Abstract: A nuclear device including a plurality of heat pipes; a first fuel configured to surround respective of the plurality of heat pipes coaxially with respect to a central axis of each of the respective heat pipes, the first fuel containing a fissile material at a first enrichment level; a second fuel configured to directly abut the first fuel on the outside of the first fuel and farther than the first fuel from the respective heat pipes surrounded by the first fuel, the second fuel containing the fissile material at a second enrichment level less than the first enrichment level; and a core including the heat pipes arranged in parallel with each other.

Abstract: A nuclear device, including: a heat pipe; a first fuel allocated around a side surface of the heat pipe parallel to a central axis of the heat pipe, the first fuel containing a fissile material at a first concentration; a second fuel allocated on an outer side of the first fuel and containing the fissile material at a second concentration less than the first concentration; and a core including a plurality of the heat pipes arranged in parallel to each of the central axis in the first fuel or in the first fuel and the second fuel.