In the late nineteen thirties two German
scientists Hahn and Strassman were doing a series of experiments to study the affects of firing
neutrons at some uranium compounds. The result was a very radioactive product that they
thought was a nucleus of a very heavy element, formed when the neutrons joined onto the
uranium nucleus. However, when they analysed the products they found not elements heavier
than uranium but two about half the mass number of the original uranium, a solution that had
been suggested in 1938 by Lisa Meitner and Otto Frisch.
The uranium nucleus had
split in half - this was the first evidence of what is called nuclear fission. If a nucleus
is made unstable it may lose the extra energy in two ways. It can emit radiation (alpha, beta or
gamma) or undergo fission. You can think of fission as rather like a wobbly jelly that has been
shaken about too much and simply split up.
In all nuclei there are two forces, the strong
nuclear force (acting between all the neutrons and protons) trying to hold the nucleus together
and the electrostatic repulsion trying to push the protons apart.
When a neutron is fired at a uranium 238 nucleus, uranium 239 is
formed, this is unstable and the nuclear fission occurs.
(This only occurs with fast
neutrons; slow neutrons are captured giving neptunium and then plutonium)
There
are many possible results of the nuclear fission of another isotope of uranium, uranium 235
nucleus, one possible reaction is:
| 235U + 1n | giving | 236U | giving | 148La + 85Br + 31n + energy |
| 235.044 + 1.0087 | 147.961 + 84.938 + 3.0261 |
This reaction has a mass defect of 0.1276u
Energy is given out by
the reaction because the mass of the products is less than the total mass of the original
nucleus and the neutron.
A full treatment of the equation shows that this energy is
118 MeV or 1.90 x 10-11J. This is a very small amount of energy but when you work out how
many nuclei there are in 1kg of uranium you can understand why nuclear fission is so
important.
Using Avogadro's number we can calculate the number of uranium atoms
(N) in 1kg of the metal.
N = [1000x6.02x1023]/235.004 = 2.56x1024
This means that if we could fission all the nuclei in 1kg of U235 we would
release 2.56x1024 x 1.90x10-11 = 4.9x1013J of energy!
Put in every day terms this is
sufficient to heat a house, with a 5kW heater, 24 hours a day, 7 days a week, 52 weeks a year
for over 300 years! It is not quite that simple because it's difficult to get ALL the nuclei to split.
However once one nucleus has fissioned neutrons are released and these can go on to split further nuclei. If this fission can be sustained a chain reaction is produced. A diagram of such a reaction is shown in the diagram above. This reaction will proceed at high speed, the time for an emitted neutron to collide with another nucleus to produce a second fission is about 0.01 microsecond.
