The chernobyl incident happened because a test made the reactor go beyond its capable limit.
By the time the mistake was realised, it was to late as the core exploded and in the second explosion lifted the roof of the building off it with the equivalent of 225tons of TNT.
It is the worst nuclear disaster to date, causing the deaths of 9000-16000 people and leaving the entire town of pripyat abandoned to this day.
By the classical physics definition, radiation is the idea of energy being radiated through air, without direct contact.
There are 2 types of radiation, non-ionizing being those that dont have enough energy to rip electrons apart from the atom, example heat.
The other is ionizing radiation, which is a type of radiation of such energy that it can detach electrons from atoms or molecules, which causes changes at the atomic level when interacting with matter including living organisms.
These are the ones that are emitted from a nuclear fission fuel/product.
These can alter dna, cause massive damages to organs and even result in death.
