Abstract
In the context of the nuclear fuel cycle, Uy-1PuyO2-x Mixed Oxides (MOx) are currently the most employed fuels. These materials have to meet accurate physico-chemical properties, known to change drastically during irradiation with the formation of Fission Products (FP). Investigating the FP behaviour is thus crucial to predict the fuel properties evolution during irradiation.Model Uranium-based materials called SIMfuel have been developed in the last decades to overcome the high radiotoxicity of spent fuel, and to enable separation-of-effect studies on its otherwise overwhelmingly complicated chemistry. Doping UO2 with selected stable FP isotopes reduces significantly the radiological risk, but the complex interaction between Pu and FP cannot be addressed. Therefore, we developed a fabrication route for U1-yPuyO2-x SIMfuel (SIMMOx), in order to study the speciation of FP inside the MOx fuel. The resulting material is representative of irradiated MOx fuel with Pu content of 24 wt.% and a burnup of 13 at.%.In order to extract in-depth information on the actinides and FP speciation, we studied the SIMMOx samples through a multi-scale approach, characterizing its structural properties, its microstructure (µm-scale), and the atomic environment of each element. The reduced radiotoxicity of such samples allowed us to employ powerful synchrotron techniques, which are not traditionally available to irradiated fuel.Through the coupling of such a variety of techniques we were able to paint a thorough picture of the FP speciation inside irradiated MOx fuel. First, we observed the effects of soluble FP (Ce, La, Nd, Y, Sr, Zr) on the (U,Pu,FP)O2 solid solution, which for example, cause a slight oxidation of the actinides. Then, we focused on the speciation of the metallic and oxide precipitates (white and grey phases), the analysis of which required the design of a protocol to safely perform SP-XRD on Pu-bearing samples. We determined that the addition of Ba in the system causes the formation of perovskite inclusions, as well as the oxidation of a fraction of the Mo inventory, and hence a shift in the speciation of the white phases.Arriving at such a complete description of the speciation of FP, was only possible through the multi-scale coupling of several techniques, some of which are not easily available to irradiated fuel studies. Furthermore, the ability to design sample batches with different compositions, allows to determine the effect of – potentially – individual FP on the entire system, performing real separation-of-effect studies, which is impossible to do on irradiated MOx.In this context, the importance and potential of SIMMOx to investigate the properties of irradiated MOx, appear therefore clear.