2, CentraleSupélec, Paris, , France
Spent nuclear fuels include a variable amount of minor actinides incorporated within the UO2 matrix as a result of the ongoing nuclear reactions during operation time. These actinides contribute to the radioactivity of the spent fuel, whose most long-living component is constituted by α-emission, lasting up to millennia and even farther. The accumulation of α-damage and radiogenic He affects the microstructure of the material and might result detrimental for the spent fuel integrity. For this reason, it is fundamental to know exactly how the spent fuel evolves, in order to accurately predict its behaviour over the long term.
To investigate such a slow evolution in a laboratory timeframe, a suitable way is the preparation of UO2 surrogates doped with highly-emitting actinides that will produce high levels of damage and radiogenic He over a shorter periods. In this work, UO2 disks containing tailored amounts of highly α-emitting 238Pu were produced by powder coprecipitation and sintering, and successively characterized. The coprecipitation route was chosen in order to obtain a homogeneous solid solution of the two oxides from the very beginning of the process, while the successive sintering process was tailored to mantain the homogeneous distribution of the α-dopant as well as a nearly complete densification (95%TD, compliant with the real nuclear fuel). The two chosen Pu percentages were 2.5 and 10% and were selected so that all the monitored evolving material properties come to saturation within 2 years.
The samples were then divided into 3 batches and stored at different temperatures: liquid nitrogen (approximately -195°C), room temperature (about 25°C) and wet storage conditions (around 200°C). Particular precaution was taken for the transportation between the storage and the actual characterisation. The choice of these three temperatures should allow differentiating the nature of the defects formed and their temperature-dependant evolution, in particular during spent fuel storage.
The samples evolutions were monitored by means of several characterization techniques: XRD (structure), TEM (microstructure), DSC (defect energy), LAF (thermal properties), Helium thermal desorption spectrometry, mechanical testing, and Raman spectroscopy. The outcome of this work should help predicting the long term behaviour of spent fuel.