The fears behind a possible chain reaction centered on the discovery of xenon gas as a possible harbinger of further fuel melts at the reactors – three of which experienced full meltdowns after the plant was hit by a 9.0 magnitude earthquake and tsunami in March.

But both the plant’s owner, Tokyo Electric Power Co (TEPCO) and nuclear physicist Katsutada Aoki have said that the presence of xenon is not an indication that the wrecked fuel in the reactors has re-achieved “criticality” – or a sustained chain reaction of nuclear fission. 

“The discovery of xenon in the reactor is no reason to fear anything serious,” Aoki, an expert in nuclear engineering who headed the reactor physics division of the Atomic Energy Society of Japan, told the Japan Times.

TEPCO said this week that it considered the source of the xenon to be spontaneous fission because technicians had injected boric acid into the reactor vessel to reduce the likelihood of chain fission reactions but was still able to detect xenon.

Temperature and pressure data from the unit also showed no change around the time of the xenon’s discovery in another indication that chain reactions were not taking place.

TEPCO slammed for spreading fear

But Aoki criticized TEPCO, saying the utility could have done a better job analyzing the implications of the newly discovered xenon gases and avoided spreading needless fear that a nuclear chain reaction might have restarted.

In a confusing and worrying announcement, TEPCO revealed last Wednesday that it found one one-hundred-thousandth of a becquerel per 1 cubic centimeter of xenon-133 and xenon-135 in gas samples from reactor No. 2, saying it might indicate the melted fuel in the reactor could have briefly reached criticality since xenon can be generated through such nuclear fission.

But one day after the announcement, TEPCO denied criticality had occurred, saying it found the amount of xenon was too small to be generated through fission via criticality.

Chances of chain reaction not zero

Spontaneous fission should not be confused with nuclear criticality, said Aoki, especially since both the temperature and the pressure levels have remained stable in the reactor.

But he did say that “the chances of criticality taking place is not zero.”

This is what concerns some environmentalists who have closely been observing developments of the ongoing crisis at Fukushima Daiichi, especially given the sheer quantity of melted fuel at the plant.

“I don’t think there is a reason to say situation [with a possible chain reaction] has improved much actually,” wrote Vladimir Slivyak, co-chair of Russia’s Ecodefence in an email interview. 

“We see the possibility of chain reaction is still there and no one can actually guarantee that no problems will again occur at Fukushima.”

An uncontrolled chain reaction occurring in the bowels of one of Fukushima Daiichi’s wrecked reactors could lead to an explosion and yet further spread of radioactivity.

One independent report released in Norway this month indicated that releases of radioactive caesium-137 from Fukushima Daiichi equal 40 percent of that which was released at Chernobyl in 1986.

Another report by the French Institute for Radiological Protection and Nuclear Safety also stated that the amount of caesium-137 that flowed into the Pacific from the coastal plant is some 30 times more than was estimated by TEPCO.