Plant MaterialsDOE-HDBK-1017/2-93CLADDING AND REFLECTORSCLADDING AND REFLECTORSNuclear fuels require surface protection to retain fission products and minimizecorrosion. Also, pelletized fuel requires a tubular container to hold the pellets inthe required physical configuration. The requirements for cladding material toserve these different purposes will vary with the type of reactor; however, somegeneral characteristics can be noted. This chapter will discuss the generalcharacteristics associated with cladding and reflectors.EO 1.5STATE the four major characteristics necessary in a material usedfor fuel cladding.EO 1.6IDENTIFY the four materials suitable for use as fuel claddingmaterial and their applications.EO 1.7STATE the purpose of a reflector.EO 1.8LIST the five essential requirements for reflector material in athermal reactor.CladdingCladdingis used to provide surface protection for retaining fission products and minimizingcorrosion. Cladding is also used to contain pelletized fuel to provide the required physicalconfiguration.Mechanical properties, such as ductility, impact strength, tensile strength, and creep, must beadequate for the operating conditions of the reactor core. Ease of fabrication is also important.It is desirable that ordinary fabrication procedures be applicable in fabricating the desired shape.The cladding must have a high corrosion resistance to its operating environment. It must havea high melting temperature to withstand abnormal operating conditions such as high temperaturetransients. Thermal conductivity should be high to minimize thermal stresses arising fromtemperature differences, and the coefficient of expansion should be low or well-matched with thatof other materials. The cladding material should not be susceptible to radiation damage.The nuclear properties of fuel cladding material must also be satisfactory. For thermal reactors,it is important that the material have a reasonably small absorption cross section for neutrons.Only four elements and their alloys have low thermal-neutron absorption cross sections andreasonably high melting points: aluminum, beryllium, magnesium, and zirconium. Of these,aluminum, magnesium, and zirconium are or have been utilized in fuel-element cladding. Rev. 0Page 11MS-05
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