Patent Number: 
Section: claims

1. A reactor vessel comprising:a neutron reflector;a lower core plate;a core vessel, the core vessel including a flange portion extending in a direction towards the neutron reflector; anda neutron reflector bolt fastening structure, the neutron reflector bolt fastening structure including:a plurality of divided stage portions composing said neutron reflector which is situated in said core vessel in said reactor vessel;a plurality of tie rods that fix said neutron reflector to said core vessel; anda plurality of bolts that fasten solely a lowermost stage portion of said plurality of stage portions of said neutron reflector to said core vessel,wherein said lowermost stage portion of said neutron reflector includes a plurality of flange portions extending in a direction towards the core vessel and positioned above the lower core plate, andwherein the plurality of bolts are configured to extend through each of the plurality of flange portions of the lowermost stage portion and the lower core plate, and the plurality of bolts extend partially into the flange portion of the core vessel positioned below the lower core plate, to secure the lowermost stage portion of the neutron reflector to the core vessel. 2. The reactor vessel according to claim 1, wherein said plurality of flange portions of the lowermost stage portion are arranged at opposing positions in the outer periphery of said lowermost stage portion of said neutron reflector. 3. The reactor vessel according to claim 1, wherein four of said flange portions of the lowermost stage portion are arranged at equal intervals in the outer periphery of said lowermost stage portion of said neutron reflector. 4. A method for fastening a neutron reflector comprising:fixing a neutron reflector composed of a plurality of divided stage portions and situated in a core vessel in a reactor vessel to said core vessel by means of a plurality of tie rods; andfastening solely a lowermost stage portion of said plurality of stage portions of said neutron reflector to a flange of said core vessel by means of a plurality of bolts, in which said plurality of bolts extend through each of a plurality of flanges of the lowermost stage portion and a lower core plate located below the plurality of flanges of the lowermost stage portion, and the plurality of bolts extend partially into a portion of the flange of the core vessel that extends in a direction of the neutron reflector and is located below the lower core plate, such that the lowermost stage portion is fastened to said flange of the core vessel.