Fresh concerns surfaced for Tokyo Electric Power Co. (TEPCO), the embattled owner of tsunami-hit Fukushima power plant, on Tuesday. TEPCO, which is struggling to bring reactors at the plant to cold shutdown by the end of the year, detected substances from a nuclide analysis of gas emitted from Daiichi 2 that showed a fission reaction had occurred.

TEPCO said it had possibly found short-half-life radionuclides Xenon 133 and Xenon 135, by-products of fission, as a result of analyses of samples taken by the gas management system of the reactor containment vessel at Daiichi 2. Xenon 133 has a short half-life of five days, and Xenon 135, just nine hours.

Although “no significant” changes of temperature and pressure in the reactor had been found. the plant operator said it had injected a boric-acid solution—an agent that helps prevent reactions— through the reactor water injection line for an hour today, “just to be safe.”

TEPCO took the measure because, it said, “it was undeniable that fission reaction had occurred.” It was too soon to say whether the finding represented a major concern.

"It can be assumed that isolated criticality took place for a short period of time judging from the presence of xenon," Reuters quoted TEPCO spokesman Junichi Matsumoto as saying today.

Sources: POWERnews, TEPCO, Reuters