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Two types of accidents (file A07)


by Ludwik Kowalski, Sylvie Leray and David Whittal


The diagram of a reactor is shown in Figure 3 ; the core and the steam generating components are located in the protected area. Control elements are made from a non-fissionable material, such as boron, which is an effective absorber of neutrons. The introduction of this material into the moderator, in the form of rods, is used to control the chain reaction. The rate of heating increases when control rods are gradually withdrawn and decreases when they are gradually inserted. Additional safety rods are also available to quickly stop the chain reaction. According to (8) the Chernobyl accident occurred when the safety complex was disabled by the operators who conducted a short test. The poorly designed reactor became overcritical (k>1) and its power output increased by a factor of 100 in three seconds. This produced an excess of heat and resulted in an explosion followed by a devastating fire.

click to see Figure 3 (use the back button to return later).

The accident in the Three Mile Island reactor was caused by a failure in the pump-powered cooling system See Figure 3 again. The flow of water removing heat from the core of the reactor stopped but the safety control system responded properly, as planned for that kind of situation. The chain reaction was turned off at once and production of heat from fission stopped immediately. But radioactive materials in the spent fuel, mostly fission products of short half-lives, kept generating heat. The rate of heating by waste fuel is always less than what is produced when the reactor is operational. But it was far from negligible and some fuel elements were melted. Fortunately, only a very small amount of radioactivity escaped from the strong protective cement structure. The Chernobyl reactor had no protective structure and a large fraction of the core material escaped into the biosphere. The tragic consequences of that event are still unfolding.

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