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.
"We cannot deny the possibility of a temporary, small-scale state of criticality," said TEPCO official Junichi Matsumoto. "However, because there was no sudden increase in the temperature or pressure of the reactor core, we concluded there was no major state of criticality."
Large scale criticality will not usually occur unless, as is found in a normal reactor core, nuclear fuel is carefully positioned and surrounded by water to stimulate nuclear fission.