Abstract: Fast reactors are disclosed that embody a central core surrounded by a reflector that each comprise a plurality of abutting hexagonal blocks of ceramic material. Each hexagonal block comprises a plurality of coolant channels disposed therethrough. Outer surfaces of each hexagonal block and surfaces of each coolant channel are coated with a material defining a containment shell. Burnable poison is disposed in the central core. The central core may comprise a graphite foam and uranium carbide matrix or uranium carbide. The containment shell may comprise silicon carbide or a carbon-carbon composite material. The reflector comprises natural uranium. Exemplary reactors operate at about 1000° C. and at a pressure of 68 bars (1000 psi) in a direct gas cycle at an efficiency of approximately 50%. Exemplary reactors also require no fuel addition or removal for more than 50 years.