\chapter{Conclusions}
The work presented in this thesis has the purpose to provide updated values for MYRRHA source term in the fuel and LBE, that are of main importance for safety related studies in the framework of the pre-licensing phase of MYRRHA. Source term analysed comprises the nuclide vector (of volatile nuclides and actinides), activity and decay heat.\\

\textit{LBE source term}\\
The activation and spallation products generated by irradiating lead-bismuth eutectic (LBE) starting from the reference composition have been determined. This study focuses on the spallation target assembly, which experiences harsh conditions in terms of particle fluence. The activity of the selected nuclides was of primary importance, and their evolution was tracked for the entire 40-year irradiation period, followed by a decay time of 70 years.\\
The main contributors to this activity are $^{3}$H, $^{197}$Hg, and $^{210}$Po, with spallation target assembly (STA) activities of 2.8e15, 1.1e15, and 8e14 Bq, respectively, out of a total activity of 6.3e16 Bq. The decay heat is primarily dominated by $^{210}$Po for the first year of decay and $^{207}$Bi for the entire 70-year observation period.
Further division of the irradiation zone reveals the impact of spectral homogenization on the nuclide inventory output. Deviations of up to 50\% were observed for the selected nuclides. Most of them are underestimated in the homogeneous approach, except for $^{210}$Po, which is exclusively produced by neutron-induced reactions.\\
Expanding the scope of this work beyond the spallation target assembly to consider the entire core and reactor would allow for a global assessment of the impact of homogenization and an evaluation of the overall production of specific nuclides during irradiation. Additionally, a thorough investigation should be conducted to validate the assumption of employing fixed fluxes in the procedure applied in this work.

\textit{Fuel source term}\\
The output of the equilibrium core is analysed to take out the nuclide inventory in every fuel batch. The buildup of volatiles suggests they can be divided in two categories based on the half life: 
\begin{itemize} 
\item short lived nuclides that reach the peak concentration in the first irradiation cycle because of the higher fluxes, since the fuel assembly is irradiated in the core center (position 101);
\item relatively long lived nuclides that build up during irradiation and reach the maximum concentration at \acrshort{EoI}, before discharge.
\end{itemize}
Overall, the short lived production is maximized in the sub-critical configuration since the fluxes near to the spallation target are higher. The long lived reach a higher concentration in critical condition since FAs are reshuffled in more positions before being unloaded and the cumulative burnup is higher.\\
Derived quantities such as activity and decay heat are analysed. It is done assuming to unload all the FAs from the core at the same time and let them decay for a prescribed period. In this way the FA contributing the most to activity/decay heat at each decay time is defined. What is observed is that the assembly irradiated for only one cycle conveys the greatest activity/decay heat at the beginning of decay up to 14/30 days while the most irradiated assemblies are the most contributing after around 2 years of decay. This phenomenon is due to the fact that the short lived nuclides (conveying activity/decay heat in the short period) are overproduced in the fresh FA irradiated in the core center while the long lived fission products and actinides are proportional to the the cumulative burnup.\\

\textit{Fuel assembly models}\\
The fuel assembly models are developed to carry out further analysis because they provide flexibility and lower the computational cost. The limits of the modeling approach are investigated and the comparison with the reference core model was carried out on the nuclide inventory, showing deviations up to 40\% for minor actinides. On the contrary, good agreement is found between 2D and 3D fuel assembly models. The influence of axial burnup is limited and it amounts to maximum 4\% discrepancy in $^{246}$Cm inventory. The deviations are induced by the spectral differences between core and fuel assembly model and this phenomenon is relevant, in particular for fast spectrum reactors, therefore it should be more investigated and understood to improve the quality of the surrogate FA  models of MYRRHA and other liquid metal cooled reactors.\\

\textit{Uncertainty analysis}\\
The main goal of the FA model is to perform uncertainty analysis. The sources of uncertainty studied are nuclear data; cross section from JEFF-3.3, ENDF/B-VIII.0 and JENDL-4.0u and neutron fission product yields from JEFF-3.3 are propagated to assess the uncertainties of the outputs, such as nuclide inventory and decay heat.\\
Actinides concentration uncertainty comes from cross sections. It is lower than 2.5\% for plutonium isotopes and it tends to linearly increase with burnup. The uncertainty on americium isotopes and $^{242}$Cm ranges between 1 and 10\% and it is conservatively estimated by JENDL-4.0u while it is strongly underestimated by JEFF-3.3, since uncertainty data for $^{241}$Am and $^{242m}$Am are not propagated. Largest uncertainty of around 60\% is found for $^{246}$Cm.\\
Relevant fission products, divided in volatiles and decay heat contributors are not sensitive to cross sections variations. Their uncertainty is mainly due to \acrshort{NFPY} and it ranges from 5 to 10\% depending on the actual input uncertainty.
$^{134}$Cs is sensitive both to cross section and \acrshort{NFPY} uncertainty since it is produced by neutron capture in $^{133}$Cs that in turn is generated by the decay of fission products.\\
The uncertainty in decay heat varies depending on the decay time, and it also differs when considering a fuel assembly irradiated for 1 cycle compared to one irradiated for 18 cycles. It is dependant on the uncertainty of the main contributors, that changes with the decay time. The correlation between the decay heat of the FA irradiated for 1 and 18 cycles shows, as function of decay time, how the uncertainty apportions to the two systems. It is almost 100\% at the beginning of the transient since in both cases decay heat is provided by the same short lived nuclides and between 3 and 70 years when decay heat is dominated by $^{238}$Pu and $^{241}$Am. 






