Analytical Characterization of Fuel Elements

A variety of fuel elanents are used for different types of reactors, but there are some common features. In most conunercial nnclear power plants (BWR [boiling water reactors] and PWR [pressurized water reactors] that are called in Russian VVER), the pellets are inserted into rods or tubes (usually zirconium alloys) that provide a barrier to prevent escape of fission products, the tubes or rods are arranged in bundles that are loaded into the reactor core. Usually a number of short rods are inserted into the sealed tube and held in place by a spring as described earlier. In some cases, like advanced gas cooled reactors (AGR), peUets are inserted into short narrow steel pins. Magnox reactors use magnesium alloys (usually with aluminum) rather the zirconium alloys. The fuel in some advanced reactors (TRISO) is in the form of microfuel particles with a UO2 (or UC (uranium carbide)) core surrounded by layers of pyrolytic carbon and 

The main tests that are required for the sealed tubes containing the fuel elements are mechanical and physical like the quality of pellet insertion and spring insertion, inspection of the welding system for the fuel rods, x-ray inspection, and rod inspection. 

The tubes themselves must be fully characterized in order to ensure that the zirconium alloys, or magnesium alloys, do not contain impurities that may affect the performance of the fuel. For example, the presence of traces of neutron absorbers, like hafnium that always accompanies zirconium in nature, or elements that modify the corrosion resistance of zirconium, must be determined. The ASTM has outlined the specifications for seamless wrought zirconium alloy tubes that are used for nuclear fuel cladding (B811 2013). The exact technical details and analytical test procedures do not directly involve uranium and are beyond the scope of this book. 

Highlights This section did not deal directly with the analytical chanistry or characterization of uranium, but is highly relevant to ensure the safety of the main commercial application of uranium—production of electric power in nuclear power plauts. Therefore, the analytical procedures were ouly briefly described. The main safety function of the fuel elements is to prevent the escape of fission products (mainly gaseous elements or volatile compounds) into the coolant and atmosphere. Therefore, very rigorous quality control measures are employed to ensure the physical integrity and chemical composition of these elements. 

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