In contrast, nuclear energy involves change within the nuclei of atoms; small amounts of matter from the nucleus are converted into very large amounts of energy. There are two different reac­tions that release nuclear energy: fission and fusion. In fission, larger atoms of certain elements are split into two smaller atoms, whereas in fusion, two smaller atoms are combined to make one larger atom. In each case, the mass of the end product is less than the mass of the starting material(s) be­cause a small quantity of the starting material h converted to energy.

    Nuclear reactions produce 100,000 times more energy per atom than chemical reactions such as combustion do. In nuclear bombs this energy is re­leased all at once, producing a tremendous surge of heat and power that destroys everything in its vi­cinity. On the other hand, in the utilization of nu­clear energy to generate electricity, the nuclear

Reaction is controlled to produce smaller amounts of energy in the form of heat, which can then be converted to electricity.

Atoms and Radioactivity

All atoms are composed of positively charged pro-hk negatively charged electrons, and electrically neutral neutrons. Protons and neutrons, which have approximately the same mass, and clustered in the center of the atom, making up its nucleus. Electrons, which possess little mass in comparison to protons and neutrons, orbit around the nucleus in distinct regions. Atoms that are electrically neutral possess identical numbers of positively charged protons and negatively charged electrons.

    The atomic mass of an element is equal to the sum of protons and neutrons in the nucleus. Each element contains a characteristic number of pro­tons per atom, called its atomic number. In con­trast, the number of neutrons in each atom of a given element may vary, resulting in atoms of one element with different atomic masses. Forms of a single element that differ in atomic mass are known as isotopes. For example, normal hydrogen, the lightest element, contains one proton and no neu­trons in the nucleus of each atom. The two isotopes of hydrogen are deuterium, which contains one proton and one neutron per nucleus, and tritium, which contains one proton and two neutrons per nucleus. Many isotopes are stable, and some are unstable; the unstable ones are said to be radioac­tive because they spontaneously emit radiation, a form of energy. The only radioactive isotope of hydrogen is tritium.

    As a radioactive element emits radiation, its nucleus changes into the nucleus of a different ele­ment, one that is more stable; this process is known as radioactive decay. For example, the radioactive nucleus of one isotope of uranium, U-235, decays over time into lead (Pb-207). Each radioactive iso­tope has its own characteristic rate of decay. The period of time required for one-half of a radioactive substance to change into a different material is known as its radioactive half-life. A radioactive material gives off negligible radiation after ten half-lives. There is enormous variation in the half-lives of different radioactive isotopes. For example, the half-life of iodine (1-132} is only 2.4 hours, whereas the half-life of uranium (U-234) is 250,000 years.

Mini-Glossary of Nuclear Energy Terms

Nuclear energy: The energy released by nuclear fission or fusion.

Fission: The splitting of an atomic nucleus into two smaller fragments, accompanied by the release of a large amount of energy.