The international VERDON-ISTP program aims to study and reduce the risks of environmental emissions of radioactive products in case of severe nuclear accident. Among others, several tests consist in heating fuel pellets to a very high temperature so as to monitor the release of fission products. The experimental data about the microstructure and the chemical composition of the nuclear fuel under final atmosphere (oxidizing or reducing) can be integrated in scenario codes to better predict its behavior in case of accident.
Offering a safe environment for manipulating and analyzing radioactive samples and delivering compositional information from sub-micron areas with excellent precision and accuracy, Shielded EPMA is a tool of choice for characterizing the distribution and chemical composition of fission products in the different phases.
In this particular series of tests, Shielded EPMA allowed to observe the interactions occurring between the nuclear fuel and the cladding material. As shown above, under oxidizing conditions, interdiffusion of U and Zr takes place at the interface between the fuel and the cladding. Quantitative profiles produced in this area reveal an interdiffusion distance over approximately 70 µm.
Shielded EPMA was also used to precisely analyze a molten zone (U, Pu, Zr)O2 that penetrates in the crack of a pellet under reducing atmosphere, and to investigate the impact of the atmosphere on fission product speciation (‘white inclusions’ composed of metallic Mo, Ru, Rh, Pd and Tc).
