Tokyo Electric Power Co. (Tepco) detected signs of nuclear fission at its crippled Fukushima atomic power plant, raising the risk of increased radiation emissions, however, after tests officials did not find an increase in radiation.

Tepco began spraying boric acid on the No. 2 reactor at 2:48 a.m. Japan time to prevent accidental chain reactions, officials said. Officials did detect xenon, associated with nuclear fission, said the Japan Atomic Energy Agency, the country’s atomic regulator.

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“Given the signs, it’s certain that fission is occurring,” said Junichi Matsumoto, a general manager at Tepco. There has 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 officials discovered another radioactive hot spot 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.

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

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 did not pass on to the prime minister’s office in a timely manner, said Chief Cabinet Secretary Osamu Fujimura.

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.

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 showed.

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.

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, 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.

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