The RBMK-1000 reactor used in Chernobyl nuclear power station
The RBMK-1000 is a type of pressure tube reactor designed in the former Soviet Union, which uses ordinary boiling water as the coolant and graphite as the moderator.
Boiling water acts as a coolant and also provides the steam used to drive the turbines. The coolant system of the reactor consisted of two circuits. Water flows through the coolant circuit I to the fuel channels, where it is boiled by the uranium fuel inside the pressure tubes, producing steam to feed two 500 MW turbines via the coolant circuit II.
The graphite moderator is to slow down neutrons to make them more efficient in producing fission in the fuel. The graphite core of the reactor is about 7 m high and about 12 m in diameter, composing holes for 1661 fuel channels and 211 control rods. The power of the reactor is controlled by raising or lowering the control rods, which, when lowered, absorb neutrons and reduce the fission rate.
RBMK-1000 has potential safety hazards due to its design defects. At low power output, the reactor becomes unstable and is prone to sudden power surges. Three safety systems, including an emergency core cooling system, the requirement for a minimum operation at an electric power of 200 MW, and a minimal insertion of 30 control rods, are included in the operating procedures.