Generation of plutonium
The capture of a neutron by a nucleus of uranium-238 leads to the formation within a few days of a nucleus of fissile plutonium-239. The nucleus of uranium-239 resulting from the capture of the neutron is radioactive beta-minus. By emitting an electron and an antineutrino, it becomes neptunium-239, which being itself unstable transforms in its turn the same way into plutonium-239.