Patent Number: 
Section: claims

1. A thermal-neutron reactor core comprising:a moderator extending to a lengthwise direction and formed in multiple concentric layers;a fuel in the moderator, parallel to the lengthwise direction of the moderator, the fuel containing a fissile material, a burnable poison, and formed in multiple concentric layers;a plurality of cooling tubes parallel to the lengthwise direction of the moderator, whereinthe multiple concentric layers of the moderator and the multiple concentric layers of the fuel alternate with each other, andthe plurality of cooling tubes are evenly distributed with circumferential intervals therebetween in at least one of the multiple concentric layers of the fuel. 2. The thermal-neutron reactor core according to claim 1, wherein the moderator contains a metal hydride. 3. The thermal-neutron reactor core according to claim 1, wherein the burnable poison is a burnable poison containing a concentration of one particular isotope of the burnable poison. 4. The thermal-neutron reactor core according to claim 1, wherein the burnable poison is cadmium-113 or europium-151. 5. The thermal-neutron reactor core according to claim 1, further comprising; a neutron multiplication material between the fuel and the moderator. 6. A design method for the thermal-neutron reactor core of claim 1 including a solid moderator, the method comprising:deciding a specification of the thermal-neutron reactor core which includes a kind of a fuel, a size of the thermal-neutron reactor core, a composition of the moderator, and a cooling system;determining a neutron energy spectrum based on the specification;selecting a plurality of kinds of burnable poison;examining a temperature dependence of an effective neutron multiplication factor in the thermal-neutron reactor core based on a proportion of the plurality of kinds of burnable poison; anddeciding whether the proportion is acceptable for an operation of the thermal-neutron reactor core or is not acceptable based on the temperature dependence of the effective neutron multiplication factor, wherein the effective neutron multiplication factor should decrease as temperature rises for the operation. 7. The design method for the thermal-neutron reactor core according to claim 6, further comprising:deciding a ratio of the plurality of kinds of burnable poison to the fuel if the proportion is proper for the operation of the thermal-neutron reactor core. 8. The design method for the thermal-neutron reactor core according to claim 6, wherein the plurality of kinds of the burnable poison include cadmium-113 and europium-151.