We design a nuclear reactor on the basis of a controlled fission reaction by the arrangement of reactor material. A steady reaction is maintained, in which each fission produces on an average of one new fission. We try to achieve a critical balance for the production of controlled nuclear energy.
The fission of uranium by neutrons is self sustaining chain reaction in which 2.5 neutrons per fission are produced on the average. The reaction can therefore proceed in a sustained and controlled way if one of these neutrons is able to cause further fission and remaining 1.5 neutron are allowed to escape. To meet these requireements the nuclear reactor is made sufficiently large to increase the (surface / volume) to the desired optimum level. Further if need be the fuel is also surrounded by a neutron reflecting material. This helps to decrease the ‘neutron loss’ if it is sufficiently fast. The essential parts of a nuclear reactor are shown in the figure.
1. Fissionable fuel (like enriched uranium) taken usually in the form of rods.
2. Control rods are made of some suitable neutron absorbing material like cadium. They are pushed in and out to control the rate of neutron absorption and hence the rate of further fission.
3. Moderator (like graphile, heavy water) that helps to slow down the fast neutrons and thus increasing the probability of their capture by uranium. It shares the energy of the fast moving nuetrons without actually absorbing them. The moderator is prefeably a material whose atomic mass is comparable to that of a neutron and which therefore causes a large energy exchange on each collision. The moderator is suitably mixed with the uranium fuel. The nuclear reactor goes into operation when some stray neutrons initiate fission in the enriched uranium fuel.
The moderator and control rods are then suitably adjused to sustain the chain reaction at a controlled rate. The energy released is used to produce steam in a steam generator. The steam then drives the turbines connected to the electric generator and thus help to produce electricity.
