Patent Number: 
Section: claims

1. A core catcher in a nuclear reactor, the core catcher comprising:a holding surface in a portion of the reactor that stores a coolant, the holding surface being configured to catch and hold corium; anda cooling unit which is configured to introduce the stored coolant into the core catcher, to cool a whole of the core catcher including the holding surface by heat exchange with the introduced coolant,wherein the holding surface and the cooling unit are constructed by arranging a plurality of corium holding structures laterally adjacent one another in a horizontally extending array, wherein each of the corium holding structures includes a polyhedron having at least one pair of parallel lateral surfaces and defining an inside space in the polyhedron, each corium holding structure further having an opening formed in one of the parallel lateral surfaces, the opening extending from the lateral surface to the inside space and being positioned on the corium holding structure to communicate the inside space of the respective corium holding structure with an inside space of and adjacent corium holding structure arranged in the in the horizontally extending array,wherein coolant in each of the polyhedrons communicates via the opening with a laterally adjacent polyhedron of the lateral array. 2. The core catcher according to claim 1, further comprising a coolant supply unit which is configured to supply a coolant other than the stored coolant to a floor surface on which the corium holding structures are arranged. 3. The core catcher according to claim 2, wherein the coolant supply unit is installed closer to a center of the core catcher as viewed from above than to an outer edge of the core catcher. 4. The core catcher according to claim 1, further comprising a riser protruding upward at an outer edge of the holding surface, to hold the corium on the holding surface in the lateral direction, wherein the riser comprises a stack of the corium holding structures. 5. The core catcher according to claim 4, wherein the corium holding structures are each configured as rectangular parallelepiped and the riser portion is constructed by a rectangular parallelepiped having an opening portion in at least two surfaces thereof. 6. The core catcher according to claim 1, further comprising a second coolant inlet unit that introduces the coolant into a corium holding structure of the plurality of arranged corium holding structures, and a vapor outlet unit that discharges vapor from inside the corium holding structure. 7. The core catcher according to claim 6, wherein the second coolant inlet unit and the vapor outlet unit are integrally constructed as a double tube structure which includes an inner tube and an outer tube, each having different diameters and being concentrically arranged with each other. 8. The core catcher according to claim 7, wherein the double tube structure includes the inner tube as the vapor outlet unit and the outer tube as the second coolant inlet unit. 9. The core catcher according to claim 6, wherein the second coolant inlet unit and the vapor outlet unit are installed directly below a control rod drive mechanism. 10. The core catcher according to claim 1, further comprising a protective portion provided on an upper surface of the corium holding structure, which integrally covers a whole of the upper surface of the corium holding structure.