Nuclear energy is energy released from an atomic nucleus during a nuclear reaction in which the atomic number, mass number, or radioactivity of the nucleus changes. Nuclear energy is produced in large amounts in nuclear reactors and nuclear weapons. The term atomic energy, also used for this energy, is nit strictly appropriate, since nuclear reactions do not involve the orbital electrons of the atom.
Nuclear energy arises from the special forces (about a million times stronger than chemical bonds) that hold the protons and neutrons together in the small volume of the atomic nucleus. Lighter nuclei have roughly equal numbers of protons and neutrons, but heavier elements are only stable with a neutron:proton ratio of about 1.5:1. If one could overcome the electrostatic repulsion between protons and assemble them with neutrons to form a stable nucleus, its mass would be less than that of the constituent particles by the mass defect Δm, of the nucleus, and the binding energy, BE, given by BE = Δmc2 (where c is the speed of light), would be released (see mass-energy relationship). Because c is large, a vast amount of energy would be released, even for a very small value of the mass defect. The binding energy (equivalent to the work needed to split up the nucleus into separate protons and neutrons) is always positive – nuclei are always more stable than their separate nucleons (protons and neutrons) – but is greatest for nuclei of medium mass, decreasing slightly for lighter and heavier elements. The low binding energy of very light elements means that energy can be released by combing e.g., two deuterium nuclei to form a helium nucleus. This combination of two protons and two neutrons is particularly stable (see fusion). For heavy elements the decrease in binding energy indicates that the more positively charged the nucleus becomes, the less stable it is, even though it contains more neutrons than protons. This sets a limit on the number of elements, and also explains why the nuclear fission process, in which a heavy nucleus splits into two or more medium-mass nuclei, with higher total binding energy, releases energy.
The first nuclear reaction was performed experimentally in 1919 by Ernest Rutherford who exposed nitrogen to alpha particles (helium nuclei) from the radioactive element radium, producing oxygen and hydrogen:
N14 + He4 → O17 + H1
But, because nuclei are positively charged and repel each other, it was found difficult to bring them close enough together to react with each other. The discovery of the neutron in 1932 helped overcome this problem. Being uncharged and heavy (on the atomic scale), the neutron has high energy even when moving slowly and is good for initiating nuclear reactions. By 1939 many nuclear reactions had been studied, but none seemed feasible as an energy source. Although energy might be released in a reaction, more energy was expended in producing particles able to initiate the reaction than could be recovered from it. Moreover, only a small fraction of the reagent particles would react as desired and any product particles would have little chance of reacting again. The situation was like trying to set fire to a damp forest with a box of matches. A breakthrough came around 1939 when the violent reaction of the heavy element uranium on bombardment with slow neutrons (first observed experimentally by Enrico Fermi in 1934) was successfully interpreted. It was realized that this was an example of nuclear fission, the slow neutrons delivering enough energy to the small proportion of U235 nuclei in natural uranium to split them into two parts. This split does not always occur in the same way, and many radioactive fission products are formed, but each fission is accompanied by the release of much energy and two of three neutrons (these because the lighter nuclei of the fission products have a lower neutron:proton ratio than uranium). These neutrons were the key to the large scale production of nuclear energy; they could make the uranium "burn" by setting up a chain reaction. Even allowing for the loss of neutrons, sufficient are left to produce other fissions, each producing two or three more neutrons, and so on, leading to an explosive release of energy.
The first controlled chain reaction took place in Chicago in 1942, using pure graphite as a moderator to slow down neutrons and natural uranium as a fuel. Rods of neutron-absorbing material kept the reaction under control by limiting the number of neutrons available to cause fission. The possibilities of nuclear energy as a weapon were exploited at once and World War II ended shortly after the United States dropped two atomic bombs on Japan.