– Intermittent increase of Krypton-85 to suggest on-going nuclear fission in reactor1 (Fukushima Diary, Sep 6, 2012):
Intermittent increase of Krypton-85 was observed in reactor1.
Tepco suddenly reported they examined the density of krypton-85 and hydrogen gas. This was the first time for them to announce Krypton-85 was increasing in reactor1. [Link]
– At 10:30 AM on September 4, in order to examine the case of the intermittent increase in hydrogen density and noble gas (Krypton-85) density measured by Unit 1 RCV gas control system, we injected the nitrogen to the upper part of the pressure suppression chamber in which hydrogen is supposed to be accumulated from the nitrogen injection line laid on the 1st floor of reactor building, and pushed out the accumulating hydrogen and the Krypton-85. We accordingly started confirming the presence of hydrogen and Krypton-85 at the upper part of the pressure suppression chamber utilizing the gas control system. At 4:37 PM on the same day, we stopped the nitrogen injection to the upper part of the pressure suppression chamber. The hydrogen density of Unit 1 RCV was 0.54% as a result of this work (as of 11:00 AM on September 5). Since it is below the flammability limit (4%), there is no problem.
Their Japanese version of report states the density of Krypton-85 was 948Bq/cm at maximum.[Link]
Krypton-85 is produced from nuclear fission most of the cases. It suggests the possibility of on-going nuclear fission in reactor1.
Wikipedia reads like this below
Krypton 85 (85Kr) is a radioisotope of krypton.
It decays into stable rubidium-85, with a half-life of 10.756 years and a maximum decay energy of 0.687 MeV. Its most common decay (99.57%) is by beta particle emission with maximum energy of 687 keV and an average energy of 251 keV. The second most common (0.43%) is by beta particle emission (maximum energy of 173 keV) followed by gamma ray emission (energy of 514 keV).[1][2]
In terms of radiotoxicity, 440 Bq of Kr-85 is equivalent to 1 Bq of radon-222, without considering the rest of the radon decay chain.
Other decay modes have very small probabilities and emit less energetic gammas.[3] The only other long-lived radioisotope of krypton is krypton-81 with a 210,000 year half-life; others have half-lives of less than two days.
Krypton-85 is produced in small quantities by the interaction of cosmic rays with the stable krypton-84 (which is present in concentrations of about 1 cm3 per cubic meter). However, since the mid-1940s, much larger quantities have been artificially produced as a product of nuclear fission. When uranium-235, or another fissile nucleus fissions, it usually splits into two large fragments (fission products) with mass numbers around 90-140, and two or three neutrons. About three atoms of krypton-85 are produced for every 1000 fissions (i.e. it has a fission yield of 0.3%).[4] This is only about 20% of the total fission product of mass 85, as most decay from a short-lived excited state of 85Kr directly to 85Rb without passing through the longer-lived nuclear isomer