## 7 Conclusions and Next Steps  
The relevance of the neutron reflector in the core design was discussed in the introduction with the view of extending it into power reactors as well as into neutronic experiments, where the role of the reflector has a much higher importance. In the following, the theoretical roles of the reflector for any kind of physical neutron installation are discussed. The investigation of the effect of different reflector materials regarding the achievable \(\mathsf{k_{eff}}\) and power distribution for a given reference configuration was performed. The study of a set  
Figure 17: Comparison of the control curve caused by moving the reflector made of NaCl and copper, with the systematics of the study described in the insert. Figure 18: Comparison of the shutdown curve caused by moving the bottom part of the core for the reflectors made of NaCl and copper, with the systematics of the study described in the insert. of classical reactor materials and more exotic choices led to a first down-selection based on the gain in k\({}_{\rm eff}\), with SS304 (as the representative for stainless steel), lead, copper, graphite, beryllium oxide, and iron-based material with a high Si content identified as promising. The following investigation of the power distribution highlighted that the strongly moderating reflectors based on graphite and beryllium oxide had a strong influence on the power distribution within the core and will deliver a non-natural power distribution with a peak at the reactor boundary instead of the reactor centre. Based on this first down-selection, the 2- critical core dimension was evaluated for the chosen reflector materials, identifying stainless steel and copper as the most attractive solutions, with lead as a potential backup and graphite as an attractive candidate in the case of a future thermal system. The study of the optimal reflector dimension identified that the reference approach of a reflector thickness of 30 cm was a good approach but, especially in the case of copper, a thinner reflector could be sufficient, while in the case of lead, a thicker reflector could be more promising. The analysis of the influence of the reflector material on the neutron flux spectrum confirmed the expected effect of the moderating reflectors BeO and graphite, while the difference in the neutron spectrum in the cases of the metal-based reflectors was found to be very small. The investigation of the 2D flux distributions split into five energy groups confirmed the expectation that there was a significant number of neutrons stored in the moderating reflectors, which led to a number of very well thermalized neutrons streaming back into the core, explaining the power increase at the core boundary observed in the study of the 2D power distribution. In addition, this insight highlighted the results that the penetration depth in the lead reflector was very high, while the penetration depth in the copper reflector was very low, confirming the result that a thicker lead reflector could be promising, while a thinner copper reflector could be sufficient. Following all the 2D studies, a 3D Monte Carlo study to investigate fuel volume savings was performed, which highlighted the potential fuel savings possible through a good choice of reflector. There is a potential for a reduction of up to 60% in the fuel volume compared with the reference case using a NaCl reflector, with the best results for stainless steel and copper, or a thicker lead reflector. The final cost comparison for different reflector configurations identified that the stainless steel reflector was the most cost efficient solution. However, this reflector has the strongest influence on the power distribution. In general, there is some additional optimization needed to make a final choice between the three most promising reflector materials: SS304, lead, and copper.