Neutron Capture.

During electron capture, an electron and proton combine and are converted to a neutron.

In a nuclear fission reaction, a freely moving neutron undergoes neutron capture and initiates the nuclear fission of a fuel atom.

Electron capture occurs when an electron from the innermost orbital of an atom is captured by a nucleus, which leads to the conversion of a proton into a neutron.

The capture of a neutron can split a nucleus - but only for certain isotopes like U-235 and Pu-239.

Two naturally occurring isotopes undergo spontaneous fission, meaning the nucleus splits without neutron capture. These are 235U and 238U. A few other isotopes undergo spontaneous fission, but these are produced by an earlier neutron capture. Spontaneous fission is the result of quantum tunnelling, which is rather difficult to explain. There are related links below.