Control rods
A nuclear reactor is controlled by controlling the number of neutrons
in the reactor core. (Neutrons are the links that connect the
reactions that release the nuclear energy.) Neutrons can be removed
(absorbed) by the materials in the control rods. Thus if the control
rods are inserted into the reactor core, most neutrons are absorbed
by the rods (rather than being multiplied by hitting U235).
If the control rods are removed, additional neutrons will end up hitting
U235, and the neutron number will grow.