In the framework of the pre-licensing phase of MYRRHA, the source term assessment is carried out for the MOX fuel and LBE coolant to provide safety-relevant pieces of information.\\

The buildup of activation and spallation products in the LBE during operation and after shutdown of the reactor is estimated. The attention is focused the sub-critical configuration, in which higher particle fluxes are expected with enhanced production of safety concerning radionuclides.\\
In the spallation target assembly, where the linear accelerator is locates, the worst conditions are expected and the focus is given to the estimation of nuclide inventory in this region.
Among safety concerning volatile nuclides $^3$H, $^{195}$Hg, $^{197}$Hg and $^{210}$Po convey the largest dose. The latter is also the main contributor to decay heat in the first decay period after shutdown. Most of the nuclides analysed are mainly generated by spallation reactions, whose relevance reduces moving further from the spallation target.\\

The source term in the fuel is assessed from the nuclide inventory in every fuel assembly present in the core after one irradiation cycle starting from the equilibrium condition. The buildup of volatile nuclides as function of burnup is estimated, showing that the peak concentration of short lived nuclides is reached when the FA is irradiated in the central position, while the long lived fission products reach the maximum concentration when the fuel assembly has the highest burnup and has been reshuffled in all core positions.\\
From the nuclide inventory, envelope activity and decay heat curves are drawn for a period of 70 y of decay. Comparison of critical and sub-critical configuration is presented for all the observables.\\

Power levels in the fuel assemblies of the equilibrium core provide the normalization condition for a fuel assembly depletion model. The FA model is built in Serpent2, it mimics the fuel assembly reshuffling in the core and it is suitable for uncertainty analysis.
Input parameter such as nuclear data (cross sections and neutron fission products yields) are perturbed to estimate the impact on the observables of the analysis: volatile fission products, actinides relevant for spent nuclear fuel and decay heat.\\
Cross sections uncertainties have an impact of plutonium and minor actinides. The only volatile with relevant statistical variation is $^{134}$Cs. In contrast, the perturbation of fission yields directly impacts the degree of ﬁnal uncertainty computed for ﬁssion products.
The uncertainty on integral parameters, such as decay heat, is determined by the concentration uncertainty of the nuclides contributing to the decay heat rate as function of decay time.