Background
The reactor at Chernobyl was a so-called RBMK-1000, Reaktor Bolshoy Moshchnosti Kanalnyy type 1000 Generation II reactor, designed by and built only in the Soviet Union [1]. There is a significant difference between the RBMK-1000 reactor the Soviets used at Chernobyl and the reactors in the rest of the world. The RBMK-1000 reactor was associated with two serious problems. A design the Soviet Union was also warned against by Western experts to build.
Lack of containment building
The containment building is a reinforced concrete shell over 2 meters thick. So strong that a jet can smash into it without damage [2]. Its purpose is if all other safety systems fail in some way, it may stop radioactive fallout. [Read more about the Containment Building here].
The Chernobyl plant was built without the metre-thick reinforced concrete jacket that encloses the core. No other nuclear power plant in the world has been built without it. The size of the RBMK-1000 reactor made the containment building too large and expensive. So it was saved away and simply put in a common hall.
If the Russian RBMK-1000 reactor had been designed with such a containment building, the 1986 accident at Chernobyl would have only destroyed the reactor itself, but not caused a harmful release to the environment as we saw.
All other nuclear power plants in the world are designed with such a containment building [3].
Combined graphite moderation with water cooling
The moderator’s task is to slow down the speed of the neutrons. The chance of fission only occurs if these move slowly [4].
The light water reactors (LWR) and all other reactors are water moderated and water cooled. This means that the system is interrupted and the water cooling is lost, the reaction stops because there is no longer a moderator. That is, the reactor is self-stabilizing. If they lose cooling, the fission process stops. They cannot burn.
The RBMK-1000 reactor used at Chernobyl combined graphite moderation with water cooling. If that cooling is lost, the fission process does not stop, on the other hand, because the graphite is still there, so it gets hotter and hotter, where the griffin eventually catches fire.
The Chernobyl accident cannot be repeated.
Because of these major design differences, most of the world’s operating reactors have and are never at risk of this type of accident as we saw at Chernobyl.
Today there are still 8 RBMK reactors left in Russia. But they are no longer built and are being closed down gradually. After the Chernobyl accident in 1986, they were all upgraded to prevent the release sequence. So there is no risk of a new Chernobyl with them.
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