Following the discovery of the neutron in the early 1930s, nuclear physicists began bombarding a variety of elements with neutrons. Enrico Fermi in Italy included uranium (atomic number 92) among the elements he bombarded which resulted in formation of nuclei that decayed by emission of negative β -rays. Such decay produces nuclei of higher atomic number, so Fermi assumed that the bombardment of uranium led to a new element with atomic number 93. By 1938 similar research had resulted in reports of the discovery of four new elements with atomic numbers 93, 94, 95, and 96. In 1938 Otto Hahn and Fritz Strassmann in Berlin bombarded uranium with neutrons to study the possibility of production of nuclides with atomic numbers less then 92 due to emission of protons and α -particles. To their surprise, they found that they had made barium (z = 56). Hahn informed a former colleague, Lise Meitner, who, with Otto Frisch, reviewed the data and reached the conclusion that the uranium atom was splitting (fissioning) into two new, smaller nuclei with the accompanying release of a large amount of energy. Many laboratories quickly confirmed the occurrence of this process of nuclear fission. Niels Bohr and John Wheeler, within a few months, published a paper explaining many features of fission using a model of nuclear behavior based on an analogy to a droplet of liquid, which, when given extra energy, can elongate from a spherical shape and split into two smaller droplets. Nuclei have two opposing energies: a disruptive energy resulting from the mutual electrostatic repulsion of the positive protons in the nucleus, and an attractive energy due to nuclear forces present between the nuclear particles (both neutrons and protons). The repulsive electrostatic energy of the protons increases as the number of protons increases and decreases as the average distance between them increases. The attractive nuclear
force energy increases with the total number of nucleons (protons and neutrons) in the nucleus. The nuclear attractive force is at a maximum when the nucleus has a spherical shape and at a minimum when the nucleus is distorted into two roughly equal fragments.
Nuclei formed in fission, known as fission products, range in atomic number (number of protons) from approximately 30 to 64. The original fissioning nuclide has a neutron to proton ration of about 1.6, whereas stable nuclei having the same range of atomic numbers as the fission products have neutron to proton ratios of 1.3 to 1.4. This means that nuclei formed in fission have too great a number of neutrons for stability and undergo beta ( β − ) decay to convert neutrons to protons. In general, fission is restricted to nuclei with atomic numbers above 82 (Pb), and the probability of fission increases as the atomic number increases. Fission produces nuclei of atomic masses from above 60 to about 150.
With very-low-energy neutrons, uranium of mass number 235 emits an average of two to three neutrons per fission event. Because more neutrons are released than absorbed, fission can result in a multiplication of successive fission events. This multiplication can reach very high numbers in about 10 −14 to 10 −17 seconds, resulting in the release of a great amount of energy in that time. This was the basis of the development of nuclear weapons. Soon after the discovery of fission it had been calculated that if a sufficient quantity of the fissionable material was assembled under proper conditions, a self-sustaining nuclear explosion could result. The critical mass of the fissionable material necessary for explosion is obtained with a spherical shape (minimum surface area per mass). The uranium isotope of mass 235 and the plutonium isotope of mass 239 are incited to fission and release energy in the use of nuclear weapons and in nuclear reactors. Nuclear reactors control the rate of fission and maintain it at a constant level, allowing the released energy to be used for power. Nuclear reactors are used in many nations as a major component of their natural energy. In the United States, approximately 20 percent of the electricity is provided by nuclear reactors, whereas France uses reactors to produce almost 80 percent of its electricity. Reactors used for power have four basic components: (1) fuel, either natural uranium or uranium enriched in 235 U or 239 Pu; (2) a moderator to reduce neutron energies, which increases the probability of fission; (3) control rods of cadmium and boron to control the rate of fission; and (4) coolants to keep the temperature of the reactor at a reasonable level and to transfer the energy for production of electricity. In power reactors the coolant is commonly H 2 O or D 2 O, but air, graphite, or a molten mixture of sodium and potassium can be used. Reactors are surrounded by a thick outer shield of concrete to prevent release of radiation.
There have been two major accidents (Three Mile Island in the United States and Chernobyl in the former Soviet Union) in which control was lost in nuclear power plants, with subsequent rapid increases in fission rates that resulted in steam explosions and releases of radioactivity. The protective shield of reinforced concrete, which surrounded the Three Mile Island Reactor, prevented release of any radioactivity into the environment. In the Russian accident there had been no containment shield, and, when the steam explosion occurred, fission products plus uranium were released to the environment—in the immediate vicinity and then carried over the Northern Hemisphere, in particular over large areas of Eastern Europe. Much was learned from these accidents and the new generations of reactors are being built to be "passive" safe. In such "passive" reactors, when the power level increases toward an unsafe level, the reactor turns off automatically to prevent the high-energy release that would cause the explosive release of radioactivity. Such a design is assumed to remove a major factor of safety concern in reactor operation.
LEO SZILARD (1898–1964)
Leo Szilard determined that the formation of neutrons occurs during the fission of uranium. This is crucial to sustaining a chain reaction necessary to build an atomic bomb, the first of which he helped to construct in 1942. Shortly thereafter, realizing the destructive power of the atom bomb, Szilard argued for an end to nuclear weapons research.
Gregory R. Choppin