Reactor

Object: To provide a pressure tube type heavy water moderated reactor which can uniform power distribution in an axial direction of a core. Structure: The outside diameter in the central portion of a core of a calandria tube is increased so as to decrease, at the central portion of the core, a volume in a region of a heavy water moderator in an outer portion of the calandria. In this manner, if the volume in the heavy water region whose neutron slowing-down power is great is decreased, the rate of thermal neutron generation decreases and power generated also decreases. Accordingly, the outside diameter of the calandria tube may be varied axially to thereby vary the volume in the heavy water region to adjust the power distribution in the core. (Ohara, T.)