In the previous section we listed four characteristics of radioactivity and nuclear decay that form the basis for the use of radioisotopes in the health and biological sciences. A fifth characteristic of nuclear reactions is that they release enormous amounts of energy. The first nuclear reactor to achieve controlled nuclear disintegration was built in the early 1940s by Enrico Fermi and his colleagues at the University of Chicago. Since that time, a great deal of effort and expense has gone into developing nuclear reactors as a source of energy. The nuclear reactions presently used or studied by the nuclear power industry fall into two categories: fission reactions and fusion reactions.

FIGURE 4.5 Cancer treatment with cobalt-60. The source moves along a circular track, rotating the radioactive beam around the patient, so that only the tumor receives continuous radiation.

A. Fission
In nuclear fission a large nucleus is split into two medium-sized nuclei. Only a few nuclei are known to undergo fission. Nuclear power plants currently in use depend primarily on the fission of uranium-235 and plutonium-239.

When a nucleus of uranium-235 undergoes fission, it splits into two smaller atoms and, at the same time, releases neutrons ( n) and energy. Some of these neutrons are absorbed by other atoms of uranium-235. In turn, these atoms split apart, releasing more energy and more neutrons. A typical reaction is:

The brackets around U indicate that it has a highly unstable nucleus. Under proper conditions, the fission of a few nuclei of uranium-235 sets in motion a chain reaction (Figure 4.6) that can proceed with explosive violence if not controlled. In fact, this reaction is the source of energy in the atomic bomb.

FIGURE 4.6 Diagram of a nuclear fission chain reaction. Each fission results in two (or more) neutrons that can react with other uranium atoms so that the number of nuclear fissions occurring soon reaches an enormous number.

 

In nuclear power plants, the energy released by the controlled fission of uranium-235 is collected in the reactor and used to produce steam in a heat exchanger. The steam then drives a turbine to produce electricity. Energy generation can be regulated by inserting control rods between the fuel rods in the reactor to absorb excess neutrons, thereby controlling the rate of the chain reaction. A typical nuclear power plant in operation today uses about 2 kg uranium-235 to generate 1000 megawatts of electricity. About 5600 tons (5.1 X 10⁶ kg) of coal are required to produce the same amount of electricity in a conventional power plant.

Uranium-235 (natural abundance 0.71%) is very scarce and difficult to separate from uranium-238 (natural abundance 99.28%). The much more abundant uranium-238 does not undergo fission and therefore cannot be used as a fuel for nuclear reactors. However, if uranium-238 is bombarded with neutrons (from uranium-235, for example), it absorbs a neutron and is transformed into uranium-239. This isotope undergoes beta emission to generate neptunium-239, which, in turn, undergoes another beta emission to produce plutonium-239:

Plutonium-239 also undergoes fission, with the production of more energy and more neutrons. These neutrons can then be used to breed more plutonium-239 from uranium-238. Thus, a so-called breeder reactor can produce its own supply of fissionable material. Several breeder reactors are now functioning in Europe.

Nuclear reactors using fissionable materials pose several serious risks to the environment. First is the everpresent danger that leaks, accidents, or acts of sabotage will release radioactive materials from the reactor into the environment. This problem has been a continuing concern since the accident at the Three-Mile Island nuclear plant in March 1979 - a concern that has increased in the wake of the disaster at the Chernobyl reactor in the former Soviet Union in April 1986, where radioactive fallout spread within days across the globe. Second, many of the products of nuclear fission are themselves radioactive. The radioactivity from spent nuclear fuel and from the products of nuclear fission will remain lethal for thousands of years; the safe disposal of these materials is a problem that has not yet been solved. Third, obsolete generating plants also present a problem to future generations, for they contain much radioactive material. One suggestion has been to encase such plants in concrete for 100 years or more. Although it may become necessary, this solution is hardly simple or permanent.

B. Fusion
Nuclear fusion, the other process currently under study for the generation of atomic energy, depends on the putting together or fusing of two nuclei to form a single nucleus. One of the most promising fusion reactions generates energy by the fusion of two deuterium (hydrogen-2) atoms to form an atom of helium-3:

Such reactions require enormously high energy to force the two positively charged nuclei close together enough to fuse. Once the nuclei fuse, however, much more energy is released than is required for the reaction. Nuclear fusion occurs in the core of the sun, where the temperature is approximately 40 million degrees Celsius. Unfortunately, scientists have not yet found a way to produce and control nuclear fusion on Earth. Controlled nuclear fusion produces almost no radioactive wastes and would therefore be a nonpolluting source of energy. A massive effort is being made in this country and abroad to find ways to harness this energy source.