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.