TEPCO says melted nuke fuel may have reached criticality

http://ajw.asahi.com/article/0311disaster/fukushima/AJ2011110216361

UPDATE: TEPCO says melted nuke fuel may have reached criticality

Boric acid injected into Fukushima reactor due to fission concerns
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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Susan Gordon

Alliance for Nuclear Accountability

903 W Alameda Street, #505

Santa Fe, NM 87501

505-473-1670

sgordon@ananuclear.org

http://www.ananuclear.org