## 3 Results and Discussion
Through interpolation, the achievable burnup for the specific case has been determined, and the third column and the result are compared to the reference solution to determine the burnup gain, which can be achieved compared to the reference case, column 4. The five leading, most influential fission product elements have been identified and marked in bold for the graphical analysis of the experimental data. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Element**} & **Criticality Change at** & **Achieved Target** & **Burnup Increase** & **Ranking Based On** \\  & EOL of Reference & **Burnup** & **R removal** & **Non-Dynamic** \\  & Case 1 ppm & [GWd/tHM] & **POLARIS Calc. [16]** \\ \hline ref-Rb & 21 & 206.3 & 1.5 \\ ref-Gd & 21 & 206.3 & 1.4 \\ ref-Cd & 19 & 206.2 & 1.3 \\ ref-Sr & 17 & 206.0 & 1.2 \\ ref-Y & 12 & 205.7 & 0.8 \\ ref-Se & 4 & 205.1 & 0.2 \\ ref-Sn & 4 & 205.1 & 0.3 \\ ref-Te & 4 & 205.1 & 0.3 \\ ref-Sb & 4 & 205.2 & 0.3 \\ ref-Nb & 2 & 205.0 & 0.1 \\ ref-In & 2 & 205.0 & 0.2 \\ ref-Tb & 1 & 204.9 & 0.1 \\ ref-Dy & 1 & 204.9 & 0.035 \\ ref-S & 0 & 204.9 & 0 \\ ref-Ge & 0 & 204.9 & 0.002 \\ ref-As & 0 & 204.9 & 0.003 \\ ref-Ho & 0 & 204.9 & 0.0004 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 3: Direct calculation without Uranium refill and all fission products in (typical lattice calc) versus MSR with scripted restart and deletion of volatile fission products (18, 35, 36, 53, 54, 85). following in Figure 4. For comparison of the gained results with the earlier published static POLARIS calculations, the ranking of the elements of the earlier calculations is given in the last column. As observed before, the main elements coincide very well between both simulation results, but the ranking is slightly different with the main difference in the ranking of Pd, which achieves in the HELIOS calculation a higher concentration resulting in this second analysis step in a stronger influence on criticality. In general, these results indicate that the major influence on criticality up to a burnup of 200 GWd/tHM is concentrated on a comparably small number of nine elements. The influence of the remaining other elements seems to be limited but could increase with a longer operation and a higher amount of energy produced. The more detailed graphical analysis of the influence of the fission products on the long-term operation is given in Figure 4 with the analysis of the criticality curve and the potential criticality gain achievable through the cleaning out of 5% of specific fission products at the end of each of the 10 GWd/tHM cycle through the script. In contrast to Figure 3, in Figure 4, only the weighted average criticality of each burnup cycle is evaluated and shown as a dot to improve the readability. This approach leads to a much smoother curve compared to the presentation of all intermediate results within the cycles. The graphical analysis indicates that the influence of the fission product clean-up is very low at the beginning, which can be explained by the limited content of each of the fission product elements; thus, the reduction by 5% does not have a strong influence until a significant amount of fission product is produced. This observation leads to the next steps of the investigation; (a) what influence can we expect when the fission product clean-up is initiated at a point where a considerable amount of the specific element is allowed to accumulate? (b) will the effect of starting later maybe reduce to a kind of asymptotic limit, which would mean there will be no major penalty for starting the clean-up later in the operation of the reactor. The investigation of the effect of initiation of the clean-up system at different stages of operation is given in Figure 5 with the comparison of a set of calculations with the clean-up system initiated from the beginning, as well as at a burnup of 50 GWd/tHM and at 100 GWd/tHM, while all calculation are compared to the reference case (blue curve).