CLADDING AND REFLECTORS
In the case of a fast reactor, neutron thermalization is not desirable, and the reflector will consist
of a dense element of high mass number.
Materials that have been used as reflectors include pure water, heavy water (deuterium oxide),
beryllium (as metal or oxide), carbon (graphite), and zirconium hydride. The selection of which
material to use is based largely on the nuclear considerations given above and the essential
neuronic properties of the materials. Most power reactors use water as both the moderator and
reflector, as well as the coolant. Graphite has been used extensively as moderator and reflector
for thermal reactors. Beryllium is superior to graphite as a moderator and reflector material but,
because of its high cost and poor mechanical properties, it has little prospect of being used to
any extent. Beryllium has been used in a few instances such as test reactors, but is not used in
any power reactors. Reactors using heavy water as the moderator-reflector have the advantage
of being able to operate satisfactorily with natural uranium as the fuel material; enriched
uranium is then not required. Zirconium hydride serves as the moderator in the Training,
Research, Isotopes, General Atomic (TRIGA) reactor. The zirconium hydride is incorporated
with enriched uranium metal in the fuel elements.