Tokyo Electric Power Co. detected signs of possible nuclear fission at its crippled Fukushima atomic power plant in northern Japan, raising the risk of more radiation leaks.

The company, known as Tepco, began spraying boric acid on the No. 2 reactor at 2:48 a.m. Japan time to prevent accidental chain reactions. Tepco said it may have found xenon, which is associated with nuclear fission, while examining gases taken from the reactor, according to an e-mailed statement today.

“Melted fuel in the No. 2 reactor may have undergone a sustained process of nuclear fission or re-criticality,” Tetsuo Ito, the head of Kinki University’s Atomic Energy Research Institute, said by phone. “The nuclear fission should be containable by injecting boron into the reactor to absorb neutrons.”

The discovery comes almost eight months after the March 11 earthquake and tsunami wrecked the Fukushima Dai-Ichi plant, causing a loss of cooling and the meltdowns of three reactors. The incident, the worst atomic disaster since Chernobyl in 1986, was responsible for the biggest discharge of radioactive material into the ocean in history, according to a study from a French nuclear safety institute. 

Parameters unchanged

Shares of Tepco slid as much as 6.8 percent, the biggest intraday loss since Oct. 21, and traded 3.9 percent lower at 298 yen 11 a.m. on the Tokyo Stock Exchange. The benchmark Nikkei 225 Stock Average slid 1.9 percent.

The amount of detected xenon is “very small” and results of the utility’s analysis may be wrong, said Hiroyuki Usami, a spokesman for Tepco. No significant changes in temperatures and pressures of the reactor and radiation levels at the site have been detected, he said.

Should fissioning have occurred the injection of boron will have stopped it, said Tadashi Narabayashi, a former reactor safety researcher at Toshiba Corp. and now a nuclear engineering professor at Hokkaido University.

Tepco and the government have said they are on track to bring the damaged reactors into a safe state known as cold shutdown by the end of the year. The company has been mixing boron with cooling water in the reactors to prevent accidental chain reactions.

Reassessing findings

Tepco and the Japan Atomic Energy Agency are reexamining the gases, said Toshiyuki Koganeya, a spokesman for the Nuclear and Industrial Safety Agency, the government regulator. Even if Tepco’s analysis is correct, nuclear fission would be taking place in a “very restricted part” of the reactor, said Koganeya. The regulator believes fuel accumulated at the bottom of the pressure vessel and containment vessel is unlikely to start melting again, he said.

Fukushima has sustained major damaged at four of its six reactor buildings at the Dai-Ichi plant, including three core meltdowns and possible damage to a spent fuel pool.

The radioactive cesium that flowed into the sea from the plant was 20 times the amount estimated by Tepco, according to the Institute for Radiological Protection and Nuclear Safety, which is funded by the French government.

The oceanic study estimates 27,000 terabecquerels of radioactive cesium 137 leaked into the sea from the plant. 

(Bloomberg)