The Chemical State of Fission Products in Oxide Fuels at Different Stages of the Nuclear Fuel Cycle

A survey of work at the Kernforschungszentrum Karlsruhe is presented on the chemical state of selected fission products that are relevant in the fuel cycle of light water reactor (LWR) and fast breeder reactor fuels. The influence of fuel type and irradiation progress on the composition of the Mo-Tc-Ru-Rh-Pd fission product alloys precipitated in the oxide matrix is examined using the respective multicomponent phase diagrams. The kinetics of dissolution of these phases in nitric acid at the reprocessing stage is discussed. Composition and structure of the residues, and the reprecipitation phenomena from highly active waste (HA W), are elucidated. A second metamorphosis of the fission products is recognized during the vitrification process. The formation of Ru(Rh) oxide and Pd(Rh,U,Te) alloys in simulated vitrified HA W concentrate and in HAW concentrate from the reprocessing of irradiated LWR fuels is interpreted on the basis of heterogeneous equilibria.