Water cooled reactors

Purpose: To reduce the deviation with time in forming power distribution when the power distribution fluctuates due to the control of reactor core coolant flow rate or the like. Constitution: A plurality of vessels each having fluid inlet and outlet are disposed at the radial outer circumference of a reactor core in a multi-layered arrangement along the height. Then, the liquid poison density such as of an aqueous boric acid injected to the vessels is varied depending on the height of each vessel to vary the neutron absorption amount thereby control the neutron density distribution in the direction of the reactor core height and control the power distribution. Since the axial power distribution can be corresponded with ease to the power fluctuations, the reactor operation can be facilitated. (Sekiya, K.)