Patent ID: 7978807

Claim:
A pebble bed nuclear reactor comprising: a reflector reflecting neutrons and containing graphite and moderator having a smaller moderating power than the graphite, the reflector being sectioned into plural parts along a direction of flow of fuel pebbles; and a flow path surrounded by the reflector, and through the flow path the fuel pebbles flow and undergo nuclear reaction to generate power, wherein a volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution of a reactor core in the direction of flow of the fuel pebbles, the reflector includes an inner reflector and an outer reflector that surrounds a reactor core, the flow path of the fuel pebbles is surrounded by an outer circumference of the inner reflector and an inner circumference of the outer reflector, the volume fraction of the moderator having a smaller moderating power than the graphite is set equal to 75% in an approximately ⅓ portion of the inner reflector from an upstream side of the flow path of the fuel pebbles, to 25% in an approximately ⅓ portion at the center of the inner reflector, and to 75% in an approximately ⅓ portion of the outer reflector from the upstream side, and the other portions are made of graphite.