Timeline of the Fukushima Daiichi nuclear disaster of caesium-137 by the Atmospheric Chemistry and Physics journal—an estimate about 42 percent of that released into the atmosphere in the Chernobyl explosion in 1986. Caesium-137 has a half-life of 30 years. TEPCO reports the completion of a cover for the reactor unit 1, which is 54 metres high, 47 metres wide and 42 metres deep. The cover has a built-in ventilation system that is supposed to filter radioactive material. Boric acid is injected into reactor number 2 after the discovery of xenon in its containment vessel. The presence of xenon may be an indication that a self-sustained fission reaction
Radiation effects from the Fukushima Daiichi nuclear disaster was reported to have reduced to 79 Bq/kg and to be safe for infants in Tokyo and Chiba but still exceeded limits in Hitachi and Tokaimura. On 27 April, "radiation in Tokyo's water supply fell to undetectable levels for the first time since 18 March." The following graphs show Iodine-131 water contaminations measured in water purifying plants From 16 March to 7 April: On 2 July samples of tapwater taken in Tokyo Shinjuku ward radioactive caesium-137 was detected for the first time since April. The concentration was 0.14 becquerel per kilogram and none was discovered yesterday, which compares with 0.21