<P> Molten salt fuels were used in the LFTR known as the Molten Salt Reactor Experiment, as well as other liquid core reactor experiments . The liquid fuel for the molten salt reactor was a mixture of lithium, beryllium, thorium and uranium fluorides: LiF - BeF - ThF - UF (72 - 16 - 12 - 0.4 mol%). It had a peak operating temperature of 705 ° C in the experiment, but could have operated at much higher temperatures, since the boiling point of the molten salt was in excess of 1400 ° C . </P> <P> The aqueous homogeneous reactors (AHRs) use a solution of uranyl sulfate or other uranium salt in water . Historically, AHRs have all been small research reactors, not large power reactors . An AHR known as the Medical Isotope Production System is being considered for production of medical isotopes . </P> <P> Uranium dioxide (UO) powder is compacted to cylindrical pellets and sintered at high temperatures to produce ceramic nuclear fuel pellets with a high density and well defined physical properties and chemical composition . A grinding process is used to achieve a uniform cylindrical geometry with narrow tolerances . Such fuel pellets are then stacked and filled into the metallic tubes . The metal used for the tubes depends on the design of the reactor . Stainless steel was used in the past, but most reactors now use a zirconium alloy which, in addition to being highly corrosion - resistant, has low neutron absorption . The tubes containing the fuel pellets are sealed: these tubes are called fuel rods . The finished fuel rods are grouped into fuel assemblies that are used to build up the core of a power reactor . </P> <P> Cladding is the outer layer of the fuel rods, standing between the coolant and the nuclear fuel . It is made of a corrosion - resistant material with low absorption cross section for thermal neutrons, usually Zircaloy or steel in modern constructions, or magnesium with small amount of aluminium and other metals for the now - obsolete Magnox reactors . Cladding prevents radioactive fission fragments from escaping the fuel into the coolant and contaminating it . </P>

Purpose of fuel rods in a nuclear reactor
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