Radioactivity - Nuclear Fission

The two main fuels for nuclear power stations are the isotopes uranium-235 and plutonium-239. They are used because, when the nucleus of either isotope is hit by a neutron, they become unstable and split. We say that the two isotopes are fissionable. During fission, the nucleus splits into two smaller nuclei (referred to as daughter nuclei) and emits some neutrons, usually either two or three. Where you had one neutron to begin with, you end up with two or three. As well as this, some energy is released in the form of heat and gamma radiation.

One of the tasks you may be asked to do in the GCSE exam is to balance an equation involving nuclear fission. This is simply testing if you understand the principle of conservation of matter. Take for example the fission of a plutonium-239 nucleus. If this was hit by a neutron, two possible products of the fission would be cerium-148 and krypton-89 so how many neutrons would be produced? To work this out, you need to use your knowledge of atomic structure. The number 239 indicates that the nucleus of plutonium contains a total of 239 particles. One neutron is needed for the fission to occur so on the left hand side of the equation there are 240 particles in total. That means there must also be 240 particles on the right hand side too. Adding together the 148 particles of the cerium and the 89 of the krypton gives 237 so 3 neutrons are released.

The neutrons released in nuclear fission reactions will then hit other atoms and cause them to split and so on. This is called a chain reaction. If this is not controlled, it very quickly gets out of hand creating a nuclear explosion, that is how the first atomic bombs worked. Later, more powerful hydrogen bombs were developed - these work differently. In the core of a nuclear power station, the neutrons released during nuclear fission are carefully controlled by absorbing them using special control rods. If the neutrons are moving too fast or too slowly, fission will not occur - water is used in some types of reactor as a moderator to ensure that as many neutrons as possible are moving at the correct speed to keep the reaction going in the core.