Automatic controller for power distribution of reactor core

In a BWR type reactor, hollow control members are disposed to a power controlling position near fuel rods. During low power operation, the inside of them is filled with liquid moderators and, during high power operation, the liquid moderators at the inside are removed by evaporated moderators. That is, since moderators in the control members disposed at a position where the power is high are evaporated and removed from the inside of the control members, the amount of thermal neutrons to be moderated is reduced to automatically control the local power and flatten the power distribution of the reactor core. (N.H.)