Nuclear Power

I

Introduction

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 per cent 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 expansion of nuclear power are 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 per cent 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 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 per cent of France’s electricity.

Worldwide 56 per cent of power reactors are PWRs, 22 per cent are BWRs, 6 per cent are pressurized heavy water reactors (mostly CANDUs), 3 per cent are AGRs, and 23 per cent are other types. Eighty-eight per cent 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 capacity to construct; cost per kilowatt delivered (to include fuel, operation, and downtime costs); operating lifetime; and decommissioning costs.