M07 Chain reactions
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Aim: To explain the chain reaction in fission of uranium-235 |
The Germans O. Hahn and F.
Strassmann in 1939 were the first to split the atom. They bombarded uranium- 235, which
constitutes only 0.7% of natural uranium, with neutrons. The uranium atoms split into two
smaller atoms (isotopes of barium and krypton) and three neutrons, with a release of
energy. This process is termed “fission” The neutrons produced, in their turn,
induce further fission which is associated with the release of more energy. The chain
reaction is:
If the sample of uranium-235 is smaller than a certain size about 5kg known as the “critical mass”, neutrons will be lost by escape from the surface. In a larger sample the neutrons are retained within the mass and an exponentially accelerating chain reaction occurs with a huge release of energy. This is where the energy of the atomic bomb comes from. An atomic bomb consists of two blocks of uranium-235, each smaller than the critical mass. Detonating the bomb involves firing one mass into the other to make a single block larger than the critical mass. The detonation is followed by an atomic explosion. Atomic energy is obtained from the controlled fission of U-235. The reactors use a mixture of U-235 and U-238. Only the U-235 isotope undergoes fission. U-238 will absorb fast neutrons but not slow neutrons. The neutrons produced from the U-235 fission are slowed down in the reactor by passing them through blocks of graphite to prevent them being absorbed by the U-238. To keep the reaction under control and avoid overheating, rods of boron, a strong neutron-absorber, are inserted into the reactor. If the fission speeds up they are pushed in further to absorb more neutrons; if it slows down they are pulled out to allow the number of neutrons to increase and the reaction to speed up. Other fuels e.g. Pu-239 can also be used.
The chain reaction shown in Illustration M07 is not the only one possible for U-235. Sometimes two neutrons are released, whereupon one nucleus can be formed with an atomic number between 51 and 57 and another with atomic number between 35 and 41.
