Determination of burnup of fuel samples at SCK-CEN

This poster describes how SCK-CEN teams determine the burnup of UO₂ and MOX fuels. In radiochemistry, burnup is expressed as FIMA (Fissions per Initial Metal Atom) and is calculated as the number of fissions relative to the number of heavy metal atoms initially present in the fuel. This last value is the sum of the number of fissions and the number of heavy atoms at the end of irradiation (as measured). The number of heavy atoms at the end of irradiation can be determined with Thermal Ionization Mass Spectrometry (TIMS) and radiochemical techniques, while the number of fissions can be derived from a fission product monitor for which the concentration in the fuel is proportional to the number of heavy atoms fissioned. Commonly used burn-up monitors are several Nd isotopes and the gamma emitters ¹³⁷Cs and ¹⁴⁴Ce. Depending on the chosen burn-up monitor, this can be determined by either radiochemical methods or mass spectrometry. First, fuel samples are dissolved in a hot-cell employing a multi-step procedure in order to successfully separate if from the cladding and obtain complete dissolution. Afterwards, a first aliquot of dissolved sample is used for alpha and gamma spectrometry, while other aliquots are dedicated for TIMS analyses. Complex chemical separations are needed in order to separate base actinides, minor actinides and the many fission products. Moreover, each technique requires specific sample preparation. SCK-CEN teams have gained experience with the burnup determination of commercial NPP spent fuels and of experimental reactor fuels in the framework of the HEU/LEU conversion