FissionFission reactions in atomic weapons and power stations are chain reactions. Each fission releases several neutrons which trigger more fissions. When chain reactions occur in "atomic bombs" the energy release is uncontrolled, but when they occur in nuclear reactors the energy release is controlled by moderators and neutron absorbers so that the energy appears as heat in the reactor core. The first controlled chain reaction was carried out in 1942 at an experimental nuclear reactor in the stands of Stagg Field at the University of Chicago.
Only a few heavy nuclei (uraniun 235, plutonium etc.) can be used to set up a self-sustaining chain reaction. Of these, only one exists in nature; the isotope 235U. Naturally occurring uranium is a mixture of two isotopes, 235U (99.3%) and 235U (0.7%); only the isotope 235U fissions when it absorbs a slow neutron.
Three types of nuclear reactors exist, all of which are based on the fission of 235U.
1 Natural uranium reactors
Natural uranium reactors are fuelled by the mixture of 238U and 235U that occurs in natural uranium. The only successful type is the CANDU reactor developed in Canada. This type has natural uranium fuel rods immersed in a moderating pool of heavy water (deuterium oxide). Commercial power stations based on this design are in operation and under construction in Ontario and Quebec. The advantage of the design the use of natural uranium. The disadvantage is the need to moderate the reaction with heavy water (D2O), which must be obtained from the very small percentage of deuterium oxide in natural water. Large extraction plants have been built to produce the many tons of heavy water needed for these reactors.
2 Enriched uranium reactors
Enriched uranium reactors are fuelled with 238U enriched with more 235Ut han occurs naturally. Enrichment to anywhere between 2% and nearly 100% 235U is possible. The higher enrichments are used in nuclear weapons and the lower in enriched uranium power reactors. Preparation of the enriched uranium is done in gas-separation plants, of which the largest is at Oak Ridge, Tennessee. Gaseous diffusion takes advantage of the small difference in average velocity of 235UF6 molecules and the slightly heavier 238UF6 molecules. Uranium hexafluoride (UF6) is a highly corrosive gas and the process requires large amounts of electric power to drive pumps and other apparatus.
3 Breeder reactors etc.
Nuclear reactors can be fuelled with 239Pu and 233U which do not exist in nature. They are produced in nuclear reactors from naturally occurring 238U and 232Th respectively. Nuclear reactors carrying out this process make more nuclear fuel than they consume. They are called breeder reactors. In a breeder reactor, some of the neutrons produced are absorbed by 238U which then undergoes the following chain of reactions:
238U + n --> 239U 239U --> 239Np + e- .... (half-life 23.5 minutes) 239Np --> 239Pu + e-.... (half-life 2.35 days) The half-life of 239Pu is long enough (24,360 years) to permit its accumulation. 239Pu undergoes slow-neutron fission similar to that of 235U. Breeder reactors based on this cycle operate best without a moderator to slow down the neutrons. They are called fast breeder reactors.
Fast breeder reactors have higher power densities and smaller cores than slow-neutron reactors. Circulating water is not an effective heat-transfer mechanism to move heat from the core to the exterior steam turbine (and water would act as a moderator). Water is therefore replaced by liquid sodium or another liquid metal.
For information on nuclear matters in a readable form try ...
http://www.universe-review.ca/F14-nucleus.htm
For detatls about plutonium bombs try ...