(CNN) -- A rare type of radioactive decay, not a renewed chain reaction, appears to have produced the radioactive xenon gas discovered this week at the Fukushima Daiichi nuclear power plant, its owner said Thursday.
Engineers from the Tokyo Electric Power Company discovered two short-lived isotopes of xenon on Tuesday inside the plant's No. 2 reactor, one of three that melted down following Japan's historic earthquake March 11. Their presence raised fears that a new chain reaction could have begun within the damaged fuel now believed to be piled up at the bottom of the reactor vessel.
Tokyo Electric responded by injecting boric acid into the reactor's coolant water to stop any renewed chain reaction that might prolong the nearly 8-month-old crisis at the plant, according to the company and the International Atomic Energy Agency. But the company announced Thursday that temperature and pressure readings from Unit 2 showed no change that would indicate an ongoing reaction.
One of the isotopes that triggered the concern, xenon-135, has a radioactive half-life of a little over nine hours; the other, xenon-133, loses half its radioactivity in just over five days. Tokyo Electric said Thursday it believed the gases were produced by "spontaneous fission" of uranium, since the shorter-lived isotope persisted after the use of boric acid.
Gary Was, a nuclear engineering professor at the University of Michigan, said spontaneous fission occurs when a heavy radioactive element like uranium splits on its own. It's an improbable phenomenon, he said -- but if it happens, "you'll see xenon fission products."
Was said the discovery came less than a week after Japan began taking new gas samples from the reactors.
"They need to keep monitoring this and keep checking it and make sure there are no changes in this xenon level, and do those calculations to make sure those xenon levels are consistent with spontaneous fission," he said.
Tokyo Electric has faced periodic questions about the possibility of a renewed chain reaction at Fukushima Daiichi, where meltdowns occurred in all three operating reactors after the earthquake and resulting tsunami knocked out the plant's cooling systems. More than 80,000 people remain displaced after the worst nuclear accident since Chernobyl.
A revived chain reaction would be a blow to efforts to bring the crisis to an end, producing more heat and radioactive waste.
The company has repeatedly said that it has no evidence of a renewed chain reaction, known as "recriticality," and Was said the damaged fuel is unlikely to produce such an event.
"It's believed to be very unlikely, if you have a fuel melt in the bottom of the vessel, that you could sustain fission," he said. "You've got the wrong geometry to sustain a chain reaction."