Chain reaction

Nuclear power

The world's first large-scale nuclear power plant opened at Calder Hall in Cumbria, England, in 1956 and produced electricity for 47 years.

Nuclear power is generated using uranium, a metal that is mined as an ore in large quantities, with Canada, Australia and Kazakhstan providing more than half of the world's supplies.

Nuclear reactors work in a similar way to other power plants, but instead of using coal or gas to generate heat, they use nuclear fission reactions. In most cases, heat from the nuclear reactions convert water into steam, which drives turbines that produce electricity.

There are different kinds, or isotopes, of uranium, and the type used in nuclear power plants is called uranium-235, because these atoms are easiest to split in two. Because uranium-235 is quite rare, making up less than 1% of natural uranium, it has to be enriched until the fuel contains 2-3%.

Inside a nuclear reactor, rods of uranium are arranged in bundles and immersed in a giant, pressurised water tank. When the reactor is running, high-speed particles called neutrons strike the uranium atoms and cause them to split in a process known as nuclear fission. The process releases a lot of energy and more neutrons, which go on to split other uranium atoms, triggering a chain reaction. The energy heats up the water, which is piped out to a steam generator.

To make sure the power plant does not overheat, control rods made of a material that absorbs neutrons are lowered into the reactor. The whole reactor is encased in a thick concrete shield, which prevents radiation escaping into the environment.

In Britain, nuclear power stations provide 19% of our electricity and account for 3.5% of our total energy use. All but one of those reactors are due to close down by 2023.

Some groups oppose nuclear power stations because they produce radioactive waste and could release radioactive material if there was an accident. But nuclear power plants do not release greenhouse gases, which cause coal and gas-fired power plants to contribute to global warming. Without nuclear power stations, UK's carbon emissions would be 5% to 12% higher than they are.

In 1957, the world's first nuclear power accident occurred at Windscale in west Cumbria. A fire in the reactor caused a release of radioactivity, which led to a ban on milk sales from nearby farms. The site was later renamed Sellafield. Modern reactors are designed to shut down automatically. The worst nuclear power accident in history took place in Chernobyl in 1986 when a reactor there exploded, killing tens of people instantly and exposing hundreds of thousands more to radiation.

In January, the government reaffirmed its plans to expand nuclear power in Britain to help it meet stringent targets to reduce carbon dioxide emissions.

Nuclear weapons

There are two main types of nuclear weapon: atomic bombs, which are powered by fission reactions similar to those in nuclear reactors, and hydrogen bombs, which derive their explosive power from fusion reactions.

The first atomic bomb was produced at Los Alamos National Laboratory in America under the Manhattan Project at the end of the second world war. An atomic bomb uses conventional explosives to slam together two lumps of fissionable material, usually uranium-235 or plutonium-239. This creates what is known as a critical mass of nuclear material, which releases its energy instantaneously as atoms inside it split in an uncontrolled chain reaction.

Atomic bombs unleash enormous shock waves and high levels of neutron and gamma radiation. In atomic bombs, uranium is enriched much more than fuel, to about 85% uranium-235.

On August 6 1945, an atomic bomb called Little Boy was dropped on the Japanese city of Hiroshima, followed three days later by another, called Fat Man, on Nagasaki.

Hydrogen, or thermonuclear bombs, work in almost the opposite way to atomic bombs. Much of their explosive power comes from fusing together hydrogen atoms to form heavier helium atoms, which releases far more energy than a fission bomb. Two types, or isotopes, of hydrogen are used - deuterium and tritium. A deuterium atom is the same as a hydrogen atom, except the former has an extra neutron in its nucleus. A tritium atom has two extra neutrons.

A hydrogen bomb has a built-in atomic bomb, which is needed to trigger the fusion reaction. Hydrogen bombs have never been used in war and are thousands of times more powerful than atomic bombs.

The first test of a hydrogen bomb was at Enewatak, an atoll in the Pacific Ocean. It released a three mile-wide fireball and a mushroom cloud that rose to nearly 60,000 feet, destroying an island in the process.

Nuclear waste

One of the biggest problems the nuclear industry faces is what to do with the radioactive waste it produces. Some of it will remain radioactive and hazardous for hundreds of thousands of years.

High-level waste is the most dangerous because it can melt through containers and is so radioactive it would be fatal if someone was near it for a few days. This type of waste makes up just 0.3% of Britain's total volume of nuclear waste, which is mostly waste from spent fuel rods. The largest amounts of radioactive waste are made up of nuclear fuel cases, reactor components and uranium.

Today, high-level waste is dealt with by cooling it in water for several years and then mixing it into a molten glass, which is poured into steel containers. These canisters are then stored in a concrete-lined building.

This is only a temporary measure, though. Scientists know that eventually they need to find a way of storing nuclear waste safely for thousands of years. Some countries, such as America and Finland, plan to store nuclear waste in deep underground bunkers. For this to be safe, scientists have to be sure the material could never leak out and contaminate water supplies or rise up to the surface.

Britain already has more than 100,000 tonnes of higher activity radioactive waste that needs to be stored. Large amounts of low-level waste are already stored in concrete vaults in Drigg in Cumbria. Other plans for disposing of nuclear waste have included dumping it at sea and blasting it into space.