Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
It is well known that most chemical reactions liberate energy, simplest instance being burning of coal. The chemical union, in this case, is that of carbon and oxygen in the form of molecular fusion. When 3000 tons of coals are burnt to ashes, the residual ashes and the gaseous products weigh one gram less than 3000 tons, that is, one three-billionth part of the original mass will have been converted into energy.
Thus oxygen (O) + carbon (C) - carbon monoxide (CO) + energy.
This reaction would give 92 units of energy per gram of mixture. If instead of molecular fusion of these two atomic species, we have a nuclear fusion between their nuclei 6C12 + 8O16 - 14Si28 + energy - the energy liberated per gram of mixture will be 14 x 109 Units, i.e. 15.00.000 times as great. In the liberation of chemical energy by the burning of coal, the energy comes from a very small mass i.e. loss of mass resulting from the rearrangement of the electrons on the surface of atoms. The nuclei of the carbon and oxygen atoms are not involved in any way, remaining exactly the same as before. The amount of mass lost by the surface electrons is one thirteenth of one millionth of one percent. On the other hand, nuclear energy involves vital changes in the atomic nucleus itself, with a consequent loss of as high as one tenth to nearly eight-tenths of one percent in the original mass of the nucleus. This means that from one to nearly eight grams per thousand grams are liberated in the form of energy, as compared with only one gram in three billion grams liberated in the burning of coal. In other words, the amount of nuclear energy liberated in the transmutation of atomic nuclei is from 30.00.000 to 2.40.00.000 times as great as the chemical energy released by the burning of an equal amount of coal. Whereas most chemical reactions would take place easily at temperatures of a few hundred degrees, corresponding nuclear transformation would not even start before the temperature reached many million degrees.
Nuclear energy can be liberated by two diametrically opposite methods. One is fission - the splitting of the nuclei of the heaviest chemical elements into two uneven fragments consisting of nuclei of two lighter elements. The other is fusion - combining or fusing two nuclei of the lightest elements into one nucleus of a heavier element. In both methods the resulting elements are lighter than the original nuclei. The loss of mass in each case manifests itself in the release of enormous amounts of nuclear energy.
At the turn of the century, Bacquere's discovery of radioactivity indicated that a break-up process of nucleus can really take place. Atoms of heaviest element uranium (and thorium) spontaneously emitted highly penetrating radiations (similar to X-rays). This process of slow spontaneous decay of the so-called radioactive elements consists in emitting a small segment of its nucleus known as 'alpha' particles and internal electric adjustment followed by emission of two electrons. A series of emission continues until we come finally to the nucleus of the lead atom.
Theoretically speaking all elements heavier than silver are radioactive and subject to the process of decay. But the spontaneous decay is so slow - say one or two atoms in a gram of gold or mercury in many centuries compared to several thousand per second per gram in uranium - that the most sensitive physical instrument cannot record it.
As we have already seen, the discovery of radioactivity proved the complexity of nuclear structure beyond any doubt and paved the way for artificial nuclear transformations. Earlier, bombardment of the nuclei by artificially accelerated charged particles such as alpha, protons, etc. was the method employed for nuclear transformations. But the electric charges carried by such particles caused them to lose much of its kinetic energy while passing through the atomic bodies and prevented them from coming sufficiently close to nuclei of the bombarded material. The bullets to be used for more efficient bombardment arc neutrons, which, because, they do not have an electric charge, can penetrate the heavily fortified electrical wall surrounding the positively charged nuclei. Just as coal fire needs oxygen to keep it going, a nuclear fire needs the neutrons to maintain it. but uncharged projectiles viz., neutrons are not easily available in free form as they are tightly locked up within the nuclei of atoms recaptured as soon as they are kicked out. There is only one way to sustain the nuclear reaction and that is to create a self-multiplication process i.e. each bombarding neutron must liberate more than one other neutron which in their turn would act as bullets.
Late in 1938, Rahn and Strassman discovered that atomic energy can be released through the fission process of uranium nuclei. Like the two pieces of a broken spring, the two halves of a broken heavy nucleus begin their existence in a state of violent vibrations. Before coming to rest, each of the fragments emits a neutron.
It must be remembered that, although the neutrons arc much more effective nuclear projectiles than the charged particles, their effectiveness in producing the fission is, however, not cent percent. The important condition for a sustained nuclear transformation is progressive neutron-production or a chain-reaction for which it is necessary that a hundred neutrons entering the substance must release many more than a hundred neutrons. There are two types of chain reactions: controlled and uncontrolled. The controlled reaction is analogous to the burning of petrol in automobile engine. The atom-splitting bullets - the neutrons - are first slowed down from speeds of more than ten thousand miles per second to less than one mile per second by being made to pass through a moderator before they reach the atoms at which they are aimed. Thus the liberation of neutrons is under complete control and acts as a slow but steady nuclear fire. The uncontrolled chain reaction is one in which there is no moderator - and no neutron-absorbers. It is analogous to the dropping of a match in a petrol tank. In the uncontrolled chain reaction the fast neutrons with nothing to slow them down or to devour them, build up by the trillion and quadrillion in a corresponding number of atoms, resulting in the release of unbelievable quantities of nuclear energy at a tremendously explosive rate.
It can be concluded from the general theory of nuclear structure that the fission effectiveness of neutrons increases with the increasing atomic weight of the element in question. Thus, the fission of 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 grows 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 where such reaction is possible. These are the nuclei of the famous isotope of uranium, U-235, the only natural fissionable substance.
Other fissionable substances, not existing in nature, have been artificially built. An artificial clement, which is known as Plutonium lies 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 radioactive element Thorium (Th-232). In fact, it is possible, in principle at any rate to turn the entire supply of natural uranium and thorium into fissionable products, which can be used as concentrated sources of nuclear energy.
