Patent Number: 
Section: claims

1. A nuclear device, comprising: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; anda core including the heat pipes arranged in parallel with each other. 2. The nuclear device according to claim 1, wherein a enrichment level of the fissile material in a first area is greater than that in a second area, and a heat transferred from the second area to the heat pipes is less than the heat transferred from the first area to the heat pipes. 3. The nuclear device according to claim 2, wherein the first area contains more heat pipes than the second area per unit area in a cross section of the core perpendicular to a central axis of the heat pipe. 4. The nuclear device according to claim 1, further comprising:a first layer that includes the heat pipes arranged in parallel to a central axis of each heat pipe with each first fuel surrounding respective heat pipes surrounded by second fuel;a first heat conductor along a side surface of the first layer and parallel to the central axis, thermal conductivity of the first heat conductor being greater than that of the second fuel. 5. The nuclear device according to claim 4, wherein the first heat conductor contains beryllium. 6. The nuclear device according to claim 4, further comprising:a layer that includes heat pipes arranged next to each other and in parallel to the central axis of each heat pipe, the second fuel configured to surround the first fuel surrounding respective of the heat pipes next to each other; anda second heat conductor disposed in the second fuel. 7. The nuclear device according to claim 1, further comprising:two overlapping layers, each layer including heat pipes arranged next to each other in parallel to a central axis of each heat pipe, the second fuel configured to surround the first fuel surrounding respective of the heat pipes next to each other; anda heat conductor connecting the two overlapping layers, wherein a first end of the heat conductor is closer to the heat pipe than a second end. 8. The nuclear device according to claim 1, further comprising:a first layer that includes heat pipes arranged in parallel to a central axis of each heat pipe, the second fuel configured to surround the first fuel surrounding the heat pipes of the first layer;a second layer that includes heat pipes arranged in parallel to the central axis of each heat pipe, the second fuel configured to surround the first fuel surrounding respective of the heat pipes of the second layer;wherein a heat pipe of the second layer is located between two heat pipes next to each other in the first layer when viewed in a direction parallel to the central axis of each heat pipe. 9. The nuclear device according to claim 1, further comprising a metal layer between the heat pipes and the first fuel, including a metal,wherein a melting point of the metal is such that the metal layer is solid at a temperature before the nuclear reactor starts operating, and melts at a temperature after the nuclear reactor starts operating. 10. The nuclear device according to claim 1, wherein the core includes a plurality of cylinders arranged concentrically,wherein each of the cylinders is made of a layer including heat pipes arranged next to each other in parallel to a central axis of each of the heat pipes, the first fuel surrounding the heat pipes arranged next to each other, and the second fuel surrounding the first fuel which surrounds the heat pipes. 11. The nuclear device according to claim 10, wherein the enrichment level of the fissile material in the core differs along the axis of each heat pipe. 12. The nuclear device according to claim 10, wherein the core includes multiple sections arranged in the central axis direction of the core, wherein the enrichment level of the fissile material in each section varies according to a position of the section. 13. The nuclear device according to claim 10, wherein a point outputting a maximum heat in one layer does not overlap a point outputting a maximum heat in a next adjacent layer. 14. The nuclear device according to claim 10, wherein the enrichment level of the fissile material in the core differs in a radial direction in a cross section of the core perpendicular to the central axis of the heat pipes. 15. The nuclear device according to claim 14, further comprising a control rod arranged in a central part of the core,wherein the enrichment levels of the fissile material in the center part of the core, adjacent a side surface of the core parallel to a central axis of the core, and in a at a half point of a length of the core parallel to the center axis, are lower than that of other parts in the core.