A level revision

Fission and fusion

Fission and fusion provide ways for us to get energy out of particles, generally we can get more energy out then we put in so nuclear power is a great source of energy.

Fission

Fission occurs with elements to the right of iron on the binding energy curve, typically Uranium-235 is used. Uranium-235 is an isotope of uranium; it has a different number of neutrons to the most common form, Uranium-238. Uranium-235 is used because it is unstable.

Nuclear fission works by firing a neutron at a uranium-235 nucleus. This causes the already unstable and large nucleus to split into two small nuclei of other elements. The binding energy of the two new nuclei combined is less than the binding energy of the original uranium nucleus, it is this missing energy which is given out in the reaction and used to turn water into stream which drives a turbine generating electricity.

Chain reaction

Together with the two small nuclei the fission reaction also produces 2 or 3 new neutrons. If these neutrons go on to cause another fission reaction a chain reaction can occur. Chain reactions can be sub-critical, critical or super-critical. A sub-critical reaction is one where all the chains die out, that is neutrons do not go on to create new fission reactions. A critical reaction is one where one neutron goes on to create one new fission reaction, this is what we want. A super-critical reaction is one in which all of the neutrons created in a fission reaction go on to create multiple new fission reactions, this process is used in nuclear bombs.

Moderators

A neutron moderator, such as graphite, is used to slow down neutrons so they can be captured by the uranium nuclei. If the neutrons are moving too fast they won't cause fission reactions.

Fusion

Fusion is more or less the opposite of fission. It involves using nuclei to the left of iron on the binding energy curve. When two light nuclei, such as helium, come together and form a new larger nucleus the mass of the new nucleus is less than the sum of the two original nuclei, this mass difference is released as energy.

Fusion on earth consists of a two stage process. Firstly two isotopes of hydrogen, H-2 and H-3, are brought to together to form a helium-4 nucleus and a neutron, this reaction releases energy. The second stage involves the neutron going on to collide with a lithium-6 nucleus to form more helium-4 and H-3.

Currently no method of creating fusion on earth exists without putting more energy in then is returned from the reaction. As the two nuclei are positively charged they need to brought close enough together so that the strong force, which keeps the nucleons together, pulls the two nuclei together. The strong force is stronger than the force between the two charged particles but it has a small range and to get the particles close enough requires enormous amounts of heat to give them enough speed. In the sun, where fusion occurs naturally, the temperature is in the range of millions of degrees.