nuclear reactor

A pressurized water nuclear power station. Water at high pressure is circulated around the reactor vessel where it is heated. The hot water is pumped to the steam generator where it boils in a separate circuit; the steam drives the turbines coupled to the electricity generator. This is the most widely used type of reactor. More than 20 countries have pressurized water reactors.

Device for producing nuclear energy in a controlled manner. There are various types of reactor in use, all using nuclear fission. In a gas-cooled reactor, a circulating gas under pressure (such as carbon dioxide) removes heat from the core of the reactor, which usually contains natural uranium. The efficiency of the fission process is increased by slowing neutrons in the core by using a moderator such as carbon. The reaction is controlled with neutron-absorbing rods made of boron. An advanced gas-cooled reactor (AGR) generally has enriched uranium as its fuel. A water-cooled reactor, such as the steam-generating heavy water (deuterium oxide) reactor, has water circulating through the hot core. The water is converted to steam, which drives turbo-alternators for generating electricity. The most widely used reactor is the pressurized-water reactor (PWR), which contains a sealed system of pressurized water that is heated to form steam in heat exchangers in an external circuit. The fast reactor has no moderator and uses fast neutrons to bring about fission. It uses a mixture of plutonium and uranium oxide as fuel. When operating, uranium is converted to plutonium, which can be extracted and used later as fuel. It is also called the fast breeder or breeder reactor because it produces more plutonium than it consumes. Heat is removed from the reactor by a coolant of liquid sodium.

Public concern over the safety of nuclear reactors has been intensified by explosions and accidental release of radioactive materials. The safest system allows for the emergency cooling of a reactor by automatically flooding an overheated core with water. Other concerns about nuclear power centre on the difficulties of reprocessing nuclear fuel and disposing safely of nuclear waste, and the cost of maintaining nuclear power stations and of decommissioning them at the end of their lives. The break up of the former USSR raised concerns about the ability of the new nation states to safely manage ageing reactors. In 1989 the UK government decided to postpone the construction of new nuclear power stations; in the USA, no new stations have been commissioned in over a decade. Rancho Seco, near Sacramento, California, was the first nuclear power station to be closed, by popular vote, in 1989. Sweden is committed to decommissioning its reactors. Some countries, such as France, are pressing ahead with their nuclear programmes. It was reported in 1997 that there were 443 nuclear power plants in 31 countries, representing a net growth of 15 since the 1992 Rio Earth Summit, and a further 36 were under construction. This compares with a total of just over 100 reactors generating electricity in 15 countries in 1972. Nuclear power generates around 17% of the world's electricity. Major accidents resulting in the release of large quantities of radiation occurred at Chernobyl, Ukraine, in 1986 and, on a far smaller scale, at Three Mile island in the USA in 1979.