## 5 Reflector Dimension  
In this section, the effect of the reflector on the critical size of the core and the efficiency of the reflector depending on its thickness were investigated to obtain a better understanding of the potential reduction of the demand on fissile material. This will also allow the determination of the ideal reflector thickness, depending on the chosen reflector material. The core dimension, and thus the required amount of fissile material, is strongly dependent on the choice of the reflector material; see Figure 10. The first comparison was between the heavy-metal-rich core and the eutectic core. The eutectic core required a \(\sim\)15% larger radius to achieve criticality when the same dimension of reflector was chosen. However, compared with the required heavy metal amount in the analysed 2D core, the eutectic composition required about 9% less heavy metal, which meant that the additional core volume or even a slightly greater volume was filled by NaCl only. Thus, in the case where the core dimension itself is not a limiting value, the choice of the eutectic composition could be favourable, even if the demand on the infrastructure (reflector material, potential control system [6], and the core housing) depends on the absolute core size. Therefore, this opportunity should be recognized and investigated in a future optimization process required for the design of a new type of zero-power reactor. The use of the chosen reflectors reduced, in all cases, the core dimension: the use of a 30 cm reflector consisting of lead reduced the radius by 23%; the use of the SS304 reflector, by 26%; the copper reflector, by 27%; and the graphite reflector, by 35%. The second case with the lead reflector demonstrated that the same reduction in the required core size could be achieved by a 50 cm thick lead reflector, but at the cost of a larger overall system size. Thus, all the investigated metal reflectors which delivered a reasonable power distribution had strong potential to reduce the required core size for a critical system, but the gain was only weakly dependent on the material. This opens a wide opportunity for choosing and analysing different optimization parameters in future core designs. With recognition of the great potential that the right choice of the reflector has in reducing the amount of fuel required to form a critical system, the next step was to obtain a deeper understanding of the options. Is there a saturation effect and is the applied first guess of using a 30 cm thick reflector really the ideal choice? The analysis was based on a  
Figure 10: To-scale variation of the 2D core dimension: reflector thicknesses and resulting system dimensions required to achieve a critical configuration for a zero-power system. comparison of the core criticality curves depending on the thickness of the reflector and the different reflector materials, while all cases were normalized on the critical system with the 30 cm thick reference reflector as calculated above. Figure 11 depicts the variation in k\({}_{\text{eff}}\) with reflector thickness for five reflector materials. For this evaluation, the critical core diameter was determined for each of the different reflector materials for the 30 cm thick reflector. In the following step, the reflector thickness was varied while the critical core size was maintained. This analysis was designed to create a deeper understanding of the trends towards the ideal reflector thickness for different materials, while it showed some different effects. First of all, all reflector arrangements tended to deliver the expected asymptotic behaviour, which confirms that at a certain thickness, the addition of additional reflector has only a very marginal effect.