Thermal and Radiation Stability of (Zr0.95241Am0.05)1-xNdxO2-x0.5 Phases: Updates from the RISE-241 ActUsLab JRC Project

The thermal and radiation stability of Zircaloy cladding material that houses spent nuclear fuel (SNF) is an important factor when considering the safe storage and eventual disposal of SNF in a geological repository. It is known that on the surface of the cladding, oxidised zirconia (ZrO2) phases are inherently present. Following fuel swelling and rim contact, the zirconia layer on the interior surface can interact with SNF elements, leading to the formation of phases such as pyrochlore, A2Zr2O7, and other zirconates, A2ZrO3 , among others. These phases essentially act as the first intermediate barrier between the SNF and the metallic cladding and consequently are important to consider in safety design, particularly for transport/release of radionuclides (RNs). A pertinent RN that contributes significantly to the radiological hazard of SNF, is the minor actinide isotope Am-241. The chemistry of Am is largely unique, being able to readily dissociate between its tetravalent and trivalent states in oxides, making it difficult to investigate via surrogate studies using lanthanides. Furthermore, Am-241 has a relatively short t1/2 of 432 years and decays via alpha emission (5.486 MeV), resulting in significant ensuing radiation damage in host materials. Consequentially, understanding the thermal and radiation stability of host material phases incorporating Am-241 is a pertinent endeavor for safety and disposal of SNF. As a part of the national project “AcE” funded by the German Federal Ministry of Education and Research (BMBF) and through the European Commission ActUsLab program, we have investigated several zirconium oxide polymorphs, including but not limited to Nd-pyrochlore and zirconia, doped with 5 mol% Am-241. The particular focus of the investigation is to understand the thermal and radiation stability of the different oxide polymorphs when Am-241 is incorporated. This presentation will highlight a number of on-going results from this research program, including high-temperature phase transformations, radiation induced lattice swelling, phase separation, and associated apparent redox activity induced by the presence of Am-241.