Fusion and Fission

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Fusion and Fission

Because iron is the most stable nucleus, it makes sense (in energy terms) for other nuclei to try to become more like iron.

So smaller nuclei will combine (and give out energy in doing so) to produce larger nuclei - more like iron! This process is called Nuclear Fusion.

Larger nuclei will split up to produce smaller nuclei (more like iron) and give out energy in doing so. This is Nuclear Fission.

You can use the Binding Energy curve to calculate the amount of energy released by a reaction by considering the total binding energy of all the nucleons involved before the reaction (remember - the curve gives energy values per nucleon, not per nucleus) and compare it to the total binding energy of all the nucleons after the reaction. The difference is the energy released.

Fusion and Fission diagram

Note: if the difference shows that the products have less binding energy than the reactants, then the reaction could only have taken place if it was induced - i.e. it was given some energy to make it happen.

In 1939 physicists realised that the energy released during fission could lead to the possibility of a nuclear bomb. The American government started the Manhattan project to develop the bomb, and Enrico Fermi set about building the world's first nuclear reactor (in a University squash court!).

The process of fission is illustrated below. Usually a neutron collides with a large nucleus, setting it into a series of violent vibrations. The deformation caused by these vibrations allows the electrical repulsion between the protons to overcome the short-range nuclear forces that hold the nucleus together. The result is a splitting of the nucleus into smaller nuclear fragments, and possibly the ejection of neutrons.

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These ejected neutrons are often absorbed by non-fissionable material; the nuclear reactions then stop. If - on the other hand - the neutrons are allowed to strike other fissionable nuclei, further fissions can occur. If a situation occurs in which each fission produces (on average) one neutron, which causes one other fission, and so on, we have a self-sustaining reaction. This is called a critical mass. If more than one fission inducing neutron on average is released, the result is an avalanche of nuclear reactions. This is called a chain reaction; this is a very dangerous situation deliberately manufactured in nuclear bombs.

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Nuclear power stations are designed to control the nuclear processes described above, to produce useful energy. The key to controlling nuclear reactions is to produce just the right number of neutrons to cause a self-sustaining reaction, but not an uncontrollable chain reaction.

The first problem to overcome, is that most of the neutrons produced during nuclear fission move too quickly to induce fissions in other nuclei (imagine the neutrons zipping through and not having time to start the nuclei vibrating). The neutrons can be slowed by using a moderator. The neutrons produced collide with the nuclei of the moderator substance and lose some of their energy without being absorbed. They then pass back towards the fuel where they are more likely to produce fissions with suitable nuclei. These lower energy neutrons are called thermal neutrons. Graphite and water are commonly used as moderators.

The second problem is controlling the number of thermal neutrons. The number of thermal neutrons (and hence the rate of the nuclear reactions) must be finely adjusted using control rods. This ensures a chain reaction cannot occur but a self-sustaining reaction is allowed. The control rods act like neutron sponges: they absorb more and more neutrons the lower they go into the reactor. These are rods made from cadmium or boron and can be dropped into the nuclear reactor to shut down the reactions completely if a dangerous situation develops.

The most common type of nuclear reactor is the pressurised-water reactor illustrated below. This design uses water as a moderator and as a coolant. The water is kept at high pressure (to stop it boiling, even at 320 centigrade) and driven by powerful pumps to the boiler where it produces steam to drive turbines and produce electrical energy.

Fusion and Fission diagram

Apart from the obvious danger of chain reaction nuclear reactions also produce high levels of radiation and radioactive by-products. Safety is a very important consideration in the nuclear industry: reactors must be properly shielded to stop emissions and waste must be disposed of carefully for the safety of workers and the public alike. Despite these problems nuclear power is considered by many to be a clean and relatively cheap form of energy; in addition it will allow us to conserve our supplies of fossil fuels. It is also a fact that some of the by-products of nuclear reactions include man-made nuclides that are useful in fields such as medicine and technology. The debate for and against nuclear power is likely to go on well into the future.

S-cool exclusive!!