The hell bomb

by William L. Laurence

When two light atoms are combined to form a heavier atom, the weight of the heavier is less than the total weight of the two light atoms. If the heavier atom could again be split into the two lighter ones, the latter would resume their original weight. As explained before, however, this is true only with the light elements, such as hydrogen, deuterium, and tritium, in the first half of the periodic table of the elements. The opposite is true with the heavier elements of the second half of the periodic table. For example, if krypton and barium, elements 36 and 56, were to be combined to form uranium, element 92, the protons and the neutrons in the uranium nucleus would each weigh about 0.1 per cent more than they weighed in the krypton and barium nuclei. It can thus be seen that energy could be gained either through the loss of mass resulting from the fusion of two light elements, or from the similar loss of mass resulting from the fission of one heavy atom into two lighter ones.

In the fusion of two lighter atoms, the addition of one and one yields less than two, and yet half of two will be more than one. In the case of the heavy elements the addition of one and one yields more than two, yet half of two makes less than one. This is the seeming paradox of atomic energy.

Three elements are known to be fissionable. Only one of these is found in nature: the uranium isotope 235 (U-235). The other two are man-made. One is plutonium, transmuted by means of neutrons from the nonfissionable U-238, by the addition of one neutron to the 146 present in the nucleus, which leads to the conversion of two of the 147 neutrons into protons, thus creating an element with a nucleus of 94 protons and 145 neutrons. The second man-made element (not yet in wide use, as far as is known) is uranium isotope 233 (92 protons and 141 neutrons), created out of the element thorium (90 protons, 142 neutrons) by the same method used in the production of plutonium.

When the nucleus of any one of these elements is fissioned, each proton and neutron in the two resulting fragments weighs one tenth of one per cent less than it weighed in the original nucleus. For example, if U-235 atoms totaling 1,000 grams in weight are split, the total weight of the fragments will be 999 grams. The one missing gram is liberated in the form of 25,000,000 kilowatt-hours of energy, equivalent in explosive terms to 20,000 tons of TNT. But the original number of protons and neutrons in the 1,000 grams does not change.

The fission process, the equivalent of the “burning” of nuclear fuels, is maintained by what is known as a chain reaction. The bullets used for splitting are neutrons, which, because they do not have an electric charge, can penetrate the heavily fortified electrical wall surrounding the positively charged nuclei. Just as a coal fire needs oxygen to keep it going, a nuclear fire needs the neutrons to maintain it.

Neutrons do not exist free in nature, all being tightly locked up within the nuclei of atoms. They are liberated, however, from the nuclei of the three fissionable elements by a self-multiplication process in the chain reaction. The process begins when a cosmic ray from outer space, or a stray neutron, strikes one nucleus and splits it. The first atom thus split releases an average of two neutrons, which split two more nuclei, which in turn liberate four more neutrons, and so on. The reaction is so fast that in a short time trillions of neutrons are thus liberated to split trillions of nuclei. As each nucleus is split, it loses mass, which is converted into great energy.

There are two types of chain reactions: controlled and uncontrolled. The controlled reaction is analogous to the burning of gasoline in an 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. Neutron-“killers”—materials absorbing neutrons in great numbers—keep the neutrons liberated at any given time under complete control in 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 gasoline 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 fraction of a millionth of a second. This leads to the splitting of a corresponding number of atoms, resulting in the release of unbelievable quantities of nuclear energy at a tremendously explosive rate. One kilogram of atoms split releases energy equivalent to that of 20,000,000 kilograms (20,000 metric tons) of TNT.

It is the uncontrolled reaction that is employed in the explosion of the atomic bomb. The controlled reaction is expected to be used in the production of vast quantities of industrial power. It is now being employed in the creation of radioactive isotopes, for use in medicine and as the most powerful research tool since the invention of the microscope for probing into the mysteries of nature, living and non-living.

In the controlled reaction the material used is natural uranium, which consists of a mixture of 99.3 per cent U-238 and 0.7 of the fissionable U-235. The neutrons from the U-235 are made to enter the nuclei of U-238 and convert them to the fissionable element plutonium, for use in atomic bombs. The large quantities of energy liberated by the split U-235 nuclei in the form of heat is at too low a temperature for efficient utilization as power, and is at present wasted. To be used for power, nuclear reactors capable of operating at high temperatures are now being designed.