:A design of a modular high temperature gas cooled reactor - gas turbine (MHTGR-GT) concept, employing a direct Brayton cycle, demonstrates the potential for achieving over 46% net thermal efficiency. The concept uses a 450 MWt reactor module designed for the DOE sponsored steam cycle version of the MHTGR (MHTGRSC). The reactor was extended to 850°C (1562°F) core outlet temperature without requiring fuel or core design modifications. The power conversion system, which includes a 3600 rpm helium turbocompressor with a submerged generator, is contained in a second vessel connected by an annular crossduct to the reactor vessel. Due to the high efficiency and elimination of the steam turbine plant, the MHTGR-GT offers the potential for a 25% power cost reduction over current large nuclear plants.