\chapter{Model simplification - From a 3D core model to a 2D fuel assembly model}
\begin{comment}
keyword - speed and fidelity tradeoff
"deve essere rappresentativo per analisi di incertezza"
Non vogliamo risultati equivalenti al 3D, ma vogliamo un modello surrogato per determinarne líncertezza\\ 
The need of a simplified model to simulate MYRRHA fuel depletion comes from the necessity to perform further analysis not feasible on a full core simulation. For this purpose a fuel assembly model of MYRRHA is built to perform depletion study of the FA being irradiated in the core. The flexibility of the model and the reduced computational burden allow to carry out uncertainty studies and other analysis such as the investigation of axial burnup, discussed in the next sections.\\
The tasks to accomplish to carry out the analysis of FA model are to build a model the most representative of the the core as possible. For this purpose the procedure used to perform SNF analysis in LWR at fuel assembly level is taken as a reference. The input of the model are taken from the critical core simulation since a FA neutronic simulation cannot be set up in subcritical configuration. This has to be taken into account for the conclusions drawn since as we already showed by the results at equilibrium the subcritical configuration gives the worst scenarion both for nuclides activation and associated activity and decay heat.\\
The modeling assumption must be pointed and the impact on the output of the simulation is assessed to understand the discrepancies arising respect to the reference core model we are trying to mimic.\\
Once the FA model is characterized the analysis relevant for the work are carried out. In particular the simplified model opens to the possibility of simulating the reshuffling of the fuel assembly in the core from loading to unloading (after a number of irradiation cycles) and simultaneously evaluating the evolution of uncertainty of the quantities under investigation. The uncertainties obtained comes from nuclear data and are used to provide confidence intervals for safety related parameters and to drive design choices. A further analysis is carried out on a very strong modeling assumption, i.e. the dependence of axial coordinate.
\\ bullet points dei 4 modelli \\
\end{comment}
In this chapter the concept of a \acrfull{FA} model is introduced.
The development of such a simplified model to simulate the depletion of the MYRRHA fuel comes from the necessity to perform a number of analyses that would demand too many resources for a full core simulation.
The main purpose of this work was to produce a model that:
\begin{itemize}
\item has a low computational cost;
\item it is flexible;
\item it mimics as much as possible  the behaviour of the 3D core model for what concerns the FA source term.
\end{itemize}
Even without providing highly accurate best-estimate results, this low-fidelity surrogate model shall be suitable to perform the uncertainty quantification studies that are deemed unfeasible for a full core model.
The same approach was used in \cite{Solis2020DepletionModel} for an uncertainty analysis in a depletion model of the MYRRHA Design Revision 1.6.
This methodology is also widely used for the characterization of spent nuclear fuel (\acrshort{SNF}) of traditional \acrshort{LWR} reactors \cite{FedericoGrimaldi2022NuclearCalculations}. \\
Given the selected approach to propagate uncertainties on input parameters --- i.e., statistical sampling --- the nuclide vector uncertainty quantification would be too time consuming using a full core model.
The propagated uncertainties come from the nuclear data and they are used to provide confidence intervals for safety related parameters and to drive design choices.\\
A second analysis is carried out to estimate the impact of some of the model assumptions that led to the FA model. In particular, the focus was on assessing how the axial power and burnup profiles in the FA affect neutronic parameters and the depleted nuclide inventory.
For this purpose both 2D and 3D FA models were developed.\\
The inputs of the FA models were taken from the results of the full core depletion simulation in critical mode.
The sub-critical configuration was not investigated for the evident difficulties in representing the external proton source.
This must be considered drawing the conclusions since, as already shown by the results in the previous chapter, the sub-critical configuration provides the worst scenario both for the FA activity and decay heat.\\

In total, four depletion models were developed in this work to study the MYRRHA fuel source term:
\begin{itemize}
\item the 3D full core models (at equilibrium) for the critical and sub-critical reaction operation: they were used to determine the buildup of volatile nuclides during irradiation and the FA activity and decay heat envelope curves.
\item a 2D FA model: it was used to propagate the uncertainty of nuclear data.
\item a 3D FA model: it was used as a reference to investigate the impact on the nuclide vector of considering an axial flux, power and burnup profile.
\item a 3D FA model with axial depletion zones: it was used as a reference to investigate the impact on the nuclide vector of considering an axial flux, power and burnup profile.
All the FA models are developed in Serpent \cite{Leppanen2015Serpent-aManual}.
\end{itemize}

\input{content/chapters/Model semplification/sections/Fuel assembly model description}
\input{content/chapters/Model semplification/sections/Fuel assembly model assumption}
\input{content/chapters/Model semplification/sections/Fuel assembly model limitations}
%\input{content/chapters/Model semplification/sections/Fuel assembly model output}
\input{content/chapters/Model semplification/sections/Fuel assembly model output}
\input{content/chapters/Model semplification/sections/Validation of fuel assembly model respect to core}




