The redistribution of fission products in UO2 nuclear fuels

During the operation of nuclear power reactors, which are fuelled with ceramic UO2 fuel rods, the fission of the U235 nuclei leads to the formation of fission products which are isotopes of elements in all of the Groups of the Periodic Table. The major fission products, present in 1-10% abundance, fall into five groups divided according to the chemical interaction of each product with the fuel  

The chemical identities of the fission products determine their subsequent redistribution, those elements which are in the gaseous state at the temperature of the operation migrating to the cooler exterior of the fuel rods, and the less voltile elements undergoing incorporation in the fuel rod in solid solution. Thus caesium and iodine migrate to the gas fill which surrounds the fuel rod, and elements such as the rare earths and zirconium are accommodated in solid solution in U02 without significant migration along the fuel rod radius. Strontium and barium oxidize to form separate islands which can be seen under the microscope. 

As a result of nuclear fission the oxygen/metal ratio increases in the fuel across the wide range of non-stoichiometry of U02+. The oxygen potential of 

Sorenson (eds). Nonstoichiometric Oxides. Academic Press New York (1981). 

Nickel (ed.). Corrosion of Advanced Ceramics. NATO ASI Series, Kluwer Academic (1994) TA 455 C43C67. 

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