## 4 Modelling & Simulation Results
### Control System Study
The axial reflector is maintained with a thickness of 30 cm in each of the cases. Figure 8: Criticality versus U-235 enrichment for the reflected and the unreflected 2D system demonstrating the increase in the control potential span with increasing U-235 enrichment (%). The correlated system radius required for the critical system is provided below the enrichment as further information. The control curves shown in Figure 11 indicate a reactivity control span of ~12,000 pcm for the 20 cm-thick reflector, ~16,000 pcm for the 30 cm-thick reflector, and ~20,000 pcm for the 40 cm-thick reflector. The curves were determined through Monte Carlo calculations with the reflector moved downwards in 10 cm steps from the original starting position until the lower end of the core was reached. All curves show the characteristic trend with a lower efficiency of the control system at the beginning and at the end of the movement path. This means the control system allows a very sensitive control at the beginning and at the end since moving one increment has a smaller influence on the overall core  
Figure 10: Cases for the investigation of the effect of different reflector thicknesses of the movable radial reflector on the control reactivity curve (left 30 cm reflector case with 140 cm diameter and 140 cm height, center 20 cm reflector case with 144 cm diameter and 144 cm height, right 40 cm reflector case with 136 cm diameter and 136 cm height). Figure 9: Different steps of reflector movement steps with increased downward movement from top left to bottom right (top from left to right 0 cm, ~30 cm, and ~60 cm, bottom left to right ~90 cm, ~120 cm, and ~150 cm) for the analysis of the reactivity curve related to the movement of the reflector. criticality than when the control system is acting somewhere around the core center where the neutrons typically have a higher importance. Thus, the increase in the leakage has a stronger influence on the criticality in this area. The control response to the thickness of the reflector is almost proportional to the thickness of the reflector, even for the 40 cm-thick reflector. This indicates that there would still be an opportunity to increase the reactivity change if required, since only a very small saturation effect can be observed. In general, the criticality change, which is effected by the control system, is very wide in all investigated cases, which will allow compensating for the large reactivity changes which are expected for molten salt systems due to the very strong feedback effects caused by the strong effect of the density change in the salt which is acting as a coolant as well as a carrier for the dissolved fuel [28]. Finally, the decision on the thickness of the reflector and the use of the reflector for controlling the system criticality will follow different objectives since the control system design will be dependent on the weight of the component which has to be moved and the ideal system should be lightweight, which points to a thin reflector. However, from a core design, control, and cost point of view, the objective is an as efficient as possible reflector, since, as shown above, the thickness of the reflector has an influence on the core size and thus on the amount of fuel required for the experiment, which is one of the costilect parts of a zero-power experiment. The results for the criticality study show the influence of the reflector thickness on the required volume, where the reference case with the 30 cm-thick reflector leads to a volume of \(\sim\)2.15 m\({}^{3}\), the case with the 20 cm-thick reflector requires \(\sim\)2.35 m\({}^{3}\) (+9% core volume), and the case with the 40 cm-thick reflector requires only 1.98 m\({}^{3}\) (\(-\)8% core volume). Moving the reflector for the control of the reactor will have some strong influence on the neutron spectrum since the core leakage will significantly change.