A reactivity control system includes a reservoir (42) containing a liquid nuclear poison (44), at least one stationary, hollow control blade (32) extending vertically into a reactor core (14), and a poison conduit (52) disposed in flow communication between the reservoir (42) and control blade (32) for channelling the poison (44) between the reservoir (42) and control blade (32). The level of the poison in the control blade is controlled for selectively varying nuclear reactivity in the core.