Japan's nuclear agency has confirmed that sustained nuclear fission did not take place at the Fukushima nuclear power plant last week.
The Nuclear and Industrial Safety Agency disclosed the results of experts' studies on a report by Tokyo Electric Power Company, or TEPCO.
The utility detected a small amount of the radioactive material, xenon-135, in the reactor's containment vessel of the damaged No.2 reactor on Tuesday of last week.
TEPCO initially feared it may signal an ongoing nuclear reaction. But it determined that the substance was produced through spontaneous fission, a form of radioactive decay, and not from sustained fission or criticality.
The nuclear agency said the density of the xenon, which did not change when a boric acid solution was injected into the reactor, proved that criticality did not occur.
The agency ordered TEPCO to regularly check the density of nuclear substances inside the vessels and to report any changes. more