Plant MaterialsDOE-HDBK-1017/2-93CLADDING AND REFLECTORSIn the case of a fast reactor, neutron thermalization is not desirable, and the reflector will consistof 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 whichmaterial to use is based largely on the nuclear considerations given above and the essentialneuronic properties of the materials. Most power reactors use water as both the moderator andreflector, as well as the coolant. Graphite has been used extensively as moderator and reflectorfor 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 toany extent. Beryllium has been used in a few instances such as test reactors, but is not used inany power reactors. Reactors using heavy water as the moderator-reflector have the advantageof being able to operate satisfactorily with natural uranium as the fuel material; enricheduranium is then not required. Zirconium hydride serves as the moderator in the Training,Research, Isotopes, General Atomic (TRIGA) reactor. The zirconium hydride is incorporatedwith enriched uranium metal in the fuel elements.Rev. 0Page 13MS-05
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