A nuclear reactor system includes a nuclear reactor core disposed in a pressure vessel. Nuclear reactor system further includes control drums disposed longitudinally within the pressure vessel and laterally surrounding fuel elements and at least one moderator element of the nuclear reactor core to control reactivity. Each of the control drums includes a reflector material and an absorber material. Nuclear reactor system further includes a control drum controller with a counterweight to impart a reverse torque on the control drum. Control drum controller includes a driven pulley coupled to the counterweight, a tension member coupled to the driven pulley to rotatably control the driven pulley and apply torque to the driven pulley, and an actuator to apply a tension force to the tension member. The actuator counteracts the reverse torque with the applied tension force, and the tension member applies the torque in response to the tension force.

A reactor control system for a nuclear reactor, the reactor control system comprising: one or more hollow tubes comprising neutron absorbing material, each having a first end and a second end; a pump connected to the first end of each hollow tube and operable to control the amount of a first fluid within the hollow tube, the first fluid comprising a neutron moderator, wherein: the pump is controlled based on a level of reactivity in the nuclear reactor, and the second end of the hollow tubes is in fluid communication with a second fluid, the second fluid having a neutron moderating capacity lower than 10% of that of the first fluid.

A reactor control system for a nuclear reactor, the reactor control system comprising: one or more hollow tubes comprising neutron absorbing material, each having a first end and a second end; a pump connected to the first end of each hollow tube and operable to control the amount of a first fluid within the hollow tube, the first fluid comprising a neutron moderator, wherein: the pump is controlled based on a level of reactivity in the nuclear reactor, and the second end of the hollow tubes is in fluid communication with a second fluid, the second fluid having a neutron moderating capacity lower than 10% of that of the first fluid.