Nuclear fission

The process of splitting a nucleus is called nuclear fission and is used in nuclear power reactors. Fission is another word for splitting.

Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors. Their atoms have relatively large nuclei that are easy to split, especially when hit by neutrons.

When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following happens:

  • the nucleus splits into two smaller nuclei known as daughter nuclei, which are radioactive
  • two or three more neutrons are released
  • some energy is released in the form of kinetic energy of the particles produced

The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it moving too quickly.

A chain reaction showing the steps involved in splitting a nucleus.The chain reaction of splitting a nucleus

Generating power from nuclear fuels

Nuclear reactors use the heat from nuclear reactions in the nuclear fuel to boil water. Just as in conventional power stations, the steam from the boiling water makes a turbine spin, which in turn makes the generator turn.

Unlike conventional power stations, however, nuclear power stations do not release carbon dioxide, so they do not contribute to global warming while in use.

Cold coolant is injected at the bottom. It passes into reactor vessel and through gaps in graphite core which hold moveable control rods.  The now hot coolant is siphoned out.Outline of a nuclear reactor

Uranium and uranium oxide are often used as nuclear fuels. Their supplies are limited, so they are a non-renewable resource.

Ionising radiation in the reactor itself is prevented from escaping by steel and concrete walls. This is important because radiation can cause cancer. The waste from nuclear power stations is radioactive so it can be harmful too.

Plutonium is a waste product from nuclear reactors. It can be used to make nuclear bombs.

Controlling the reaction

So that nuclear reactions, like those seen in nuclear bombs, do not happen in power stations the nuclear reaction must be controlled.

In a nuclear power station only one neutron is allowed to collide with other uranium-235 or plutonium-239 nuclei after a collision. The other neutrons that were released by the collision would cause a nuclear explosion if they were not absorbed by moveable control rods. The control rods mean that the reaction remains constant and explosions do not occur.

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