Dynamic control over neutron reflectivity characteristics of the neutron reflection assembly 110 permits selection of a desired reactivity level in reactor core section 106. For example, molten fuel salt 108 requires a minimum level of thermal neutron contact to remain critical in reactor core section 106. The dynamic neutron reflector assembly 110 may be adjusted to provide the reflectivity characteristics for maintaining or contributing to the criticality in the molten fuel salt 108 within the reactor core section 106. As another example, it may be desired to operate the MSR system 100 at full power, which would motivate an increased thermalization of neutrons in the reactor core section 106 to increase the fission rate. The reflectivity characteristics of dynamic neutron reflector assembly 110 could be therefore increased to provide more moderation until a desired reactivity level representing full power for the reactor core section 106 has been reached.