CLADDING AND REFLECTORSDOE-HDBK-1017/2-93Plant MaterialsAluminum, such as the 1100 type, which is relatively pure (greater than 99%), has been used inlow power, water-cooled research, training, and materials testing reactors in which the operatingtemperatures are below 100C. Magnesium, in the form of the alloy magnox, serves as claddingfor the uranium metal fuel in carbon-dioxide cooled, graphite-moderated power reactors in theUnited Kingdom. The alloy zircaloy, whose major constituent is zirconium, is widely used asthe fuel-rod cladding in water-cooled power reactors. The alloys in common use as claddingmaterial are zircaloy-2 and zircaloy-4, both of which have mechanical properties and corrosionresistance superior to those of zirconium itself. Although beryllium is suitable for use ascladding, it is not used due to its high cost and poor mechanical properties.The choice of cladding material for fast reactors is less dependent upon the neutron absorptioncross section than for thermal reactors. The essential requirements for these materials are highmelting point, retention of satisfactory physical and mechanical properties, a low swelling ratewhen irradiated by large fluences of fast neutrons, and good corrosion resistance, especially tomolten sodium. At present, stainless steel is the preferred fuel cladding material forsodium-cooled fast breeder reactors (LMFBRs). For such reactors, the capture cross section isnot as important as for thermal neutron reactors.In 1977 the Carter Administration deferred indefinitely the reprocessing of nuclear fuels fromcommercial power reactors. This led the electric utility industry to conduct research onhigh-burnup fuels and programs that would allow an increase in the length of time that the fuelrods remain in the reactors. High integrity and performance of fuel cladding will become evenmore important as these high-burnup fuel rods are designed and programs for extended burnupof nuclear fuels are placed into operation.ReflectorMaterialsA reflector gets its name from the fact that neutrons leaving the reactor core hit the reflector andare returned to the core. The primary consideration for selecting a reflector material is itsnuclear properties. The essential requirements for reflector material used in a thermal reactorare:Low macroscopic absorption (or capture) cross section to minimize loss ofneutronsHigh macroscopic scattering cross section to minimize the distance betweenscattersHigh logarithmic energy decrement to maximize the energy loss per collision dueto low mass numberTemperature stabilityRadiation stabilityMS-05Page 12Rev. 0
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