# On the Dimensions Required for a Molten Salt Zero Power Reactor Operating on Chloride Salts
## 3 Results and Discussion
### 2-D Integral Neutron Spectrum Studies  
The next part shows an investigation of the influence of different system specifications: heavy metal content of the salt, use of the reflector, and enrichment on the neutron spectrum in the system. The general integral neutron spectrum (integrated over the whole system) in the NaCl-UCI system is highly comparable with a fast reactor, where almost no neutron flux below 100 eV is observed and there is a clear peak between 200 keV and 1 MeV. The spectrum is slightly harder than in a classical sodium cooled fast reactor fuel assembly of EFR type [23]; see the grey line in Figure 9. The molten salt reactor spectrum is characterized by a much smaller, but still visible, low energy tail formed by the sodium with a clearly lower number of neutrons below 100 keV and a higher number of neutrons above this value.  
The change from the eutectic to the heavy metal rich system leads to a slightly harder neutron spectrum with a decrease in the number of neutrons below \(\sim\)100 keV and a slight increase in the number of neutrons around 1 MeV; see Figure 9. However, the change in the neutron spectrum is only very limited; the general spectral distribution stays almost constant.  
Figure 8: 2-D system thermal feedback effects (salt density and fuel temperature) for the eutectic and the heavy metal rich system depending on uranium enrichment (calculated through POLARIS).  
The influence of the NaCl reflector on the neutron flux spectrum is much stronger than the influence of the salt composition; see Figure 10. The reflector creates a new peak around 1 keV, which disappears when the reflector is removed. This low energy peak reflects neutrons that are undergoing several collisions without leading to neutron reactions, which happens mainly in the reflector. Besides this, the neutron spectrum of the system is clearly softened by the reflector creating a higher neutron flux below \(\sim\)200 keV and a lower neutron flux around 1 MeV.  
The strongest change in the neutron spectrum is caused by the change in the U-235 enrichment; see Figure 11. The use of highly enriched uranium leads to a clear reduction in system size; see Figure 6. The small system size leads, on the one hand, to a significantly higher neutron leakage and less collisions, while on the other hand, the strong reduction  
Figure 10: Neutron spectrum 20% enriched for the reflected and reflected eutectic system (calculated through POLARIS).  
Figure 9: Integral neutron spectrum 35% enriched for the eutectic and the heavy metal rich system in comparison with the neutron spectrum in a classical sodium cooled fast reactor (SFR) fuel assembly of EFR type (calculated through POLARIS, the SFR case is based on HELIOS [24]).  
of the U-238 amount diminishes the parasitic absorption of neutrons in the system since most of the heavy metal is in the 90% enriched system fissile material. Overall, these changes lead to a decrease in the number of neutrons in the energy range below 200 keV and an increase of the number of neutrons above 500 keV. The neutron spectrum of the 90% enriched system is much closer to the fission neutron spectrum.