Nuclear properties of the actinoids
All the isotopes of actinoid elements are unstable toward radioactive decay; the reason that actinium, thorium, protactinium, and uranium are found in nature at all is because some of their isotopes are unusually stable and others are being formed constantly by decay of the long-lived isotopes. It is convenient to divide the naturally occurring isotopes into families on the basis of the relationships of their atomic masses to each other. The mass numbers of all isotopes of the so-called thorium series, for instance, turn out to be multiples of four, and the series is known as the 4n series. In the uranium series, masses have been shown to be such that they are represented by 4n + 2; in the actinium series, by 4n + 3. Not found in nature to any significant extent, but which can be artificially produced, is the neptunium (4n + 1) series, named for its longest-lived member, neptunium-237.
In the various radioactive decay processes, several “rays” are emitted, the term ray being a holdover from the time when all these emissions were thought to be rays. They are (1) electrons, called beta particles to indicate their origin in radioactive decay and designated as negative beta, or β−, particles, (2) helium nuclei, called alpha particles and designated as α particles or as helium with a plus-two charge, He+2, (3) gamma rays, which are electromagnetic waves of very high frequency, designated as γ rays, and (4) positrons, which are positively charged electrons and are designated as positive beta, or β+, particles. Finally, an orbital electron in a radioactive atom may be captured by the nucleus and taken into it. This radioactive event is called K-capture. Except for the emission of gamma rays, each of these processes leads to an isotope of a different element—that is, to a substance with a different atomic number. The emission of an alpha particle leads to a change in atomic weight as well, because this emission has significant mass. The changes occur in sequence, each decay process leading to still another unstable element, until a region of stable isotopes of the elements lead and bismuth has been reached.
The most important nuclear reactions, however, involve the capture of neutrons by an actinoid nucleus, followed by splitting, or fission, of that nucleus into two unequal parts, with the liberation of enormous quantities of energy plus two or more extra neutrons. Nuclear reactors and atomic bombs depend upon the chain reaction set up by this process: the resulting neutrons react further, inducing more fission reactions, which produce more neutrons, which lead to still more fission reactions, with the result that, without control, a great deal of energy is liberated very quickly.
One gram of burning coal yields fewer than 10,000 calories of heat. The fission of one gram of uranium-235 produces 2 ×1010 calories, or about 2,000,000 times as much energy. With adequate control the energy can be released in useful form to make electricity.
The most important actinoid by far, because of its fissionability, is uranium, which has several isotopes. Natural uranium consists mostly of uranium-238, a nonfissionable isotope. The fissionable isotope uranium-235, which occurs to the extent of only seven-tenths of 1 percent, is the valuable substance that causes the chain reactions with neutrons. Methods that separate isotopes by virtue of their slightly different masses are used to enrich natural uranium with respect to uranium-235. Such enriched uranium was used in the first atomic bombs tested in New Mexico and exploded over Japan in the summer of 1945. The oxide UO2 is now the fuel for almost all nuclear reactors.
Two other fissionable actinoid isotopes are important. The first is plutonium-239, which may be prepared by neutron bombardment of uranium-238 and thus is formed as a by-product in uranium reactors in which uranium-238 is exposed to neutrons. Plutonium-239 can be used in place of uranium-235 in atomic weapons or in reactors. The second is potentially even more important, because it is produced from the element thorium, of which there are enormous reserves on Earth. When the isotope thorium-232 is bombarded by neutrons, it captures one neutron and is converted to thorium-233. This isotope decays by beta emission to protactinium-233, which again emits a beta particle to give uranium-233, also a fissionable isotope of uranium. Since uranium is a relatively scarce element, development of nuclear power on a large scale is expected to deplete rapidly the uranium that can be produced economically. The use of thorium thus could extend the supplies of fissionable material by about threefold, and the use of plutonium as well could more than double them. Breeder power reactors are designed so that very few neutrons are lost through the surface or from absorption by impurities, and more fissionable material (either uranium-233 or plutonium-239) is produced than is consumed. Should such breeder reactors be made practicable, never-ending supplies of fissionable isotopes would be available.
Although no other fissionable isotopes of the actinoids occur in significant amounts, there are practical uses of the nonfissionable isotopes because of the amounts of heat they produce by nuclear decay. For special power sources that require great dependability, thermoelectric generators using these isotopes have been considered. Alpha-emitting isotopes with radioactive half-lives in the range of several months to 100 years or more are suitable candidates. (Beta- or gamma-emitting isotopes require too much shielding to be usable.) Curium-244, which has a half-life of 18.1 years, produces 2.83 watts of heat per gram. Plutonium-238 produces 0.57 watt per gram and has the much longer half-life of 87.74 years. Plutonium-238 power sources have been planted on the Moon and have been used in space missions to the outer planets to provide electrical energy for transmitting messages back to Earth. Plutonium-238 also was used as the power source for pacemakers in the 1970s, before high-efficiency chemical batteries became available.