Krypton-85, with a half-life 10.76 years, is formed by the fission process with
a fission yield of about 0.3%. Only 20% of the fission products of mass 85 become Kr itself; the rest passes through a short-lived nuclear isomer and then to stable Rb. If irradiated reactor fuel is reprocessed, this radioactive krypton may be released into the air. This krypton release can be detected and used as a means of detecting clandestine nuclear reprocessing. Strictly speaking, the stage which is detected is the dissolution of used nuclear fuel in nitric acid, as it is at this stage that the krypton and other fission gases like the more abundant xenon are released. Despite the industrial applications of Krypton-85 and the relatively high prices of both Krypton and Xenon, they are not currently extracted from spent fuel to any appreciable extent even though Krypton and Xenon both become solid at the temperature of liquid nitrogen and could thus be captured in a cold trap if the flue gas of a voloxidation process were cooled by liquid nitrogen.
Increase of fission gases above a certain limit can lead to fuel pin swelling and even puncture, so that fission gas measurement after discharging the fuel from the reactor is most important to make burn-up calculations, to study the nature of fuel inside the reactor, behaviour with pin materials, for effective utilization of fuel and also reactor safety. In addition to that, they are a nuisance in a nuclear reactor due to being neutron poisons, albeit not to the same extent as isotopes of xenon, another noble gas produced by fission.