Updates with confirmation of xenon in second paragraph.)

Nov. 2 (Bloomberg) -- Tokyo Electric Power Co. detected signs of nuclear fission at its crippled Fukushima atomic power plant, raising the risk of increased radiation emissions. No increase in radiation was found at the site and the situation is under control, officials said.

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, according to an e-mailed statement today. The detection of xenon, which is associated with nuclear fission, was confirmed today by the Japan Atomic Energy Agency, the country's atomic regulator said.

“Given the signs, it's certain that fission is occurring,” Junichi Matsumoto, a general manager at Tepco who regularly talks to the media, told reporters in Tokyo today. There's been no large-scale or sustained criticality and no increase in radiation, he said.

Fission taking place in the reactor can lead to increases in radiation emissions and raises concerns about further leaks after another radioactive hot spot was discovered in Tokyo on Oct. 29. It's possible there are similar reactions occurring in the No. 1 and No. 3 reactors, the other cores damaged at the station, Matsumoto said.

“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.”

Loss of Cooling

Trade and Industry Minister Yukio Edano delivered a warning to Hiroyuki Fukano, the head of the Nuclear and Industrial Safety Agency, because the information on the discovery of xenon wasn't passed to the prime minister's office in a timely manner, Chief Cabinet Secretary Osamu Fujimura told reporters today.

Shares of Tepco declined 2.6 percent to close at 302 yen on the Tokyo Stock Exchange. They've fallen 86 percent since the disaster. The benchmark Nikkei 225 Stock Average was down 2.2 percent.

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, Tepco is trying to prevent further leakage of radiation that has spread across the world.

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.

Evaluating Reactions

“We are evaluating whether there are many reactions or not or whether its stopped,” Matsumoto said. The incident won't affect its schedule of bringing the plant under control by the end of this year, Matsumoto said.

Yasuhiro Sonoda, a member of the ruling Democratic Party of Japan, on Oct. 31 drank a glass of filtered water from the Fukushima plant to demonstrate the situation is being brought under control.

Sonoda denied reporters' claims it was a publicity stunt. “I drank it because there shouldn't be any concerns about the water,” he said. “I didn't intend to say it's completely safe by drinking it.”

No significant changes in temperatures and pressures of the reactor and radiation levels at the site have been detected, said Hiroyuki Usami, a spokesman for Tepco.

Reactor Readings

The temperature of the bottom of the No. 2 reactor pressure vessel was 76 degrees Celsius (167 Fahrenheit) at 5 a.m. today, compared with 77.4 degrees a day earlier and 77.5 degrees two days ago, according to Tepco's data. Radiation levels taken near the west gate of the plant have been stable at about 11 microsieverts per hour for the past few days, the data shows

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.

Fissioning involves the splitting of atoms, which, in the case of certain uranium isotopes, can lead to an uncontrolled reaction and emittance of radiation.

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.

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.

Damaged Reactors

Fukushima sustained major damage at four of its six reactor buildings at the Dai-Ichi plant, including the 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. The

Fukushima station may have emitted more than twice the amount of radiation than estimated by the Japanese government at the height of the Fukushima accident, according to another study by the Atmospheric Chemistry and Physics journal.

Tepco has declined to comment on both studies.

--With assistance from Taku Kato, Sachiko Sakamaki and Takehiko Kumakura in Tokyo. Editor: Aaron Sheldrick

Nov. 2, 2011

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