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 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 nuclear power system has an open volume between containment vessel and a reactor vessel containing a reactor core. Located in the open volume is a container holding a neutron-absorbing chemical in solid form. The container is configured to release the chemical in solid form directly into the open volume in response to a predetermined temperature and/or a predetermined pressure within the open volume. The released chemical can assist in maintaining the reactor core in a sub-critical state.

A nuclear power system has an open volume between containment vessel and a reactor vessel containing a reactor core. Located in the open volume is a container holding a neutron-absorbing chemical in solid form. The container is configured to release the chemical in solid form directly into the open volume in response to a predetermined temperature and/or a predetermined pressure within the open volume. The released chemical can assist in maintaining the reactor core in a sub-critical state.