The amount of heat they are exposed to has a lot to do with the inferences and observations that they make on the density of the earth's core. The inner core is the most compact due to the fact that they have a lot of direct exposure to heat and the gravity of the earth pressung down on it. The inner core is less subjected to those conditions.
The answer is: convection of the core
The site of the chernobyl nuclear accident in 1986, was at the Chernobyl Nuclear Power Plant, which was located in Ukraine. In terms of cost and casualties, it was the worst nuclear power plant disaster in history.