## 3 Results and Discussion
Figure 4: Effect on criticality caused by cleaning 5% of different fission product elements with the strongest influence on criticality, based on a simulation using 10 GWd/tHM steps for the scripted calculation. The observation of the very low influence of the clean-up in the early stage of the operation taken above is confirmed in Figure 5. There is a slight advantage in the maximum achieved criticality for the case of the initiation of the clean-up at the start of the system (red curve), but the difference in the final burnup and the criticality at the end of the observation period is very limited compared to the case with the initiation of the clean-up at 50 GWd/tHM (grey curve). The later imitation at 100 GWd/tHM yields a slightly smaller final burnup (yellow curve), but the gaps seem to close rapidly, which leads to the expectation that in the longer-term operation, the influence of the initiation of the clean-up has a limited influence and we will observe an asymptotical behaviour. This observation is supported by the analysis of fission product concentration in the system, as analysed in Figure 6 for Ruthenium. The general Ruthenium concentration in the system increases in a slightly exponential manner in the reference case without any clean-up (blue curve). The reduction of the Ru content is very limited in the beginning, up to 50 GWd/tHM, since the overall amount of Ru is still very limited, even if clean-up is already initiated (red curve). The initiation of the clean-up in the step after 50 GWd/tHM (grey curve) leads to a stronger reduction in the first steps since the concentration has already grown to a certain level. At the end of the observation period, the results of the initiation at 0 and at 50 GWd/tHM have almost converged, with a red and grey curve. The later initiation (yellow curve) leads to a higher final concentration, but the difference between the other results reduces over time. This supports the conclusion that with a longer operation, the effect of the initiation of the clean-up system will anneal. Based on this observation, we can get to the conclusion that there should be an ideal time point to start the clean-up. This time point will be a result of the accumulation of the fission product, the criticality effect, the efficiency of the chemical process, dependent on the concentration, and maybe the design volume for the clean-up stream. The cleaning of 5% of the volume after each calculation cycle, here 10 GWd/tHM, is just to be seen as a model for a future continuous clean-up system. However, in the current case, the Ru concentration is still growing, even if the clean-up is initiated. It will possibly achieve some asymptotic value, but obviously after a much longer time than the observed one. Based on this observation, it seems logical to investigate the influence of the cleaning stream amount in the next step. Figure 5: Effect on criticality caused by different initiation of the clean-up system on the example of Ruthenium separation, based on a simulation using 10 GWd/tHM steps and a 5% reduction of the Ruthenium content at each step. The analysis of different clean-up volumes is given in Figure 7 (influence on criticality) and Figure 8 (influence on element concentration). Obviously, the clean-up volume has a clear influence on the criticality curve and as expected, increasing the clean-up volume leads to a higher criticality peak, but the effect seems to be far from linear. The gain from no clean-up to 5% volume being cleaned is \(\sim\)40 GWd/tHM, and the gain between 5% and 10 % volume is \(\sim\)25 GWd/tHM. Another doubling of the clean-up stream gives about 25 GWd/tHM; the step from 20% to 50% gives only \(\sim\)20 GWd/tHM. Thus, we face another optimisation problem, again, especially in this case, where the gain is absolutely not linear with the salt clean-up stream.