## 6 Analysis of the Influence of the Reflector Material on the Neutron Flux  
### Neutron Spectrum  
To create a deeper understanding of the effects of the different reflector materials, the neutron spectrum of the whole system and the spatial neutron flux distribution of five energy groups were analysed. The five energy groups was mainly chosen to reflect the neutron energy distribution of a fast system. The first insight into the global neutron spectrum indicated that the highly reflective materials BeO and graphite led to a neutron spectrum close to a light water reactor spectrum, with a significant thermal peak, while the metal reflectors Cu, Pb, and SS304 led to typical fast reactor spectra, which were similar to the spectrum of the absorber-containing reflector, ferroboron, and the unreflected system using He to fill the reflector region (see Figure 12). This insight into the neutron spectrum for BeO and graphite correlated very well with the observations for the power distributions; see Figure 8. Even if the fuel in the core represents a fast reactor configuration, there is a strong thermal peak which is formed in the strongly moderating reflector. The thermal neutrons then penetrate back into the fast reactor core, having very high importance due to the significantly higher reaction cross-section in the low energy range. Thus, these neutrons very efficiently undergo fission reactions in U-235. This leads to the undesired power peaking at the boundary of the core, caused by a strong self-shielding effect, which limits the depth of the penetration of the thermal neutrons. Thus, these moderating reflectors are not promising for fast reactor investigations, but they would have to be investigated in more detail at a later stage when thermal configurations could be investigated in a multi-purpose facility. For a more detailed look into the effects of the metallic reflectors, Figure 12 is narrowed down to the energy range that is relevant to the neutron energies expected in a fast reactor. The resulting plot, Figure 13, shows some very important details created by the different metallic reflectors, which correlate once again very well with what is shown for the power distribution in Figure 9. Obviously, SS304, Cu and Pb formed a small low-energy tail in the system as well, and the interesting point is that this low-energy tail exactly coincided with the power increase at the boundary, which was highest for SS304 and lowest for lead. All the metal reflectors tended to soften the neutron spectrum slightly, with an increase in the number of neutrons below \(\sim\)200 keV and a decrease in the number of neutrons above \(\sim\)500 keV. In contrast to this, the use of a reflector containing a strong absorber for low energy neutrons, such as ferroboron, tended to harden the neutron spectrum, even compared with an unreflected system, by reducing the leakage of fast neutrons above 1 MeV, while there was no effect on the low-energy tail below 10 keV. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & \multicolumn{3}{c}{**System Radius [cm]**} & \multicolumn{3}{c}{**Deviation [\%]**} \\ \hline
**Code** & **NEWT** & **mg MC** & **ce MC [ref]** & **NEWT** & **mg MC** \\ \hline SS304 & 36.8 & 35.5 & 37.9 & \(-\)2.9\% & \(-\)6.3\% \\ \hline Pb & 38.25 & 37.65 & 38.1 & 0.4\% & \(-\)1.2\% \\ \hline Cu & 36.1 & 35.45 & 36.05 & 0.1\% & \(-\)1.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Systematic comparison of the TRITON/NEWT multi-group results with KENO VI in the multi-group approach as well as the continuous energy approach. Figure 12: Whole-system integrated 252 energy group neutron spectra in the 2D system, depending on the different reflector materials. Figure 13: Selected fast reactor neutron spectra in the integrated 2D system, depending on a selection of potential reflector materials.