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.
Tokaimura nuclear accident happened in 1997.
Nuclear War MUD happened in 1992.
It would become an atom of a different element. This can only happen during radioactive decay, nuclear fusion, or nuclear fission.
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.
Trinity - nuclear test - happened on 1945-07-16.
Kramatorsk nuclear poisoning incident happened in 1989.
Nuclear War - video game - happened in 1989.
Timeline of the Fukushima nuclear accidents happened in 2011.
There are emergency safety system that could be used to bring the nuclear plant again under control.