UPDATE: TEPCO says melted nuke fuel may have reached criticality

November 02, 2011

Officials of Tokyo Electric Power Co. (TEPCO) pumped water mixed with boric acid into the No. 2 reactor at the Fukushima No. 1 nuclear power plant early on Nov. 2 after finding traces of xenon, a radioactive gas that might indicate nuclear fission has taken place.

Officials of the company said that some parts of the reactor may have reached criticality, a state of self-sustaining nuclear fission. Fuel believed to have melted in the accident triggered by the March 11 earthquake may have caused the fission. The boric acid was pumped into the reactor to suppress the reactions.

Radiation levels near the Fukushima plant have not shown any unusual surge, but the latest development may hinder plans by the central government to bring the Fukushima nuclear accident under control by the end of the year.

TEPCO workers began operating a gas control system at the No. 2 reactor from Oct. 28 to purify gases within the containment vessel in an attempt to reduce the leaking of radioactive material.

TEPCO officials said a test on Nov. 1 of the gases processed found traces of what appeared to be radioactive xenon 133 and xenon 135.

The identification of the gas as xenon is now being checked by another research institute. Radioactive xenon is produced when uranium in fuel in a reactor core undergoes nuclear fission.

Xenon 133 has a half-life of about five days while xenon 135 has a half-life of about nine hours.

If the gas found in the reactor was xenon, it would mean nuclear fission is continuing in the reactor.

Because of such results, TEPCO officials mixed boric acid into water pumped into the reactor for cooling purposes from 2:48 a.m. on Nov. 2.

However, no radioactive iodine, which is also produced during nuclear fission with xenon, was detected in the gas checked at the No. 2 reactor.

Moreover, neutron rays, which are emitted during nuclear fission, were not detected in the vicinity of the reactor building.

"We cannot deny the possibility of a temporary, small-scale state of criticality," said TEPCO official Junichi Matsumoto. "However, because there was no sudden increase in the temperature or pressure of the reactor core, we concluded there was no major state of criticality."

Large scale criticality will not usually occur unless, as is found in a normal reactor core, nuclear fuel is carefully positioned and surrounded by water to stimulate nuclear fission.

Through a constant pumping of water into the cores of the No. 1 to No. 3 reactors, temperatures at all three reactors had fallen under 100 degrees by late September, six months after the nuclear accident, indicating stable cooling of the reactors.

For that reason, central government and TEPCO officials had thought they were on course to bring the situation at the Fukushima plant under control by the end of the year.

The temperature at the bottom of the pressure vessel of the No. 2 reactor was 76 degrees as of 5 a.m. on Nov. 2.

During the measurement of gases by the gas control system, the concentration of hydrogen rose from 1 percent to 2.7 percent on Oct. 30. Hydrogen is produced when water is broken down by radiation from the nuclear fuel, and it is possible that the increase in hydrogen may have been related to nuclear fission in the reactor.

Additional nitrogen was pumped into the reactor to prevent an explosion of the sort triggered at the Fukushima plant in March.

TEPCO officials said it was the first time xenon had been detected. However, that may be due to the failure to take appropriate measurements. It is possible that nuclear fission has been producing xenon in the No. 2 reactor since the accident and that similar reactions are occurring in the No. 1 and No. 3 reactors, where nuclear fuel has also melted.

Officials of the Nuclear and Industrial Safety Agency (NISA) held a news conference on the morning of Nov. 2.

"While we cannot deny that nuclear fission may have occurred in some places, (the No. 2 reactor) is, overall, in a stable condition," said Yoshinori Moriyama, NISA deputy director-general for nuclear accident measures.

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Graphic by The Asahi Shimbun

Graphic by The Asahi Shimbun

  • Graphic by The Asahi Shimbun
  • Steam rises from the No. 2 reactor building at Fukushima No. 1 nuclear power plant in this photo taken on March 24. A nuclear fission reaction appeared to have occurred at the reactor on Nov. 2. (Provided by Air Photo Service)

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