"Officials of Tokyo Electric Power Co. (TEPCO) pumped water mixed with boric acid into the No. 2 reactor at the Fukushima No. 1 nuclear power plant early on Nov. 2 after finding traces of xenon, a radioactive gas that might indicate nuclear fission has taken place.
Officials of the company said that some parts of the reactor may have reached criticality, a state of self-sustaining nuclear fission. Fuel believed to have melted in the accident triggered by the March 11 earthquake may have caused the fission. The boric acid was pumped into the reactor to suppress the reactions.
Radiation levels near the Fukushima plant have not shown any unusual surge, but the latest development may hinder plans by the central government to bring the Fukushima nuclear accident under control by the end of the year." Asahi Shimbun