Light Water Reactors

The nuclear fission reactors used in the United States for electric power production are classified as "light water reactors" in contrast to the "heavy water reactors" used in Canada. Light water (ordinary water) is used as the moderator in U.S. reactors as well as the cooling agent and the means by which heat is removed to produce steam for turning the turbines of the electric generators. The use of ordinary water makes it necessary to do a certain amount of enrichment of the uranium fuel before the necessary criticality of the reactor can be maintained.

The fission of a U-235 nucleus in one fuel rod releases an average of 2.4 fast neutrons per fission. These neutrons are slowed down or "moderated" by the water between fuel rods, increasing the cross-section for neutron capture and fission by a U-235 nucleus in a neighboring fuel rod. The two varieties of the light water reactor are the pressurized water reactor (PWR) and boiling water reactor (BWR).

Uranium Enrichment

Natural uranium is only 0.7% U-235, the fissionable isotope. The other 99.3% is U-238 which is not fissionable. The uranium is usually enriched to 2.5-3.5% U-235 for use in U.S. light water reactors, while the heavy water Canadian reactors typically use natural uranium. Even with the necessity of enrichment, it still takes only about 3 kg of natural uranium to supply the energy needs of one American for a year.

Uranium enrichment has historically been accomplished by making the compound uranium hexaflouride and diffusing it through a long pathway of porous material (like kilometers!) and making use of the slightly higher diffusion rate of the lighter U-235 compound. There have been tests of centrifugal separators, but modern efforts are directed toward laser enrichment procedures.

The uranium fuel for fission reactors will not make a bomb; it takes enrichment to over 90% to obtain the fast chain reaction necessary for weapons applications. Enrichment to 15-30% is typical for breeder reactors.

Uranium Diffusion Enrichment

To produce the highly enriched uranium-235 needed for the development of nuclear weapons, a huge diffusion plant was built during World War II at Oak Ridge, Tennessee. Two other massive plants for uranium enrichment were built at Paducah, KY and Portsmouth, OH after the war. The compound uranium hexafluoride was produced and allowed to diffuse through thousands of stages of porous material, making use of the fact that the slightly lighter U-235 compound would diffuse faster than the U-238 compound. While electric power reactors require only enrichment from the 0.7% of natural uranium ore to about 3% U-235, the weapons applications required enrichment to over 90% U-235. Part of the enriched uranium was used to breed plutonium-239 for the more widely used plutonium devices.

Heavy Water Reactors

Nuclear fission reactors used in Canada use heavy water as the moderator in their reactors. Since the deuterium in heavy water is slightly more effective in slowing down the neutrons from the fission reactions, the uranium fuel needs no enrichment and can be used as mined. The Canadian style reactors are commonly called CANDU reactors.

Heavy water (D₂O) is 10% heavier than ordinary water and has a neutron moderating ratio 80 times higher than ordinary water. As of January 2002, 32 of the 438 nuclear reactors in operation around the world were of CANDU type.

References:
CANDU reactors