Nuclear power production is based on the energy released when an atomic nucleus such as
uranium undergoes fission following the absorption of a neutron to form a compound nucleus. This
compound nucleus is unstable and may break into two or three smaller atomic nuclei with the simultaneous
emission of several neutrons together with the release of considerable amount of energy. These
neutrons may themselves be absorbed by other nuclei, and if enough of these are uranium nuclei, it is
possible for a chain reaction to develop. Chain reactions form the basis of the operation of a nuclear
reactor. Fission of a single atom of uranium yields 200 MeV (= 3.2e–11 J), whereas the oxidation of one
carbon atom releases only 4 eV. Natural uranium consists of 99.3% 238U and only 0.7% of lighter
isotope 235U, but it is the latter that provides the most readily available fission energy in nuclear reactor.
The maintenance of chain reaction, with exactly one neutron (on average) eventually causing another
fission, is the design objective of any nuclear reactor.
Nuclear Energy Generation. If the ratio of 235U to 238U in a mixture is low, it is necessary to
arrange that the neutrons be slowed down by a moderator (a light material like water, heavy water,
helium gas, beryllium, carbon, mixed, usually in homogeneously, with the fuel) in order to take advantage
of the increase in fission cross section for low energy neutrons. If the ratio is high, it is possible to
design reactors that are based on fission caused by fast (high energy) neutrons. Reactors using slow
neutrons are called thermal reactors in contrast to fast reactors whose design makes use of fission
caused by fast neutrons. To reduce the size and increase the options for the choice of materials for a
reactor, it is possible to enrich the uranium that is to enhance the fissile 235U in some portion of the
available natural uranium at the expense of the remainder. Higher the enrichment, easier it becomes to
maintain the chain reaction, so the volume of the reactor may be reduced and a moderator with a lower
moderating ratio may be used. Light water reactors use uranium enriched from 0.7 % to about 3%.
In a thermal reactor, the production of fissile isotopes is lower than burn-up of the fissile component
of uranium 235U in the fuel. However, in a fast reactor, using high-energy neutrons, the number of
neutrons produced per fission is higher than in a thermal reactor, and some fission of 238U also occurs,
so that there are more spare neutrons available for absorption by the common uranium isotope 238U,
giving a higher rate of fissile decay products. By suitable design the conversion gain can be chosen so
that more fissile material is produced than is consumed. Reactors of this type are called fast breeder
reactors. Practically all power reactors in operation use 235U as a fuel.
Resources. About 150 tonnes per year of natural uranium is required to meet the current demand.
Proven resources are 2191000 tonnes there may be additional resources of 2177000 tonnes
available. It is expected that FBR will takeover the future requirements and hence the future needs may
not increase drastically.
0 Comments