## 4 Modelling and Simulation Results
Therefore, as a next step for this wider consideration, it is advisable to recheck this given list with an inventory code, such as ORIGEN of the SCALE package [25] or FISPIN [35], to ensure that a complete set of fission products is considered. This would then lead to more comprehensive studies which aim to model, understand then confirm experimentally this chemical speciation, any chemical reactions and the resulting chemical and physical properties. Using an inventory code will then inform the best possible approaches for future salt clean-up based on the chemical nature and environments of the different elements. One simple example would be Iodine, for which its formation in the \(-1\) oxidation state should result in retention in the molten salt and potential complex formation, whilst its formation in the zero oxidation state as I\({}_{2}\) should result in ready volatilisation without the need for chemical separation. Going further, it must be recognised that identifying the evolution of elemental oxidation states \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & **Concentration** & **Conc. Percentage** & \(\Delta k_{\mathit{eff}}\) & \(\Delta k_{\mathit{eff}}\)**Percentage** & \(\Delta k_{\mathit{eff}}\)**Concentration** & \multicolumn{2}{c}{**Range of**} \\  & **[ppm]** & **Fraction [\%]** & **[pcm]** & **Fraction [\%]** & **[pcm/ppm]** & \multicolumn{2}{c}{**Oxidation States**} \\ \hline Rh & 696 & 2.19 & 324 & 5.26 & 0.466 & 3+ & 3+ \\
**Pd** & **1139** & **3.58** & **503** & **8.16** & 0.442 & 2+ & 2+ \\ H & 647 & 2.03 & 251 & 4.07 & 0.388 & 1+ & 1+ \\ I & 179 & 0.56 & 62 & 1.01 & 0.346 & 1\(-\) & 0\(-\) \\
**Ru** & **2512** & **7.90** & **739** & **11.99** & 0.294 & 2+ & 3+ \\
**Cs** & **3061** & **9.62** & **603** & **9.78** & 0.197 & 1+ & 1+ \\
**Mo** & **3890** & **12.23** & **725** & **11.76** & 0.186 & 2+ & 6+ \\
**Nd** & **3122** & **9.81** & **569** & **9.23** & 0.182 & 2+ & 3+ \\ Pr & 904 & 2.84 & 122 & 1.98 & 0.135 & 3+ & 3+ \\ Rb & 471 & 1.48 & 52 & 0.84 & 0.110 & 1+ & 1+ \\ Xe & 3685 & 11.58 & 346 & 5.61 & 0.094 & 0 & 0 \\ La & 999 & 3.14 & 88 & 1.43 & 0.088 & 3+ & 3+ \\ Zr & 4491 & 14.12 & 364 & 5.91 & 0.081 & 3+ & 4+ \\ Ce & 1944 & 6.11 & 135 & 2.19 & 0.069 & 3+ & 4+ \\ Ba & 1309 & 4.11 & 89 & 1.44 & 0.068 & 2+ & 2+ \\ Sr & 1067 & 3.35 & 58 & 0.94 & 0.054 & 2+ & 2+ \\ sum & 31,815 & 100 & 6163 & 100 & & & \\ whole & 34,179 & & 6478 & & 0.190 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._and chemical speciation is only the important first step in then proposing and developing suitable reprocessing approaches for the identified fission product poisons and that the feasibility of such approaches may well in turn inform the final choice of neutron poisons to reprocess. Whilst not the subject of this paper, we acknowledge this to be a major challenge to be addressed in future publications, informed by this work. Therefore, although initial inspection of Table 1 suggests the development of separation technologies for Ruthenium and Molybdenum should be prioritised for salt clean-up, due to their preferential formation during burnup and their highest adverse effect on criticality; practical considerations, such as feasibility, economics, viability at scale and the separation rates of separation technologies also need to be considered. Ultimately, the final decision of which elements to separate from the fuel salt can only be made after evaluation of the possible separation technologies available for the elements present in their oxidation state(s). This is challenging but opens up a new optimization opportunity within this work. A further challenge in molten salt reactor operation will also be in the control and monitoring of the physical and chemical state of the reactor and the associated clean up processes. One application will be in salt clean-up and its feedback on reactor operation.