\section{Source term assessment}\label{obervables}
The source term refers to the magnitude and mix of the radionuclides released from the different components of the reactor. Determining the source term is a critical component in the licensing process of the reactor since it estimates the amount of risk associated to a certain accidental condition in term of health and environmental consequences. The source term analyzed in this work comes from MYRRHA fuel and LBE, after being irradiated by particles fluxes. The work carried out for the thesis follows the structure of the source term assessment performed for the previous MYRRHA design versions \cite{Stan12}, \cite{Stan16}.\\

\textit{Volatile nuclides }\\
Volatile nuclides produced pose the major concern in term of effective dose since they can be released during accidental conditions and spread in the environment outside the reactor envelope. Postulated events bringing to release of volatiles together with the major contributors to the effective dose are already identified and extensively discussed in \acrshort{SCK CEN} internal reports. They take as an input the source term produced in previous MYRRHA design versions and new evaluation of the source term is requested to update the safety studies.\\

 
\textit{Decay heat}\\
The source term is employed to derive the decay heat, i.e. the power generated in a certain component after the shut down of the reactor. It is the energy associated to the decay of the radionuclides composing the source term and it must be equally assessed to establish the cooling requirements of a component after it is unloaded from the reactor. A valuable piece of information is the magnitude of decay heat produced in the fuel assembly when unloaded from the core, used to design and dimension the handling equipment.

\subsection{Reference accidental conditions and related observables}\label{AccCondObservables}
\textit{Accidental conditions}\\
The conditions taken into accounts for the assessment are those in which potentially concerning radionuclides spread in the environment as a consequence of an accidental situation. The accidental conditions are fully analysed in specific SCK CEN internal reports.\\
\begin{itemize}
\item Release of volatile activation/spallation/fission products from the core through the cover gas is assumed in case of large break of the cover barrier or proton beam misalignment causing the beam to burn through the beam tube at cover gas level. The release of fission products happens only if fuel pin crack is assumed \cite{BisogniCoverGas}.

\item LBE volatile activation/spallation products also spread in the outer environment in case of failure in the LBE Conditioning System (\acrshort{LBECS} failure) \cite{BisogniLBECS}.
\item FA mechanical failure during unloading procedure in which the failure of all the fuel pins is assumed for conservatism \cite{BisogniMechFailure}.\\
\end{itemize}

\textit{Radionuclide observables in the accidental conditions}\\
The previous source term assessments provided the results necessary to identify the nuclides responsible to the largest dose in case of accidental release. The present work aims to estimate their production in the new MYRRHA design release and to include other nuclides if relevant.\\
The exhaustive list of nuclides relevant in each condition is available in the SCK CEN reports carefully describing the accidental sequences.\\
A reduced list containing the priority nuclides is selected and analysed in this work:
\begin{itemize}
    \item LBE activation, spallation products: $^{210}$Po, that is not only contributing to the dose in case of accident but it is also the driver of LBE decay heat in the short term, $^3$H, $^{197}$Hg, $^{194}$Hg, $^{203}$Hg, $^{195}$Hg, $^{16}$N;
    \item volatile fission products produced in the fuel. Noble gases, such as xenon and kripton isotopes, $^{131}$I and $^{133}$I, $^{134}$Cs and $^{137}$Cs contributing to the activity after discharge, $^{3}$H, $^{125}$Sb and tellurium isotopes such as $^{127m}$Te, $^{129m}$Te and $^{133}$Te.
\end{itemize}

The activity and decay heat are observables derived directly from the source term. The activity level of fuel and LBE is estimated to assess their radiotoxicity and the major contributors are identified. The same analysis is carried out for the decay heat.\\

A deeper analysis is performed at fuel assembly level: the activity and decay heat rate of a FA unloaded from the core must be evaluated to establish the safety limits and dimensioning the handling system. Fuel assemblies unloaded from different positions in the core will have different compositions, as a result they will provide different level of activity and decay heat. The unloading of all the FA present in the core at a certain time was simulated to study their decay separately.\\
The evolution of the decay heat rate is evaluated for a period of 70 years after discharge in analogy with the other outputs, with particular interest to the value after 90 and 420 days of decay, corresponding to the possible moments in which the FA is removed from the \acrshort{IVFS}. Moreover the major contributors to decay heat as function of decay time are determined, which contribute the most to decay heat uncertainty. 


\begin{comment}
In this work, source terms are reported for the irradiated MOX fuel. The contribution of the fuel cladding to a FA decay heat is several orders of magnitudes lower than that of spent MOX fuel and can be considered negligible for these specific applications as already discussed before.

The calculated decay heat rates will allow assessing the level of cooling to be implemented for the FA during handling. The results of this study will also be used as inputs for the cooling requirements for the spent MOX FA stored in the buffer in the Spent Fuel Hot Cell.
\end{comment}