To obtain energy from a nuclear reaction it is necessary to somehow control the reaction, and stop a chain reaction, and hence a huge fireball of death, from occuring.
This was first done during the Manhattan project in Chicago. The trick was to make sure that not too many reactions occured, and therefore not too many neutrons were produced to sustain a chain reaction. This is achieved with the use of control rods.
The control rods are made of materials which absorb neutrons, such as cadmium and graphite. The first controlled nuclear reaction used cadmium rods, however graphite is more commonly used now.
The rods are inserted into the core to slow down the reaction (by absorbing neutrons), and removed to speed up.
Cadmium, like Uranium-238 undergoes neutron capture when bombarded with neutrons. It is said to have a "High Neutron Capture Cross Section", and as such is perfect for controlling nuclear fusion reactions.