Nuclear Fission and Fusion - Part III

4. Energy released during fission reaction, chain reaction
When 1 gram of Uranium-235 undergoes fission, it is seen that 8.30 x 10⁷ kilojoules of energy is released. This is equivalent of burning nearly 2.5 tons of coal with high efficiency. This is the reason why uranium-235 is used as fuel in atomic power stations.

In an uranium ore, the most abundant isotope of uranium is uranium-238 (²³⁸₉₂U). 99.3 % or uranium comes as uranium-238 and only 0.7 % of uranium is in the form if uranium-235, ²³⁵₉₂U. It is seen that ²³⁸₉₂U isotope does not fission easily. Scientists have found a way out of this. They bombard ²³⁸₉₂U with - particles and initiate a nuclear reaction, whereby ²³⁸₉₂U is converted to ²³⁹₉₄Pu (plutonium-239). This fissions easily.

The energy released when ²³⁹₉₄Pu fissions is about 30% less than the ²³⁵₉₂U fissioning process.

When a slow moving neutron enters the nucleus of uranium-235, it creates a turmoil in the structure of the nucleus. All the nucleons get disturbed and nuclear structure starts oscillating. The oscillations first give rise to a waist like structure. This proceeds to narrow and there develops a neck like structure. Further rapid movements make the neck even more elongated, which ultimately breaks. The break up leads to two smaller nuclei around the masses of barium and krypton.

From the picture it is clear that the fission products are not always barium or krypton. They can be cesium and rubidium also. It is as yet a mystery why the fissioning of U-235 does not give rise to two smaller nuclei that have mass 235/2. Research has shown that if energetic neutrons induce the fission, then the reaction products are two small mass nuclei that have equal masses.

When a ²³⁵₉₂U undergoes fission, it generates three neutrons. These neutrons in turn go and initiate fission in other ²³⁵₉₂U nuclei. Once one single slow neutron starts a fission in ²³⁵₉₂U, the reactions in other ²³⁵₉₂U are sustained by the first fissioning phenomenon itself.  This is called a chain reaction.

A nuclear reaction whereby a particle that starts a reaction is also produced during the reaction and thus starts further reaction is called the chain reaction.

Once started a chain reaction will go on continuously, till all the ²³⁵₉₂U in the nuclear reaction sample are over. The fission of ²³⁵₉₂U produces more neutrons, which start other fission reactions of more ²³⁵₉₂U nuclei. This process is continued like an unending chain till all the ²³⁵₉₂U in the sample is over. Very large amounts of energy is released with each fission, therefore it can be imagined how much energy is released in a continuous chain reaction.

It has been observed that to sustain a chain reaction, a minimum quantity of fissionable material is needed. Neutrons produced during fission may escape the sample and may not be able to keep the chain reaction going. To contain the neutrons produced and to increase the probability of emitted neutrons to start next fission, some minimum amount of fissionable material is required. This minimum mass is called the critical mass. The minimum mass of fissionable material that can support and sustain a chain reaction is called the critical mass of the fissionable material. If the chain reaction is not sustained, then the mass of fissionable material is called sub-critical mass.  

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