TEPCO: Reactor may have gone critical; Xenon-135 Detected at Fukushima

The Tokyo Electric Power Company, or TEPCO, said on Wednesday that it detected xenon-133 and xenon-135 in gas taken from the reactor's containment vessel on the previous day. The substances were reportedly in concentrations of 6 to more than 10 parts per million becquerels per cubic centimeter.

Xenon-135 was also detected in gas samples collected on Wednesday.

Radioactive xenon is produced during nuclear fission.
The half-life of xenon-133 is 5 days, and that of xenon-135 is 9 hours.

TEPCO says the findings suggest that nuclear fission may have occurred recently, not just after the March 11th accident, and that a state of criticality could have occurred temporarily in some areas.

TEPCO workers poured a boric acid solution into the reactor on Wednesday to suppress nuclear fission.

The utility says it has not found any significant change in temperature and pressure of the reactor, and that large-scale criticality did not occur.

TEPCO says the reactor's cooling process is continuing and that the firm expects to achieve cold shutdown at the plant this year as planned. But the utility also says it wants to take a close look at the situation of the plant's No.1 and 3 reactors.