## 4 Modelling and Simulation Results
However, the ultimate decision criterion as which to elements should be separated should be those with the highest effect on criticality (combined through the content and the neutron capture cross section), which is readily assessed through \(\Delta k_{eff}\). This indicates Ruthenium and Molybdenum as elements with the highest influence on criticality with more than 700 pcm effects after 100 GWd/tHM of burnup at reasonably high concentrations of 2500 to almost 4000 ppm. The next elements are Caesium, Neodymium, and Palladium with a criticality effect between 500 and 600 pcm, with both Nd and Cs each having a reasonable concentration of more than 3000 ppm, while the concentration of Palladium is only about 1000 ppm. The next level would be Technetium and Samarium with a criticality effect of slightly below 500 pcm, but both elements have relatively low concentrations below 1000 ppm. The strong relative effect of the Europium is highlighted. This element has by far the strongest effect on criticality per unit mass, as a result of the high capture cross section. However, the challenge of separating Europium would be high due to its relatively low concentration. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & **Concentration** & **Conc. Percentage** & \(\Delta k_{eff}\) & \(\Delta k_{eff}\) **Percentage** & \(\Delta k_{eff}\)**/**Concentration** & \multicolumn{2}{c}{**Range of**} \\  & **[ppm]** & **Fraction [\%]** & **[pcm]** & **Fraction [\%]** & **[pcm/ppm]** & **Oidation States** \\ \hline Eu & 53 & 0.17 & 80 & 1.30 & 1.509 & 2+ & 3+ \\ Pm & 89 & 0.28 & 69 & 1.12 & 0.775 & 3+ & 3+ \\ Sm & 663 & **2.08** & **491** & **7.97** & 0.741 & 2+ & 3+ \\ Tc & 894 & **2.81** & **493** & **8.00** & 0.551 & 3+ & 4+ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Concentration of the leading fission product poison elements accumulated after a burnup of 100 GWd/tHM assessed through their correlated influence on the system criticality, with the most relevant elements marked in **bold**. The overall criticality effect of all evaluated elements accounts for 6478 pcm in criticality loss, which correlates well with the result obtained without considering the fission products, see Figure 4, which gives a criticality difference of 6448 pcm. Thus, both approaches deliver results that agree with the calculation accuracy expectations. In addition, it has to be mentioned that in the current criticality loss calculations all gaseous fission products are still considered dissolved, thus the real criticality loss of a molten salt reactor would be lower due to the release of at least a part of the gaseous fission products (e.g., Xe) and fission products (like I\({}_{2}\)) with low melting points which will volatilise. The results given in Table 1 now form a basis for discussion of the interplay between reactor physics and chemistry. It should be noted that to date simulation results have been delivered through the use of a lattice code that is optimised solely for the determination of criticality, and which results in the highlighting of those major isotopes with the most effect on criticality through \(\Delta k_{\mathit{eff}}\). However, this only returns those elements whose removal has the largest effect on criticality, without consideration of the ease of their separation chemistry. Building this in requires additional consideration of their chemical properties in terms of the range of oxidation state(s) possible in the molten salt (Table 1), the resulting speciation (their chemical environment, which may be burnup dependent) and the process(es) by which they may then be separated. Building in such considerations in a more holistic judgment of physics and separation chemistry is likely to require a wider set of fission products to be considered, more than the criticality relevant fission products which are typically followed in a lattice code.