Abstract: A pressure vessel for nuclear energy powered thermionic fuel elements is disclosed. An inner cylindrical nuclear fuel heat source is surrounded by cylindrical layers of, in outward order, an emitter, a gap, a collector, an insulator and a cladding layer. A hexagonal pressure vessel surrounds the other parts of the thermionic fuel element and forms liquid metal coolant passages between the inside corners of the hexagon and the cladding. Longitudinally, each thermionic fuel element comprises a middle active zone between two 20% enriched uranium zones to reduce the critical mass of the system for safety. Beryllium zone endcaps act as neutron reflectors and further reduce the critical mass. A plurality of thermionic fuel elements is arrayed into a reactor core with brazed together pressure vessel hexagon sides. In the event of a leak, or other coolant flow failure, the pressure vessel sides act as thermal conduction fins and transfer waste heat to adjacent pressure vessels where coolant flow has not failed.