BOILING WATER TYPE NATURAL CIRCULATION REACTOR

PURPOSE: To enable controlling reactor power not only by control rods but by intentionally changing core flowrate independently, facilitate manual operations such as startup, shut-down and daily load following and improve operational capability of efficiently compensating the reactor power in the case of disturbance such as transients. CONSTITUTION: This boiling water type natural circulation reactor is constituted of an annular plate 8 with drilled flow path holes 9 placed in the annular part 6 formed by a reactor pressure vessel 4 and a shroud 5, an opening control means 10 for choking a part of or whole flow path holes 9, and a drive means 11 for controlling to an arbitrary opening by moving the opening control means 10 up and down to vary the flow path area.