But in certain cases the initial capture is rapidly followed by the fission of the new nucleus. In this example, U-239 becomes Np-239 after emission of a beta particle (electron). The new nucleus may decay into a different nuclide. A simple example is U-238 + n => U-239, which represents formation of the nucleus U-239. Capture involves the addition of the neutron to the uranium nucleus to form a new compound nucleus. When a neutron passes near to a heavy nucleus, for example uranium-235 (U-235), the neutron may be captured by the nucleus and this may or may not be followed by fission. Neutrons in motion are the starting point for everything that happens in a nuclear reactor. Isotope separation to achieve uranium enrichment is by physical processes.
