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ATOM IN MODERN SCIENCE 33 uranium nuclei gives on the average a cent percent result, plutonium, a still heavier element would give a better result If with each new generation the number of neutrons grow by, say about 50 percent, there will be enough of them to attack and break-up any single nucleus of the material This is called the progressive branch chain reaction and the substance in which such a reaction will take place is called fissionable substance Among all the variety of nuclear species existing in nature, there is only one type of nuclei to which such reaction is possible These are the nuclei of the famous isotope of uranium, U-235, the only natural fissionable substance
U-235 does not, however, exist in nature in a pure form and is always found to be strongly diluted by the heavier unfissionable isotope of uranium U-238, (07 percent of U-235 and 99 3 percent of U-238), which hinders the progressive chain reaction in natural uranium Two methods are adopted to successfully utilise the atomic energy of U-235 (1) the straight forward separation of the two isotopes and (2) artificial reduction of the distributing action of the heavier isotope by the use of moderators
Other fissionable substances, not existing in nature, have been artificially built An artificial element which is known as Plutonium has the atomic number 94 and is even more fissionable than U.235 It is obtained by transforming the nucleus of natural uranium
Another artificial fissionable substance U-233 is obtained by similar transformation of the nuclei of natural radio active element Thorium (Th-232) In fact, it is possible, in principle at any rate to turn the entre supply of natural uranium and thorium into fissionable products which can be used as concen
