## 6 Analysis of the Influence of the Reflector Material on the Neutron Flux
### Neutron Spectrum
The spatial and energy resolved neutron flux were investigated through a comparison of two sets of neutron distributions for a five-group neutron flux using the energy boundaries shown in Figure 12. Figures 14 and 15 show the neutron flux distribution for different materials (left to right) with decreasing group energy from top to bottom. At the bottom of the figures, the colour scale based on the normalized neutron flux for each group and each material is given for comparison. To comparing the groupwise spatial flux distribution of the He reflected system, see Figure 14, which represents an unreflected system; with the system using FeBo as an absorbing reflector, the effect of the reflector is visible in each group. In the first three groups, the reflector led to a wider spread of the high flux area in the centre of the core and a very thin area of increased neutron flux outside the steel shell of the core. This effect was more pronounced in Group 4, where an almost flat flux distribution appeared through the whole core, while in Group 5, the flux maximum moved to the outer area of the core, but there are only very few neutrons in this group at all. However, when looking into the absolute values of the neutron flux for the different groups given at the bottom of the figure, it is clear that the absolute number of neutrons in each group has changed only very slightly. All five groups show only a very small penetration depth of the neutron flux into the reflector, which indicates that a much thinner reflector would most probably deliver almost the same result. This is in strong contrast to the case using a lead reflector; here, the penetration depth was already high in the first group and, starting from Group 2, the full reflector was used. Thus, a thicker reflector could be promising in this case. This confirms the results given in Figures 10 and 11, demonstrating that increasing the reflector thickness over the 30 cm standard still has a good influence on the criticality. The lead reflector flattened the spatial neutron flux in Groups 1 to 3 significantly, while it led, compared with the He and FeBo cases, to a decrease in the absolute number of neutrons in the first group and an increase in the second, third, and fourth groups. The maximum neutron flux in the fourth and fifth group appeared in the lead case already in the reflector, leading to an increase in the power production at the core surface. The general tendency in the copper reflector was comparable with the lead reflector, but with a significantly lower penetration depth in the first three groups. The maximum neutron flux in the copper case was slightly lower in Groups 1 to 3, but higher in Groups 4 and 5, as already expected from Figure 13. Groups 4 and 5 indicated a spatial flux maximum in the reflector feeding back into the core and leading to the slight power increase at the core boundary, as shown in Figure 9, with a slightly stronger effect than in the case using the lead reflector. The groupwise spatial distribution of the neutron flux for the cases with the more moderating reflectors (graphite, BeO, and SS304) are given in Figure 15, with the case of the copper reflector added for comparison. Both the strongly moderating reflectors (graphite and BeO) show that in the first groups, there was a significantly lower penetration of the neutron flux into the reflector when compared with the metal-based reflectors. This indicates that for the BeO reflector especially, there seems to be room for a strong reduction of the reflector thickness. The low penetration depth coincides with the clearly reduced maximum neutron flux in these three groups in the moderating reflectors. The real effect of the moderating reflector is seen in Group 4 and even more in Group 5. In Group 4 the neutron flux peak has already completely moved into the reflector, but in contrast to the metal reflectors, there was only a very thin outer core layer which was boosted by the lower energy neutrons from the reflector.