FIG. 17 also illustrates a secondary dynamic neutron reflector sub-assembly 1716. Secondary dynamic neutron reflector sub-assembly 1716 may be used to incrementally shift neutron spectrum in nuclear fuel region 1704 by selectively filling each of the channels 1702 with a volume of neutron reflector material. Secondary dynamic neutron reflector assembly 1716 may include a plurality of refractory-clad sleeves 1702 to conduct a flowing neutron reflector material therethrough. In FIG. 17, the neutron reflector material flows upward through a refractory clad channel 1702 toward the viewer. In an implementation, neutron reflector material may circulate in channels 1702 (e.g., cells, sleeves, conduits, etc.) with input and output ports above the nuclear fuel region 1704 such that no fixtures or ports are needed beneath the reactor. In other implementations, the neutron reflector material may flow in only one direction, either in an upward or downward direction, through the channels 1702 with one port above the nuclear fuel region 1704 and another port below fuel region 1704. In yet other implementations, the neutron reflector material may comprise a semi-stagnant or creeping flow through the channels 1702. In yet other implementations, the neutron reflector material may flow through radial input and output ports.
