Africa.
The use of nuclear reactors to generate process heat is an important development, particularly if the heat is used for desalination. An integrated nuclear reactor producing electricity and clean water could produce water at between 0.7 dollars and 1.1 dollars per cubic metre. There is considerable interest in this technology from North Africa, the Arabian Peninsula states, Turkey, and northern China.
C Further Ahead
The existing types of nuclear reactor are not particularly efficient in their use of uranium. An alternative is the fast breeder reactor (FBR), which uses uranium some 60 times more efficiently than today’s PWRs and BWRs, although it is more expensive and is not yet a mature technology. Russian scientists have successfully operated the BS 600 fast reactor for 18 years with over 75 per cent availability. FBRs in other countries have been less successful and they eventually closed down because it was thought that the technology would not be required for 30 years and uranium and plutonium are readily available at the present time. In the long term, fast reactor technology could effectively increase world energy resources by a factor of ten and its time will no doubt come unless nuclear fusion can be engineered into a power station. Research on fusion continues, with the time horizon constantly receding, but it is expected that the prize will be worth the effort.
The world is poised to make much more use of nuclear power, provided the public perception that nuclear power is too dangerous to contemplate ultimately alters. It is possible that destabilization of weather systems, resulting from global warming, may persuade people that nuclear power is the lesser of two evils.
The biggest drivers for new nuclear construction are the security of supply, the steadily increasing prices of natural gas and oil, the likely interruptions of gas and oil supply for political reasons, and the absence of carbon dioxide emissions from nuclear stations.
The way ahead seems to be represented by an increasing mixture of nuclear power and renewable energy.
Nicholas S. Fells