In the safety assessment of geological disposal of spent fuel, the potential release of radionuclides upon breach of the safety barriers needs to be taken into account. For decades already, knowledge has been acquired in the field of radioactive waste management and especially geological disposal which has allowed to progress towards licensing of geological disposal facilities. Further R&D efforts are needed to maintain and increase knowledge about the development, deployment and operation of such geological disposal facilities over the decades to come, to address evolving regulatory concerns, to ensure optimization of waste management routes and disposal solutions, … [1, 2].
A multi-year Spent Fuel Autoclave Leaching Experiment (SF-ALE) has started in the LHMA hot cells in 2017. The main objective of the SF-ALE is to tackle the unknowns in radionuclide release using state-of-the-art autoclave instrumentation which allows accurate sampling under carefully monitored fuel dissolution conditions. The main objective of the proposed PhD topic is to support the interpretations of the data produced in the SF-ALE and to gain deeper understanding on the fundamentals of fuel degradation under repository conditions. Early research efforts by SCK•CEN in the field include research on the slow evolution of irradiated MOX and UO2 fuel exposed to dry and wet atmospheres [3,4].
Nuclide release upon fuel dissolution is determined by sampling the dissolution liquid and gas, but this provides only indirect information from where the release takes place. As the fuel microstructure varies significantly in all directions, information on release fractions with an truly three dimensional information and adequate spatial resolution is crucial for understanding the fundamentals of the nuclide release. The proposed PhD work aims to clarify this relationship by taking the full advantage of the recently installed focused ion beam (FIB) system at LHMA. FIB combined with analytical scanning-electron microscopy (SEM) allows to obtain 3D information at submicron scale from spent nuclear fuel samples. The FIB observations will be complimented by other in-house methods, such as transmission electron microscopy (TEM) , X-ray methods (XPS, XRD)[6, 7], and Raman spectroscopy. It is foreseen that in the course of the PhD project, the hot cell Raman instrumentation will further be developed to allow measurements directly for the spent fuel samples which have experienced prolonged dissolution experiment. Raman can provide important data regarding changes in the crystal geometry, such as oxidation and secondary phase formation .
 Fast / Instant Release of Safety Relevant Radionuclides from Spent Nuclear Fuel - FIRST-Nuclides, Final scientific report (2014) www.firstnuclides.eu
 E. Gonzales-Robles et al., “Determination of fission gas release of spent nuclear fuel in puncturing tests and in leaching experiments under anoxic conditions,” Journal of Nuclear Materials 479 (2016) 67.
 A. Leenaers et al., “ Oxidation of spent UO2 fuel stored in moist environment,” Journal of Nuclear Materials 317 (2003) 226.
 A. Leenaers et al., “Microstructure of spent MOX fuel stored under dry air for 25 years,” Proc. of ICEM 2 (2001) 1093.
 G. Leinders et al., “Low-Temperature Oxidation of Fine UO2 Powders: A Process of Nanosized Domain Development,” Inorganic Chemistry 55 (2016), 3915.
 S. Van den Berghe et al., “XPS investigations on cesium uranates: mixed valency behavior of uranium,” Journal of Nuclear Materials 277 (2000) 28.
 A. Baena et al., “Lattice contraction and lattice deformation of UO2 and ThO2 doped with Gd2O3,” Journal of Nuclear Materials 467 (2015) 135.
 C. Jégou et al., “Oxidizing dissolution of spent MOX47 fuel subjected to water radiolysis: Solution chemistry and surface characterization by Raman spectroscopy,” Journal of Nuclear Materials 399 (2010) 68.