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.
Tokaimura nuclear accident happened in 1997.
Nuclear War MUD happened in 1992.
Nuclear fission, the fission or "splitting" of an atomic nucleus, can happen in two basic ways. One is a form of natural radioactive decay called spontaneous fission. The atom just "splits" apart. The other involves the absorption of a neutron, and fission can occur as a result of this as well. Both spontaneous fission and neutron absorption are described in separate questions here on WikiAnswers. You'll find links to them below.
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.