Alumina or Al2O3 is a chemical compound made up of Aluminum and Oxygen. It possesses good chemical and thermal stability, thermal and electrical insulation and good relative strength, which makes them useful in various engineering applications. It is available in various purity levels including 99.8% and 99.0%. The required level of purity, grain size and density depends on its target application and the specification of the product. Being the most natural form of aluminum oxides, it is abundantly available and hence preferred across diverse applications, from manufacture of glasses to nuclear power plants. Alumina tubes are normally used in controls rods in nuclear power plants. These tubes or spacers are used to maintain inter-element spacing between the fuel rods and hold them in their proper location. They also act as a thermal insulator for the fuel pin end caps and welds.
Due to its various physical, chemical and irradiation properties, alumina spacers are used in nuclear reactors to create various levels of separation between fuel rods. These spacers are made out in the form of grids which help position the source material within the cladding. Alumina is chosen for the spacer rods because it has a low absorption cross section and is chemically inert. These spacers are provided with cut-offs for the flow path. Spacers are fixed to the flow tube by the means of pins.
It is important that these spacers are ground to a very high level of accuracy to maintain high efficiency of the fuel rod. The constant heat up and cooling of the fuel may cause wear on the welded end caps and thus alumina is used for abrasion resistance and metallic component protection. This protection is possible due to high thermal conductivity and small thermal expansion of alumina. The spacer does not produce heat and serves as thermal protection to the fuel end pins. The spacer pellet also limits the region of plastic deformation at the end caps to several surface elements.