FIGS. 30A-30C illustrate a reconfigurable reactor design that can be operated as either a fast neutron reactor or a thermal neutron reactor. In the embodiment shown, the MCFR reactor begins operating with a large core diameter and fast reflectors, which may comprise any fast spectrum reflector including, without limitation, lead, lead bismuth, etc. Thermal reflectors may be provided and which comprise any thermal reflector material including, without limitation, graphite, etc. Then, thermal reflectors may be inserted directly into the core, inside the fast reflectors, to reduce the critical core diameter and increase the power density. In this way, the MCFR reactor is capable of operating in multiple phases, where the first phase operates in a fast-reflected fast neutron spectrum and the second phase operates in a thermal-reflected thermal neutron spectrum. Additionally or alternatively, the reactor may be operated with both the thermal and fast reflectors in place for a later transition from a thermal reactor after removal of the thermal reflectors which would then modify the reactor to a fast spectrum with only the fast reflectors in place. Thus, the reactor is reconfigurable so that can be operated as either a fast neutron reactor or a thermal neutron reactor as needed. The same heat exchanger circuits are used in either configuration and/or additional heat circuits may be provided when the inner (thermal) reflectors are removed.
