The signs that fission reactions are occurring come 35 years nearly to the date when the Chernobyl nuclear power plant in what is now Ukraine exploded on April 26, 1986. It was the worst-ever nuclear disaster in the world to date.
The nuclear fission reactions are taking place in a room that is sealed with concrete and contains a large part of the uranium fuel of the former reactor.
Ukrainian scientists are now trying to assess whether the detected nuclear fission reactions will die out or create a larger problem that will require some type of extraordinary intervention.