The fission processes that occur within nuclear reactors are accompanied by the release of neutrons that sustain the chain reaction. Fission occurs when a heavy nuclide such as uranium-235 absorbs a neutron and breaks into nuclides of lighter elements such as barium or krypton, usually with the release of additional neutrons. Like all nuclides with a high atomic number, these uranium nuclei require many neutrons to bolster their stability, so they have a large neutron-proton ratio (N / Z). The nuclei resulting from a fission (fission products) inherit a similar N / Z, but have atomic numbers that are approximately half that of uranium. Isotopes with the atomic number of the fission products and an N / Z near that of uranium or other fissionable nuclei have too many neutrons to be stable; this neutron excess is why multiple free neutrons but no free protons are usually emitted in the fission process, and it is also why many fission product nuclei undergo a long chain of β decays, each of which converts a nucleus N / Z to (N − 1)/(Z + 1), where N and Z are, respectively, the numbers of neutrons and protons contained in the nucleus.