BWR type reactor

There are disposed a water rod passing through a fuel assembly of a BWR type reactor and a fluid inertia pipe connected to the lower portion of the assembly to introduce coolants from a pump to the fuel assembly. In addition, there are disposed, in the fluid inertia tube, a power detection device capable of longitudinally moving in the water rod for detecting the power of the fuel assembly and a means for driving the power detecting device. During operation, the power detecting device moves in the water rod by a driving device to measure the power of the reactor core directly. The measured value and a calculated value correspond with each other each one by one to minimize the factor of errors. The monitoring performance for the reactor core can be improved, in addition, the operation margin can be reduced, and the power can be reliably increased even with the facilities of the same scale. (N.H.)