Radioactive xenon has been detected at the Fukushima nuclear plant, indicating that nuclear reactions are still occurring and that the worst and totally unreported nuclear disaster is going to haunt Japan for a long time.
BusinessWeek notes that the Japanese government has confirmed the existence of radioactive xenon:
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.
NHK reports:
The operator of the crippled Fukushima Daiichi nuclear
power plant says it found in the facility’s No.2 reactor radioactive
substances that could have resulted from continuous nuclear fission.
The Tokyo Electric Power Company, or TEPCO, said on Wednesday that it
detected xenon-133 and xenon-135 in gas taken from the reactor’s
containment vessel on the previous day. The substances were reportedly
in concentrations of 6 to more than 10 parts per million becquerels per
cubic centimeter.
Xenon-135 was also detected in gas samples collected on Wednesday.
Radioactive xenon is produced during nuclear fission.
The half-life of xenon-133 is 5 days, and that of xenon-135 is 9 hours.
***
The utility also says it wants to take a close look at the situation of the plant’s No.1 and 3 reactors.
Bloomberg writes:
“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.
***
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.”
The New York Times points out:
The unexpected bursts — something akin to flare-ups after
a major fire … threaten to increase the amount of dangerous
radioactive elements leaking from the complex and complicate cleanup
efforts, raising startling questions about how much remains uncertain at the plant….
The plant’s owner admitted for the first time that fuel deep inside three stricken plants was probably continuing to experience bursts of fission.
***
It is impossible to determine exactly what state the fuel is in,
given that even an intact reactor can offer only limited gauges in the
form of temperature, pressure readings and neutron flow, but not visual
observation. That lack of clarity is one of the most resonant lessons
of the Fukushima disaster, where those trying to guide the response and
assess the danger operated by what amounted to educated guesswork.
In reactors of the design used at Fukushima, that chain reaction is
normally stopped when the operator gives a command to insert control
rods, which rise up from the bottom of the core and separate the fuel
assemblies. But when the cores of three reactors at Fukushima melted, a
large part of the fuel presumably formed a jumbled mass in the bottom
of the vessel, and without a strict gridlike geometry, the control rods
cannot be inserted. Some of the fuel has escaped the vessel,
experts believe, and is in spaces underneath, where there is no way to
use control rods to interrupt the flow of neutrons.
***
The three reactors — together with spent fuel rods stored at a fourth
damaged reactor — have been leaking radioactive material since the
initial disaster, and new episodes of fission would only increase their dangers.
“Re-criticality would produce more harmful radioactive
material, and because the reactors are damaged, there would be a danger
of a leak,” said Hiroaki Koide, assistant professor at Kyoto
University’s Research Reactor Institute, whose prescient warnings about
nuclear safety have won him respect in Japan.
Mr. Koide holds that the nuclear fuel at the three reactors
probably melted through containments and into the ground, raising the
possibility of contaminated groundwater. If much of the fuel was
indeed in the ground early in the crisis, the “feed and bleed” strategy
initially taken by Tokyo Electric — where workers pumped cooling water
into the reactors, producing hundreds of tons of radioactive runoff —
would have prevented fuel still in the reactor from boiling itself dry
and melting, but would not have done anything to reduce danger from
fuel already in the soil — if it got that far.
***
Tokyo Electric does not deny the possibility that the fuel may have
burrowed into the ground, but its officials say that “most” of the fuel
likely remains within the reactor, albeit slumped at the bottom in a
molten mass.
But even in their most dire assessments, some experts had not
expected even bursts of re-criticality to occur, because it was unlikely
that the fuel would melt in just the right way — and that another
ingredient, water, would be present in just the right amounts — to allow
for any nuclear reaction. If episodes of fission at Fukushima
were confirmed, Mr. Koide said, “our entire understanding of nuclear
safety would be turned on its head.”
Some nuclear experts have debated for months whether nuclear
reactions might be continuing, either in the fuel inside the reactors,
or in the spent fuel pools at the plant. They have pointed, for
example, to the continued reports of short-lived iodine in the spent
fuel pool at Reactor No. 3.
A former nuclear engineer with three decades of experience at a major
engineering firm … who has worked at all three nuclear power complexes
operated by Tokyo Electric … said that tiny fuel pellets could have
been carried to different parts of the plant, like the spaces under the
reactor during attempts to vent them in the early days. That would
explain several cases of lethally high radiation readings found outside
the reactor cores.
“If the fuel is still inside the reactor core, that’s one thing,” he said. But if the fuel has been dispersed more widely, then we are far from any stable shutdown.”
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