What happens in an accident in a nuclear power plant?

The reactor of a nuclear power plant (NPP) contains radioactive substances that are very dangerous for human beings. Such plants are monitored very carefully. If a technical fault goes unnoticed there can be uncontrolled chain reactions in the reactor. The internal pressure rises enormously within a few minutes and there is an explosion. This is exactly what happened in 1986 in Chernobyl, an NPP in Ukraine. A huge cloud of radioactive substances formed over Europe; it first settled over the Scandinavian countries, and then travelled to the Mediterranean countries via Poland and Germany. The radioactive substances reached the soil through rain. People, who switched off the reactor and later cleaned up the area, died from the effects of radiation.