Nuclear Chemistry
Nuclear Reactors

Dr. Frank Settle

    There are different types of nuclear reactors. Most are used for power generation, but some can also produce plutonium for weapons and fuel. Two components are common to all reactors, control rods and a coolant. Control rods determine the rate of fission by regulating the number of neutrons. These rods consist of neutron-absorbing elements such as boron. The coolant removes the heat generated by fission reactions. Water is the most common coolant, but pressurized water, helium gas, and liquid sodium have been used.

    (Courtesy of the Uranium Information Center)

    Slow-neutron reactors operate on the principle that uranium-235 undergoes fission more readily with thermal or slow neutrons. Therefore, these reactors require a moderator to slow neutrons from high speeds upon emerging from fission reactions. The most common moderators are graphite (carbon), light water (H2O), and heavy water (D2O). Since slow reactors are highly efficient in producing fission in uranium-235, slow-neutron reactors operate with natural or slightly enriched uranium. The reactors at Hanford, which produced plutonium for U.S. nuclear weapons, and the one at Chernobyl were water-cooled, graphite-moderated, slow-neutron reactors. Today, most U.S. reactors used for generating electric power employ light water as both moderator and coolant. Light-water reactors are classified as either pressurized-water reactors (PWR) or boiling-water reactors (BWR), depending on whether the coolant water is kept under pressure or not. The long time periods, typically 12 to 18 months, between refueling of light-water reactors make it difficult to use them as a source of plutonium.

    Hanford Weapons Reactor
    (Courtesy of the Department of Energy)

    There is a problem with light-water reactors: while slowing some neutrons, light water also absorbs many others. This means that light-water reactors require slightly enriched (up to 20% U-235) uranium fuel to sustain the fission reaction. Heavy-water reactors do not suffer from this problem and can thus use natural uranium as fuel. However, the task of isolating the small amount of D2O present in natural water requires considerable amounts of electricity. The Canadian nuclear program emphasizes heavy-water reactors. Heavy water is classified as a "sensitive material" because a nation possessing it can produce plutonium directly from natural uranium, thus eliminating the need for enrichment.

    Fast-breeder reactors are distinguished from other reactors by the fact that they produce more fuel than they consume. For each fission, more than one neutron is absorbed by U-238 to produce Pu-239. Breeding, therefore, requires lots of neutrons. At least one neutron is required to sustain the chain reaction and more than one is required to breed Pu-239. Since a fission event produces between two and three neutrons, the reactor requires use of all available neutrons. More neutrons are produced when fission is produced by fast-neutron fission in Pu-239. Thus breeder reactors use plutonium or highly enriched uranium as fuel with no moderator present to slow the neutrons. The design of fast-breeder reactors poses greater safety problems than those of other reactor types. The challenge is to develop a safe fast-breeder reactor that is economically competitive with thermal (slow-neutron) reactors, even when the lower fuel costs are accounted for. Currently the price of natural uranium and enrichment is not high enough to justify the additional costs associated with the use of breeder reactors. Thus, breeder reactors are more complex than other types of reactors and raise concerns about the proliferation of plutonium.

    Approximately 22% of the electricity used in the United States is generated by nuclear reactors, although no new reactors have been placed in operation for the past 15 years. France leads the world in the generation of electricity with more than 70% coming from nuclear sources. France and Great Britain reprocess spent reactor fuel from commercial reactors. Concerns about nuclear weapons proliferation led President Carter to ban reprocessing of spent fuel rods in the United States in 1977. In 1981, President Reagan lifted this ban, but the nuclear power industry has shown little interest in reprocessing, as there are abundant reserves of uranium in this country. Of course, reactors at Hanford, Washington, and Savannah River, South Carolina, processed irradiated uranium to obtain weapons-grade plutonium from 1945 to 1988.

    Chernobyl Nuclear Power Plant following the 1986 accident
    (Courtesy of the Department of Energy)

    Reactor safety has been called into question by two major accidents, one in 1979 at the Three Mile Island (TMI) nuclear power plant in Pennsylvania and the other in 1986 at Chernobyl in the former Soviet Union. The latter was the worst reactor accident in history, with 31 people dying of direct radiation poisoning and thousands more exposed to high doses of radiation. However, although much smaller in scope, with no deaths, TMI also had a profound impact on the public’s view of commercial nuclear power. The TMI plant was a pressurized light-water reactor, whereas Chernobyl involved a water-cooled graphite core similar to the Hanford weapons reactors. Both accidents involved human error overriding safety features built into the reactor systems. No new nuclear power plants have been built in the United States since the late 1970s.

    Complete Bibliography on Nuclear Reactors from ALSOS Digital Library for Nuclear Issues

©2003 Kennesaw State University
Principal Investigator Laurence Peterson
Project Director Matthew Hermes