The reactor is controlled by three control rods which contain boron carbide as absorber material. When these rods are fully inserted into the reactor core, the neutrons continuously emitted from a start-up source (Sb-Be photo-neutron source) are absorbed by the rods and the reactor remains sub-critical.
If the absorber rods are withdrawn from the core, the number of fissions in the core and the power level increases. The start-up process takes a few minutes for the reactor to reach a power level of 250 kW from the sub-critical state. The reactor can be shut-down either manually or automatically by the safety system. It takes about 1/10 of a second for the control rods to fall into the core.