What's the Difference Between Spontaneous and Neutron-Induced Fission?

Spontaneous fission occurs without a nucleus first absorbing a neutron, whereas neutron-induced fission begins when a heavy nucleus captures a neutron and becomes unstable. Both processes split a heavy nucleus into smaller nuclei and release energy.

The Mechanism

In spontaneous fission, an unstable heavy nucleus splits through a random nuclear process. Like radioactive decay, it is spontaneous because no external trigger is required, and it is random because the exact time at which a particular nucleus splits cannot be predicted.

In neutron-induced fission, a heavy nucleus absorbs a neutron. This produces an excited, unstable compound nucleus, which deforms and divides into two smaller nuclei called fission fragments. For uranium-235, the general process is:

$${}^{235}_{92}\mathrm{U}+{}^{1}_{0}\mathrm{n}\rightarrow{}^{236}_{92}\mathrm{U}^{*}\rightarrow\text{fission fragments}+\text{neutrons}+\text{energy}$$