NUCLEAR ENERGY, BASIC PROCESSES OF

Nuclear energy, sometimes referred to as atomic energy, originates in the atomic nucleus, which is the extremely dense core at the heart of an atom. A large amount of energy can be released by nuclei in two different ways: fission, in which a very large nucleus is induced to break apart into two smaller ones, and fusion, in which two very small nuclei combine.

A great deal of energy is released when a large nucleus undergoes fission, but for most nuclei the fission process is not easy to initiate. There are very few nuclei—uranium-235 and plutonium-239, in particular—that are relatively easy to fission. At present, commercial nuclear reactors use fission of uranium-235 as the energy source. A uranium-235 nucleus can be induced to undergo fission through interactions with a slowly-moving neutron. The uranium nucleus absorbs the neutron, thus becoming a uranium-236 nucleus, which then breaks apart into two smaller nuclei called fission fragments. In addition, neutrons are also released, and these neutrons can then induce fission of other uranium nuclei in a chain reaction. Neutron-absorbing materials are located in the reactor core to ensure that the chain reaction proceeds at the proper pace. If too few neutrons are available for further fissions, then the reaction slows down or stops. If too many neutrons are available, then the reactor core can overheat.

The energy from nuclear fission is released mainly as kinetic energy of the new, smaller nuclei and neutrons that are produced. This kinetic energy is essentially heat, which is used to boil water to generate steam that turns turbines to drive electrical generators. In a nuclear power plant, the electrical generation area is essentially the same as in a plant that burns fossil fuels to boil the water.

Nuclear fission is also involved in nuclear weapons. To create a bomb, the concentration of the isotope uranium-235 must be increased to at least 85 percent from its natural concentration of only 0.7 percent. This increase of concentration is difficult and expensive. In a typical nuclear reactor the uranium-235 concentration in the fuel is only 3 to 4 percent, and hence a nuclear reactor cannot explode like a bomb. In a nuclear bombthe chain reaction is uncontrolled, and a large number of uranium nuclei undergo fission in a very short period of time, producing a nuclear explosion.

Generation of electricity by nuclear fission power reactors has many advantages. A primary advantage is that very little uranium fuel is required—only about two-hundred tons annually for a typical reactor. To generate the same amount of electricity, a coal-fired plant needs 3 million tons or 15,000 times as much coal. Another advantage is that nuclear power produces no air pollution such as nitrogen oxides and sulphur dioxide, both of which contribute to acid rain, and no greenhouse gases such as carbon dioxide and methane that contribute to global warming. Nuclear power also produces no ozone and no particulate matter in the air, and hence, in terms of air pollutants, a nuclear plant is much "cleaner" than a fossil-fuel electrical plant.

Nuclear reactors, however, do generate highly radioactive waste. This waste, which consists primarily of the fission fragments and their radioactive-decay products, must be stored for many years before its radioactivity decays to a reasonable level, and the safe long-term storage of this waste is a matter of great concern and debate. Fortunately, the volume of waste that is created is only about 20 cubic meters annually from a reactor, compared with 200,000 cubic meters of waste ash from a coal-fired plant. When nuclear weapons were tested in the atmosphere, the radioactive products from the nuclear explosions were released into the air and fell to Earth as radioactive fallout.

Another concern about nuclear power plants is their decommissioning. Nuclear reactors have useful lifetimes of about thirty years, after which they need to be shut down permanently. The remaining fuel, which is very radioactive, will have to be removed and stored for many decades. The entire reactor building structure has also been made somewhat radioactive, as well as all the pipes, valves, etc., in the plant. There are a number of options for dealing with this radioactive material, ranging from immediate dismantling of the plant (with some of the work probably done by robots) to using the normal reactor containment structure as a long-term storage facility. In the United States, the Nuclear Regulatory Commission has required that plant owners set aside sufficient funds for dismantling.

The greatest concern that most members of the general public have about nuclear energy is the possibility of a catastrophic accident such as occurred in 1986 at Chernobyl in the Ukraine. If reactors have been properly designed and the staff trained with safety in mind, then the chance of a major accident is very slight. The Chernobyl disaster was caused by a combination of poor reactor design and insufficient training of the reactor operators, who violated many of the operating procedures related to safety.

Nuclear fusion is the energy-producing process that occurs in the sun and other stars. Small nuclei such as those in hydrogen and helium fuse together to produce larger nuclei, releasing energy. Nuclear fusion is not yet commercially viable as an energy source, since extremely high temperatures are required to initiate fusion, and containment of the fusing nuclei at these temperatures is difficult. However, if nuclear fusion ever becomes a usable energy source, a typical fusion power reactor would require less than a ton of fuel annually.

It is often stated that nuclear fusion will produce no radioactive hazard, but this is not correct. The most likely fuels for a fusion reactor would be deuterium and radioactive tritium, which are isotopes of hydrogen. Tritium is a gas, and in the event of a leak it could easily be released into the surrounding environment. The fusion of deuterium and tritium produces neutrons, which would also make the reactor building itself somewhat radioactive. However, the radioactivity produced in a fusion reactor would be much shorter-lived than that from a fission reactor. Although the thermonuclear weapons (that use nuclear fusion), first developed in the 1950s provided the impetus for tremendous worldwide research into nuclear fusion, the science and technology required to control a fusion reaction and develop a commercial fusion reactor are probably still decades away.

Ernie McFarland

BIBLIOGRAPHY

Ahearne, J. F. (1993). "The Future of Nuclear Power." American Scientist 81:24–35.

Atomic Energy of Canada Limited (AECL). (1999). Nuclear Sector Focus: A Summary of Energy, Electricity and Nuclear Data.Mississauga, Ontario, Canada: AECL

Atomic Energy of Canada Limited (AECL). (2000). <http://www.aecl.ca/english/energy/energy_f.html>.

Cordey, J. G.; Goldston, R. J.; and Parker, R. R. (1992). "Progress Toward a Tokamak Fusion Reactor." Physics Today45(1):22–30.

McFarland, E. L.; Hunt, J. L.; and Campbell, J. L. (1997). Energy, Physics, and the Environment, 2nd ed. University of Guelph, Ontario: Department of Physics.