Nuclear Fission

Nuclear Fission

• The word FISSION means split.

• The process of nuclear fission involves the splitting of the atomic nucleus.

• There are some very large nuclei that are unstable and undergo this splitting naturally.

• The two most important examples are:

235 23992 94U Puand

How Fission Happens

• The uranium-235 nucleus is unstable.

• If a free neutron hits this nucleus it becomes much more unstable and splits into two releasing a small amount of energy as heat and gamma rays.

• In the process it also releases three more neutrons.

Free neutron Large U-235 nucleus

Where does the first free neutron come from?

• Among the billions of atoms in a piece of uranium 235 an atom or two will split spontaneously anyway.

• The point is there is always the occasional free neutron to start this process.

• Once the process has started neutrons are produced in the process of fission.

The Fission Process

• Although the fission of a single nucleus produces a small amount of energy. Billions of nuclei undergoing fission produce very large amounts of energy.

Controlled Nuclear Fission

• When the U-235 is mixed with other materials as in nature the fission process happens occasionally.

• If we increase the purity of the U-235 more fissions happen because neutrons produced in the fission process go on to split other U-235 nuclei. This is called a chain reaction .

• .

• In a nuclear reactor the concentration of U-235 is large enough to generate lots of heat in a chain reaction. This heat can be used usefully to generate electricity

Uncontrolled Chain Reaction

• If the U-235 is very pure and there is enough of it ,around 56kg ( a smaller lump than a football) the chain reaction will go out of control. This leads to a nuclear explosion

The first atomic bombs (one a U-235 and the other Pu-239) obliterated two cities in Japan in

1945.

The destruction of 2 cities in Japan in 1945.

The size of each blast was equivalent to around 40 000 large conventional bombs. (Very low explosive yield compared to modern bombs of this type)

The devastation produced by the explosion was the beginning of the problem only as much deadly nuclear material was spread far and wide leading to injury deaths and birth defects.