Spontaneous Fission is the process of Elements and Induced Fission is the process of firing neutrons at heavy Atoms
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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.