Kr-92 FIG. 31.24 The fission of U-235 into its fission fragments
Natural uranium is made up of roughly 99.3% U-238 and 0.7% U-235 isotopes. Both of these isotopes have useful properties. The U-235 is needed for nuclear fission and the U-238 can be bombarded with neutrons to artificially produce plutonium for fuel rods in reactors and nuclear weapons. U-235 is generally stable with a half-life of approximately 7.1 × 108 years. But if we bombard it with neutrons, we can induce fission.
U-235 undergoes fission (Fig. 31.24) in the following way:
• A sufficient mass of uranium is brought together. This is termed critical mass. There needs to be at least a critical mass of nuclear material to be fissionable.
• When a neutron strikes the U-235, it briefly turns into U-236. This is very unstable and will quickly undergo fission.
• The U-236 splits into two smaller nuclei of krypton (Kr) and barium (Ba), along with three neutrons.
• The neutrons carry most of the kinetic energy liberated from the fission. This can be calculated by E = mc2.
• Each of these released neutrons can cause further fission in more U-235 nuclei. This is called a chain reaction, as the reaction carries on automatically until it is slowed or stopped by human intervention.
A chain reaction is a fission reaction in which at least one neutron is emitted in order to carry on the reaction.
Critical mass is the minimum mass of fissile material required in order to sustain a chain reaction.
