## 4 Modelling & Simulation Results
### Control System Study  
The boundary conditions of ideally not disturbing the homogeneous arrangement of the core described for the shutdown system are even more important for the control system which will be used to start up the reactor and to keep the reactor in the critical state. The additional request on the applicability for the solid as well as the hot liquid state is also essential for the control system. Based on these requirements, the control of the criticality via the movement of the radial reflector seems to be a promising option, especially if the system would be efficient enough to assure that most of the core can stay reflected to obtain good opportunities for the evaluation of the reflected configuration. However, this way of control could, in addition, allow the investigation of the neutron flux distribution in the unreflected core (the reflector space in the model is filled with a vacuum), too. The potential core criticality change, investigated in the 2D system, is presented in Figure 8 as a function of the fuel enrichment. From this evaluation of the criticality change, the limitation of the radial reflector control is obvious. The system using the reflector for control will strongly increase in efficiency with decreasing reactor core size. The reactor size coincides in the investigated system with the uranium enrichment, while the effect of the salt configuration, and thus the amount of the NaCl carrier salt, is only very minor. The figure indicates that a sufficient criticality change by removing the radial reflector can only be achieved for systems with at least 20% enrichment and thus an estimated 2D system radius of about 80 cm. For the chosen system with a 35% enrichment, the expected value of this initial investigation will be in the range of more than 20,000 pcm. Thus, a broad range of configurations and even a significant temperature range with the resulting reactivity change due to the strong feedback effects [4] can be evaluated and/or compensated with the proposed radial control reflector holding the reactor in a critical status. Figure 7: Negative reactivity effect (shutdown reactivity) caused by moving the reflector and different sized layers of the core away from the main core for the 4 described cases. Based on this initial 2D evaluation, the approach of a control system using a vertically moving radial reflector will be investigated in more detail in a 3D analysis using KENO-VI. The first step for the 3D analysis is to determine the appropriate \"slightly over critical\" configuration--a cylinder with a diameter of 140 cm and a core height of 140 cm delivers a \(\mathrm{k_{eff}}\) of 1.02033 in the KENO-VI multi-group calculation. The used core model is presented in the left upper corner of Figure 9, using a 10 mm-thick stainless-steel wall and a 30 cm layer to split off for the shutdown system. The following images from left to right in the upper and then the lower row visualize the concept of the radial reflector control accomplished by moving the radial reflector downwards in a stepwise manner. The visualized pictures are only a few of the used steps investigated for the analysis of the control effect curve of the control system. The radial reflector in the undisturbed reference system reaches 6 cm further than the core at the top and the bottom to assure that the effect of moving the reflector on the core criticality starts already after a limited amount of movement. The analysis of the control reactivity curve is accompanied by the analysis of the effect of the thickness of the reflector by investigating not only the reference case with the 30 cm-thick reflector but also a case with a reflector reduced to 20 cm and one with an increased reflector thickness of 40 cm (see Figure 10). The 20 cm reflector case requires a core with a 144 cm diameter and a height to assure a critical reactor with some reserve criticality margin, while the increase in the thickness of the radial reflector to 40 cm allows reducing the core size to 136 cm in diameter and height.