\section{MYRRHA project}
MYRRHA is the Multi-purpose hYbrid Research Reactor for High-tech Applications being developed at SCK CEN since 2004 \cite{Abderrahim2010MYRRHAApplications}. It is the world's first large scale \acrfull{ADS} that consists of a sub-critical nuclear reactor driven by a high power linear accelerator. The proposed cut view of the reactor vessel is shown in Fig. \ref{mirrha_model}; the picture is taken from \cite{Abderrahim2020}.
\begin{figure}[h]
\centering
\includegraphics[width=0.75\textwidth]{images/pictures/MYRRHA reactor.png}
\caption{MYRRHA reactor glimpse with emphasis to the features analyzed during the work. The picture is taken from \cite{Abderrahim2020}.}\label{mirrha_model}
\end{figure}

\textit{Linear accelerator}\\
The linear accelerator accelerates the protons up to the energy of 600 MeV. At the end of the \acrshort{linac} the 4 mA proton beam is injected into the reactor, generating fast neutron fluence by hitting the heavy nuclei in the spallation target of the reactor.\\

\textit{LBE coolant and spallation target}\\
MYRRHA is a pool type reactor and it operates near to the atmospheric pressure. It is cooled by \acrfull{LBE}, that also serves as spallation target.
LBE has been selected as for several reasons:
\begin{itemize}

\item Wide operating temperature range: LBE has a relatively low melting point (125 ºC) when compared with its individual components (lead 327 ºC, bismuth 271 ºC), while it boils only above 1670 ºC; 
\item Excellent heat conductivity: LBE enables the reactor’s primary system to operate under normal pressure;
\item Radiation shielding: LBE is an excellent radiation shield that blocks gamma radiation.
\item Virtually transparent to neutrons: LBE is an ideal medium as spallation source inside the reactor where the linac’s proton beam is converted into neutrons required to sustain the fission reaction;
\item Fuel mix: LBE allows a wide variety of fuels to be used besides uranium 235 and 238, including mixed oxide fuels. In particular, simulations indicate that \acrshort{linac} driven, LBE cooled reactors may contain up to 30\% of long-lived minor actinides, such as neptunium, americium and curium;
\item LBE based reactors use far less nuclear fuel and produce far less nuclear waste.

\end{itemize}

\textit{MOX fuel}\\
Recycled spent nuclear fuel reprocessed into \acrfull{MOX} is the target fuel of MYRRHA. MOX pellets with plutonium content of 30\% will be loaded in the reactor. \\

The combination of the three features makes MYRRHA the perfect facility where to perform trasmutation of minor actinides since they guarantee fast spectrum and higher fission-to-capture ratio increasing the fuel utilization and creating less by-products by neutron capture. This will have a positive impact on both safety and economics of the final disposal by reducing the waste's total radiotoxicity and the final repository size.


