## 3 Results and Discussion
The idea of this approach is to operate the reactor until the criticality falls below unity; every time this happens, an additional element will be added to the list of elements to be cleaned out of the fuel by 20% at the end of each cycle. Thus, the calculation now operates on the reference case without clean-up as a start. After 180 GWd/tHM, the Ru clean-up is initiated, at  
Figure 3:4 Development of the fission product elementary content over burnup as produced in HELOS, considering the stepwise separation of different elements at 50 GWd/tHM with a 20% rate of salt being cleaned every step, based on a simulation using 10 GWd/tHM steps. 240 GWd/tHM, the Mo clean-up is initiated, at 350 GWd/tHM, the Pd clean-up is initiated, at 540 GWd/tHM, the Cs clean-up is imitated, and at 700 GWd/tHM the Tc clean-up is initiated, leading to the final burnup of \(\sim\)890 GWd/tHM. This staggered approach helps to limit the excess reactivity, which has to be compensated through material exchange through the feeding system otherwise. Figure 3:5 Change in the elementary content of heavy metals over burnup for the simulation based on using 10 GWd/tHM steps and an initiation of the clean-up system after 50 GWd/tHM as shown above. Figure 3:6 Development of the system criticality over burnup, considering the staggered separation of elements every time the system falls below criticality, based on a simulation using 10 GWd/tHM steps and a clean-up of 20% of the salt after each cycle. Analysing the elementary contents in Figure 13 indicates the stepwise initiation of the different element clean-up approaches. The concentration of the specific elements grows almost linearly until the initiation of the clean-up and falls then rapidly following the initiation of the clean-up system. The final asymptotic value achieved using this approximation model will finally only depend on the production rate. A more realistic, sophisticated simulation, including a chemical process model, would maybe allow producing a closer-to-reality result, where the chemical separation rate and maybe different flow rates through the clean-up system will be possible and allow a different level of optimisation. One of the optimisation parameters which comes to mind first would be to have in all cases a reasonably high content of the element to be separated, but the amount which would be ideal will depend on the chemical process and the efficiency of the separation; thus, even this amount could be element specific and should thus be subject of a discussion and optimisation between reactor physics, nuclear chemistry, and process engineering. The most important outcome of this simulation is it will be possible to start the reactor without having the salt clean-up operating from the first criticality since the elements have first to accumulate to a reasonably high concentration before a real chemistry-based clean-up should be initiated. Finally, there will be a strong demand for close interaction between the specialists to leverage the optimisation potential, which will be essential to obtaining a cost-optimised solution for this challenging problem. Closing the study with a final reactor physical view into the problem is given by the analysis of the evolution of the fissile material content of the reactor system, Figure 14, the figure clearly demonstrates the change of the leading fissile material from U-235 to Pu. The very tiny feed of U-235 through the depleted Uranium in the U-235 curve is only visible in the inserted detail. However, this tiny feed is responsible such that the U-235 content will never achieve an asymptotic level of 0. In contrast, the Pu curves are completely smooth, which reflects that there is no change in the Pu production and consumption inserted via the script, only through the internal breeding processes.