Presently, nuclear power is developed by fission reactions in nuclear reactors. Mainly, it is used for generating of electricity. The reactors in operation are all burner reactors. They use only the Uranium U-235. This isotope is present to the extent of less than 1 per cent in naturally occurring Uranium.
Nuclear fission is a special type of nuclear reaction in which an excited compound nucleus breaks up generally into two fragments of comparable mass numbers and atomic numbers. Fission usually occurs amongst the isotopes of heaviest elements known e.g., Uranium, Theorium etc.
Nuclear fission was discovered by the two German chemists Otto Hahn and F. Strassmann in 1939.It happened to be one of the most important discoveries in nuclear physics, since it paved the way for the utilization of the internal energy of the nucleus for practical purposes. The nuclear fission discovered by Hahn and Strassmann can be symbolically written as
235U92 + 1n0 → 236U92* → 141Ba56 + 92 Kr36 + 3 1n0
Nuclear fission is a highly exoergic reaction. Various experiments have established that the total kinetic energy of the two fission fragments is about 167 Mev, which shows the enormity of the energy release in the fission process, compared to the energy release in an ordinary nuclear reactions. Beside, the fission fragments, some energy is also carried by the γ-rays and a few prompt neutrons emitted along with the fragments during fission.
The energy release per gram of 235U in reaction like above is 2.29×104kWh. A thermal power generator having a capacity of 1 Mev (heat) would have to be run for 229 hours to generate this amount of energy. The mass of coal which must be burnt to produce an equivalent amount of energy is 2.56×103 Kg. The above estimates clearly show the advantage of using Uranium for power generation.
To utilize the enormous amount of energy, the first nuclear reactor to be built for central station power generation was completed in 1954 in the U.S.S.R. About 5 MW electrical power was generated by this reactor. In the
Naturally occurring Uranium contains three isotopes, U234, U235 and U238. The relative percentages of these isotopes are as follows:
U234 0.006 per cent,
U235 0.711 per cent,
U238 99.283 per cent .
Of these isotopes, only U235 undergoes spontaneous fission when subjected to bombardment by slow neutrons. It is in fact the only naturally occurring fissile materials. The break-up of U235 when subjected to neutron bombardment yields fission products, neutrons and the release of a large amount of energy as heat (8.2×107 KJ per gram of U235). The neutrons are slowed down by a moderator, and used to bombard the U235 nucleus again, thereby setting up a controlled chain reaction. A nuclear reactor working on the above reaction and utilizing only U235 is called a burner reactor. All reactors working in present day nuclear power plants are essentially burner reactors and thus waste the abundant U238 isotopes present in naturally occurring Uranium.
Although U238 is not a fissile material, it is a fertile material, i.e. it can be converted by neutron bombardment into a fissile material, Plutonium 239. Similarly, naturally occurring Thorium 232 is also a fertile material. It can be converted into U233 which is fissile material. The breeder reactor is the reactor in which these fertile materials are used to achieve fissile materials and the fissile materials break-up into fission products, neutrons and energy. Power breeders are considered the best possible solution for the production of economical nuclear power.
Reactors working on various breeder cycles have been built. However, the major effort has been on liquid-metal cooled, fast breeder reactors working on the U238 to Pu239 cycle. In the seventies, it appeared that breeder reactors would be in commercial operation by the turn of the century. However, fears of nuclear accidents, difficulties associated with radioactive waste disposal and the possibility of Plutonium being misused for weapons have caused the breeder development program to be closed or to be slowed down in some countries.
However, the world’s resources of Uranium are estimated to lie in the range of 3.5Mt to 6.6Mt. Of this, 2.315Mt is classified as reasonably assured reserves. A nuclear power plant uses 0.15 to 0.2t of fuel per year (in the form of Uranium Oxide) for every megawatt of capacity. Thus the reserves would be adequate for running the present installed capacity of 350 000 MW for a period of only about 35 years. It is evident that a difficult situation will arise by 2020AD or 2030AD if the present technology continues to be used.
It is thus fairly evident that a need exists for developing alternative energy sources. The immediate need would be to alleviate the problems caused by the depletion oil and natural gas, while the long term need would be to develop means to replace presently used nuclear fission technology and then coal.
