Power Plant Engineering

Nuclear Energy Generation. If the ratio of 
235
U to 
238
U 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 advan-
tage 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 
235
U 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 compo-
nent of uranium 
235
U 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 
238
U also occurs,
so that there are more spare neutrons available for absorption by the common uranium isotope 
238
U,
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 
235
U as a fuel.
Resources. About 150 tonnes per year of natural uranium is required to meet the current de-
mand. 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.

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POWER PLANT ENGINEERING