## 2 Codes, Models and Methods
To improve the modelling of both procedures, a new strategy has been developed, based on the use of a reduced burnup per cycle (5 GWd/tHM and 10 GWd/tHM using five burnup steps in HELIOS), coinciding with full removal of gaseous and volatile fission products the elements 18, 35, 36, 53, 54, 85 are not carried forward through ZENITH), while the dissolved fission products can be removed, based on a cleaning efficiency providing the opportunity to set this efficiency elementwise for all considered elements. The use of the described process has already been validated and used in several peer-reviewed publications [17; 18]. However, the modelling and simulation quality will be significantly improved due to the new code version and the increased computational power, which allow now the use of the 173 energy group cross-section set instead of the 47 group set in the earlier publications. However, due to the characteristics of HELIOS, some approximations still have to be accepted. There is no fuel salt movement; thus, an undesired burnup distribution arises during each of the calculation cycles, while the materials are only redistributed when new user input is defined. HELIOS is an LWR code, and an LWR spectrum is used for the weighting of the master libraries inside each energy group. However, this error will be significantly reduced compared to earlier publications since the number of energy groups is tripled; thus, the width of each energy group has significantly reduced. Comparisons with other codes in the EVOL benchmark [25], in a fast reactor isotope accumulation test for SFR against SERPENT [31], as well as comparisons with SCALE/POLARIS [26; 32], have shown good agreement. During the work on the transmutation studies under the boundary conditions of the nuclear phase-out in Germany, see, e.g., [17; 18], the script has been extensively validated against analytic models for the operation of fertile free cores. This is what is currently available in terms of modelling techniques and solvers; therefore, to judge the reliability of the results, a real reactor physics experiment for molten salt reactors would be required, as discussed in [32]. The approximations and the use of the HELIOS code package seem to be adequate for the approximation level required for this kind of long-term investigation of isotope accumulation to support the development of a clean-up system. The results of the influence of different elements on the system criticality have been evaluated against earlier publications [15; 16]. The accumulation of isotopes is evaluated through a row of calculations of the system using the described dynamic calculation, which incorporates the effect of the removal of the gaseous and volatile fission products as given above. All other fission products are left in the system, and the accumulation is observed in 10 GWd/tHM steps. This is followed by a more sophisticated evaluation of the effect of the partial removal of different fission product elements to observe the potential increase in criticality compared to the reference case, as well as the increase in the final burnup achievable through the removal of a specific element. The same approach is used to investigate the effect of the initiation of the fission product clean-up and the effect of different separation efficiencies,  
Figure 2: Description of the calculation cycle for the simulation of an MSR, based on the HELIOS package. representing a different throughput of salt through the clean-up system. In the next step, this approach is used to investigate the effect of the separation of different fission product elements in parallel. In a final approach, the fission product removal is applied in a staggered way, where the clean-up of one additional fission product is initiated every time the criticality of the test system is approaching k = 1.