This assembly is in the center of the core and consists of a hollow assembly tube with a thin membrane covering the top of the assembly. Connected to the top of the assembly tube is a wedge shaped plunger and container filled with thousands of boron-carbide balls.
Boron absorbs fast neutrons very well and thus makes an ideal control material in nuclear reactors.
One problem with this system is the difficulty and length of time involved in cleaning up the central assembly should the system be activated intentionally during an accident or unintentionally during refueling procedures.