RBMK Design
The core of an RBMK reactor is made of graphite blocks, it is the moderator which will slow down the neutrons produced by the fission of uranium. This is obviously a huge structure, big as a house.
This mass of graphite is pierced with 1661 vertical holes in which will be inserted pressure tubes of about 3.5 inches (9 centimeters) in diameter, containing uranium and leading the cooling water under a pressure of 1000 psi (70 kg/cm) from bottom of the the reactor upwards. This water intended to cool down the tubes and the graphite boiling out from the top of the tubes and provide steam to drive turbines.
In each pressure tube, uranium is contained in 18 rods about 0.5 inch (13 millimeters) in diameter. The height of the core is 21 feet (6.5 meters).
The structure of graphite is locked in a steel tank with a diameter of about 42 feet (13 meters). A mixture of helium and nitrogen is injected to increase the quality of caloric transfer between graphite and tubing pressure and perhaps reduce the oxidation of graphite.
On the outside, around the core has been constructed a building, intended only to prevent radioactive leaks. This concrete building contains screens of sand and water to absorb radiation. The problem of graphite is that its temperature reaches about 700 C (1,300 F) in the reactor in operation. At this temperature graphite is glowing red and does not ignite because an inert gas (helium/nitrogen) is sent continuously. But if the air (thus oxygen) enters the enclosure, it catches fire.
Block diagram of an RBMK reactor
(There are two identical sets in a reactor)
The core of the reactor is covered with a concrete slab weighing about 1,000 tons. On this slab are set all the pressure tubes.
In an RBMK reactor, there are two separate pressure loops pressure feeding each a turbine installed in a building next to the reactor. In Chernobyl, the RBMK 1000 reactors produced each 925 MW of energy.
The RBMK reactors did not have a surrounding wall for full containment. The pipes beneath the reactor was placed in what the Soviets called anti-leak boxes. These boxes were connected to a vast pool beneath the entire building. That pool was used to trap steam and radioactive particles if a pressure tube or a pipe broke. But all higher tubing were only protected by an standard industrial building. If a pipe breaks on the top, this would result in an leak of radioactive steam whose importance depends on the speed at which systems emergency stop the reactor.
The accident at Three Mile Island's nuclear power plant in 1979, confirmed to the world that a complete confinement was necessary for the safety of a nuclear power plant. The first RBMK reactors, including those in the Leningrad power plant, did not even have a partial confinement.
But the RBMK is enormous, the building is very large, about 71 meters high, and achieve a complete confinement as on the U.S. reactors would cost a lot. Also, the Soviets continued to have confidence in security systems for their plants, rather than investing in passive safety of a quality confinement.
The combination of a graphite moderator and a cooling with water does not exist in any other reactor. This design makes the reactor becomes unstable at low power, and this is confirmed in the Chernobyl accident. This instability was due mainly to the design of the control rods and the positive void coefficient.
A positive void coefficient has the consequence of increasing the power of the reactor. If the quantity of boiling water increases, the bubbles generated in boiling water decrease the power of moderation of the water. And a loop begins, the water boils, the reactor increases its power, water boils more and so on.
The RBMK reactors have a high positive void coefficient because they work with few enriched uranium and uses light-water instead of heavy-water.