boiling water reactor  A boiling water reactor (BWR) is a type
of light-water nuclear reactor developed by the General Electric
Company in the mid 1950s.   The BWR is characterized by two-
phase fluid flow (water and steam) in the upper part of the
reactor core. Light water (i.e., common distilled water) is the
working fluid used to conduct heat away from the nuclear fuel.
The water around the fuel elements also "thermalizes" neutrons,
i.e., reduces their kinetic energy, which is necessary to
improve the probability of fission of fissile fuel. Fissile fuel
material, such as the U-235 and Pu-239 isotopes, have large
capture cross sections for thermal neutrons.  Comparison with
other reactors  Light water is ordinary water. In comparison,
some other water-cooled reactor types use heavy water, such as
the Canadian made CANDU reactor series. In heavy water, the
deuterium isotope of hydrogen replaces the common hydrogen atoms
in the water molecules (D2O instead of H2O, molecular weight 20
instead of 18).  The Press