World Nuclear Association

RBMK light water graphite reactor

March 2001

The Soviet designed RBMK is a pressurised water reactor with individual fuel channels and using ordinary water as its coolant and graphite as its moderator. It is very different from most other power reactor designs as it was intended and used for both plutonium and power production. The combination of graphite moderator and water coolant is found in no other power reactors. The design characteristics of the reactorwere shown, in the Chernobyl accident, to cause instability when low power. This was due primarily to control rod design and a positive void coefficient. A number of significant design changes have now been made to address these problems.

RBMK reactor

Fuel rods
Pellets of enriched uranium oxide are enclosed in a zircaloy tube 3.65m long, forming a fuel rod. Two sets of 18 fuel rods are arranged cylindrically in a carriage to form a fuel assembly of about 10 m length. These fuel assemblies can be lifted into and out of the reactor mechanically, allowing fuel replenishment while the reactor is in operation.

Pressure tubes
Within the reactor each fuel assembly is positioned in its own pressure tube or channel. Each channel is individually cooled by pressurised water.
Graphite moderator
A series of graphite blocks surround, and hence separate, the pressure tubes. They act as a moderator to slow down the neutrons released during fission. This is necessary for continuous fission to be maintained. Conductance of heat between the blocks is enhanced by a mixture of helium and nitrogen gas.
Control rods
Boron carbide control rods absorb neutrons to control the rate of fission. A few short rods, inserted upwards from the bottom of the core, even the distribution of power across the reactor. The main control rods are inserted from the top down and provide automatic, manual or emergency control. The automatic rods are regulated by feedback from in-core detectors. If there is a deviation from normal operating parameters (e.g. increased reactor power level), the rods can be dropped into the core to reduce or stop reactor activity. A number of rods normally remain in the core during operation.
Coolant
Two separate water coolant systems each with four pumps circulate water through the pressure tubes. Ninety-five percent of the heat from fission is transferred to the coolant. There is also an emergency core cooling system which will come into operation if either coolant circuit is interrupted.
Steam separator
Steam from the heated coolant is fed to turbines to produce electricity in the generator. The steam is then condensed and fed back into the circulating coolant.
Containment
The reactor core is located in a concrete lined cavity that acts as a radiation shield. The upper shield or pile cap above the core, is made of steel and supports the fuel assemblies. The steam separators of the coolant systems are housed in their own concrete shields.

RBMK Core