A criticality event, and high risk of loss of life and property.
Nuclear reactor
Yes, all natural radiation (in the rocks) is a result of fission (but this fission is not part of a chain reaction like in a fission bomb). However, it is theoretically possible for natural processes to concentrate radioactive elements (uranium) to the extent where a natural nuclear fission reactor (a chain reaction like in a nuclear power plant) will form. Oklo in in Gabon is the only known location for this to have happened and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago.
Nuclear Dawn happened in 2011.
It would become an atom of a different element. This can only happen during radioactive decay, nuclear fusion, or nuclear fission.
Nuclear War MUD happened in 1992.
Tokaimura nuclear accident happened in 1997.
Fission is a nuclear reaction where the nucleus of an atom splits into two or more smaller nuclei, along with the release of a large amount of energy and more neutrons. This process is triggered by bombarding the atom with a neutron, causing it to become unstable and break apart. Fission is the principle behind nuclear power plants and nuclear weapons.
Timeline of the Fukushima nuclear accidents happened in 2011.
Trinity - nuclear test - happened on 1945-07-16.
Kramatorsk nuclear poisoning incident happened in 1989.
Nuclear War - video game - happened in 1989.
There are emergency safety system that could be used to bring the nuclear plant again under control.