Most nuclear power plants in operation or under construction in the world today are using, and
will continue to use for the near future, ordinary (light) water cooled and moderated reactors: the Pressurized
Water Reactor (PWR) and the Boiling Water Reactor (BWR). A small number use the heavy
water cooled and moderated reactor (PHWR). The expectations are that the fast-breeder reactor power
plant and perhaps an improved version of the gas-cooled reactor power plant will come on line in
increasing numbers in the twenty-first century. Almost all-current water reactors use slightly enriched
uranium dioxide, UO2, fuel. The fuel has to go through a cycle that includes prereactor preparation,
called the front end, in-reactor use, and post reactor management, called the back end.