Abstract: In response to a change in a thermal operational parameter in a nuclear fission reactor, reactivity is controlled by driving a neutron absorption parameter modifying material to or from a region of the core of the nuclear fission reactor. The apparatus includes a driver material disposed in a first reservoir portion and a second reservoir portion of a first reservoir and connected by a first conduit. A high-Z material may be distributed in the driver material. The neutron absorption parameter modifying material is disposed in both a second reservoir and the second reservoir portion of the first reservoir. A portion of the neutron absorption parameter modifying material is in physical contact with a portion of the driver material in the second reservoir portion. The neutron absorption parameter modifying material is driveable by the driver material between the second reservoir portion and the second reservoir through a second conduit.