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Nuclear Fission Explained

Author: Maggie Wakefield - Updated: 25 August 2012 | commentsComment
 
Nuclear Fission Explained

Britain has been using nuclear power to generate electricity for just over half a century. Currently, 19 reactors are in operation. Taking an average over recent years, they have produced around one-fifth of the UK’s electricity; their peak came in 1997, at 26 per cent. Nuclear reactors have a fixed lifespan and most of the UK’s existing nuclear power stations are now approaching the end of their lives.

The nation’s long-term energy strategy relies on nuclear power to help it achieve its target of an 80 per cent reduction in greenhouse gas emissions by the year 2050. Third-generation nuclear reactors are planned, and these, like their predecessors, will release energy through nuclear fission.

How a Nuclear Power Plant Works

In conventional fossil-fuelled power plants, the generator is powered by turbines driven by steam. The steam is produced by water that is heated to boiling point by the combustion of fossil fuels. Nuclear power plants work on the same principle, but instead of combustion, the energy source used to heat the fluid is nuclear fission.

Nuclear fission is a means of releasing atomic energy. Atoms contain energy that binds them together. In nuclear fission, the nucleus of an atom is split; therefore some of the energy that was used in binding the atom together, is now released.

Uranium

The substance used in nuclear reactors is uranium. Uranium is a metallic element, silvery grey in colour, and weakly radioactive in its natural state. Uranium ore is mined in a number of countries including Canada, Australia, Russia, South Africa and the US. Extensive processing must be done to extract, refine and prepare the material required to fuel a nuclear reactor.

Uranium occurs naturally in three isotopes – in other words, with three different atomic configurations. Over 99 per cent of naturally-occurring uranium is composed of the isotope uranium-238, but the only isotope that is active in reactors is uranium-235. This isotope has unusual characteristics; it is unstable, and it is the only isotope, of any available element, that can be split by thermal neutrons. However, only about 0.71% of natural uranium is uranium-235. After being mined, uranium is therefore ‘enriched’ to increase the concentration of the uranium-235 isotope.

Nuclear Reactors

The one significant difference between a nuclear power plant and a conventional fossil-fuel power plant is the fuel that is used. In a nuclear plant, the thermal energy necessary to power the process is obtained from nuclear fission – the splitting of the nuclei of atoms of uranium.

Uranium is delivered to the power station in the form of solid ceramic pellets, packed into long metal tubes. Bundles of these tubes are placed inside the nuclear reactor. There, the uranium is bombarded by neutrons; the uranium-235 atoms, which are naturally fissile, split and release their own neutrons, which strike other atoms and split those, and once the chain reaction has begun the process is self-sustaining. Each time an atom is split, heat is released. The fuel tubes are surrounded by water, which is turned into steam by the heat.

What Safety Measures are in Place?

It is essential to monitor the temperature of the reactor. This can be done by maintaining a constant flow of gas to transfer the heat, and the most suitable gas for this purpose is carbon dioxide. The rate of fissile activity can be decreased or increased by a set of control rods, which are lowered inside the reactor to slow the reaction down and raised to accelerate it.

Reactors also have a separate set of emergency control rods that can be dropped to shut the reactor down completely. To minimise the risk of human error, UK reactors are equipped with a computerised system that automatically implements shutdown if necessary to prevent overheating, and it is also possible in an emergency to flood the reactor with nitrogen or water, which will absorb the neutrons and therefore bring the process of nuclear fission to a full stop.

Risks and Benefits

Ensuring that the reactor does not overheat is only one of the precautions that must be taken. It is also essential to monitor carefully for any radioactive leaks, and perhaps the aspect of nuclear power generation that causes most concern is the disposal of the spent fuel rods, which are highly radioactive.

It is undeniable that there are risks involved in the use of nuclear reactors. But nuclear fission represents an effective, affordable, low carbon energy source, and one that we expect to rely on for the foreseeable future.

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