epithermal reactor

The open-cycle gas-core reactor (OCGCR) is typically a thermal/epithermal reactor.

Her liquid-sodium cooled epithermal reactor was more thermally efficient than a light water-cooled system, quieter, and presumably better system, but posed, presumably and arguably, several safety hazards for the ship and crew.

The Gen IV MSR is more accurately termed an epithermal reactor than a thermal reactor due to the average speed of the neutrons that would cause the fission events within its fuel being faster than thermal neutrons.