Small traces of radioactive gas associated with nuclear fission have been detected at the troubled Fukushima Dai-ichi Power Plant, reigniting concerns about the stability of its reactors.

 Xenon 133 and Xenon 135, both substances that are produced during nuclear fission of uranium, were first detected inside the containment vessel of reactor number 2 Tuesday. The Tokyo Electric Power Company, or TEPCO, began injecting water and boric acid overnight to prevent a chain reaction.

TEPCO maintains there has been no change to the reactor’s temperature, pressure or radiation levels.

Japan’s nuclear watchdog said another meltdown was unlikely, considering the low density of xenon, and unchanged temperatures at the reactor.

“We have confirmed that the reactor is stable and we don’t believe this will have any impact on our future work,”  TEPCO spokesman Osamu Yokokura told the Associated Press.

The Nuclear and Industrial Safety Agency said the small amounts of radioactive gas released from reactor 2 this time, did not pose a public health risk.

The setback comes nearly eight months after a massive earthquake and tsunami triggered Japan’s worst nuclear disaster. Three Fukushima reactors suffered a core meltdown in March, spewing large doses of radiation within a 12-mile radius, and displacing more than 80,000 residents.

The short half life of xenon 133 (5 days) and xenon 135 (9 hours) indicate the nuclear fission occurred recently, but TEPCO dismissed the possibility of a “major criticality incident” with a sustained nuclear reaction.

The recent activity raises questions about the reactors’ stability, and the utility’s ability to reach a state of cold shutdown by the end of the year, its stated goal. On Tuesday, nuclear crisis Minister Goshi Hosono announced he would allow media into the Fukushima plant for the first time since March 11, saying the reactors had stabilized.

A government panel has said it will take more than 30 years to safely close the plant.