Accessible fraction of the radionuclide inventory of the spent nuclear fuel (AFI)

When the fuel oxide is burned in reactor, a large amount of radionuclides (RN), composed of fission gases (FG), fission products (FP), activation products (AP) and actinides (and actinide-decay products) are created in the fuel assembly due to different processes (fission, neutron capture, decay), resulting in a complex dynamic system between the produced RN, the fuel matrix itself and the cladding materials during and after the irradiation. For the evaluation of the safety of the potential geological disposal of spent nuclear fuel in Belgium and elsewhere, it is necessary to have sufficient knowledge about the mechanisms and the rate by which the spent fuel will release its radionuclide inventory into the environment. Most of the inventory is confined in the fuel and will be released congruently with the dissolution of the oxide matrix, but a certain fraction, which is present in more accessible structures of the fuel assembly, may be released at a higher rate shortly after contact with ground water. This fraction, is called ‘accessible fraction of inventory’ (AFI) and contains a fraction of the inventory of the fuel and a fraction of the inventory of the cladding. The AFI is thus dependent on the location of the RN in the fuel and in the cladding, which is strongly dependent on the nuclide itself and on the conditions at which this nuclide is generated and its mobility in the fuel. Based on the chemical and physical characteristics of the nuclides during and after irradiation, the RN are grouped. Radionuclides belonging to the same group are expected to show similar behaviour in terms of AFI. Not all RN are relevant for the safety studies. A list of RN having a possible impact on the dose rate has been established by the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (NIRAS/ONDRAF). The concerned RN are 10Be, 14C, 36Cl, 41Ca, 59Ni, 79Se, 93Zr, 93Mo, 94Nb, 99Tc, 107Pd, 108mAg, 126Sn, 129I and 135Cs. Experimentally, the AFI of these critical RN is determined through leaching experiments performed on today’s fuels. This report presents an exhaustive compilation of experimental AFI of the fuel, available in the literature for these relevant RN and limited data on the AFI of RN of the cladding. If experimental data is lacking for safety-relevant nuclides, data on other elements belonging to the same group can provide relevant information for these nuclides.