## 3 Results and Discussion
The detailed look into the development of the elementary concentrations of Ru resulting from the different clean-up streams (Figure 8) helps to obtain a deeper understanding of the criticality and burnup effect when correlating them with the development of the Ru concentrations. It seems the greatest reduction/change in the Ru content in one step is achieved through the step from no clean-up to the 5% volume case, which correlates to the achieved burnup increase compared to the last scheme. The reduction in the other cases seems to be slightly lower, e.g., for the step from 5 to 10% and from 10 to 20%, which deliver about the same reduction and the same burnup increase, while the last step, 20 to 50%, leads to the lowest absolute reduction and the lowest burnup increase. In general, all cases seem to tend to an asymptotic value for the Ru content with a lower amount for the higher volumes in the clean-up system, which seems to be logical. However, the asymptotic value could be correlated to the ideal point to start the clean-up process, with the view that a clean-up system with a lower volumetric capacity should be started later. This could maybe lead to another optimisation approach, using a slightly larger system with stronger breeding while accepting a higher fission product content in the core versus a smaller core with less breeding and a stronger clean-up system--this will be a question of balancing the economics of the process, the cost of the reactor and the source term in the core versus the cost of the clean-up system and the source term in the clean-up system. Figure 6: Development of the Ruthenium concentration as produced in HELIOS during the reactor operation investigating the effect of the time of the start of salt clean-up withdrawing each time 5% of the elementary concentration. Following the optimisation of the separation on the hand of a single fission product element, the next is now to look into the effect of the separation of different elements as analysed in Table 2. The most interesting and important point here will be to investigate the additivity of the effect of the separation of different elements. For this, the reference case will be calculated several times, each time separating one element more, first Ru, then  
Figure 8: Development of the Ruthenium concentration as produced in HELIOS during the reactor operation investigating the effect of different percentage of the concentration at each step of the calculation. Figure 7: Effect on criticality caused by different reduction efficiencies of the clean-up on the example of Ruthenium separation, based on a simulation using 10 GWd/tHM steps and an initiation of the clean-up system after 50 GWd/tHM. Ru and Mo, followed by Ru, Mo, and Pd, and finally, Ru, Mo, Pd, and Cs, see Figure 3:0 The results show not only that the effect is additive, and it seems even that the effect of cleaning out additional elements seems to be slightly self-amplifying since the burnup gain of the last steps is clearly larger than in one of the earlier steps, which is surprising since the elements which are separated first have the strongest criticality effect following Figure 3:1 This effect should be investigated in more detail as soon as a clean-up strategy is developed in collaboration with specialists in nuclear chemistry. Currently, the case is still a bit artificial due to the comparably high \(\mathrm{k_{eff}}\) which is achieved and would not be useful in a molten salt reactor which ideally should not rely on an active control system, e.g., by relying on control rods, since the system can be controlled through temperature feedbacks [27] and through the feeding/clean-up stream. If this operational regime is achieved, there is another problem to be thought about using the current calculation approach; in the current mode of calculation relying on criticality normalisation, the breeding will be underestimated since the normalisation artificially reduces the neutron source.