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 Nobody knows exactly how much fuel melted after the March 2011
earthquake and tsunami knocked out cooling systems. Or where exactly
the fuel went — how deep and in what form it is, somewhere at the
bottom of reactor Units 1, 2 and 3.
The complexity and magnitude of decommissioning the Fukushima
Dai-ichi plant is more challenging than Three Mile Island or
Chernobyl, say experts such as Lake Barrett, a former U.S. regulator
who directed the Three Mile Island cleanup and now is an outside
adviser to Fukushima operator Tokyo Electric Power Co.
One core melted at Three Mile Island in 1979, versus three at
Fukushima, and it didn't leak out of the containment chamber, the
outer vessel that houses the reactor core. At Fukushima, multiple
hydrogen explosions caused extensive damage, blowing the roofs off
three reactor buildings and spewing radiation over a wide area.
Chernobyl was a worse accident in terms of radiation emitted, but
authorities chose an easier solution: entombing the facility in
cement.
At Fukushima, TEPCO plans a multi-step process that is expected to
take 40 years: Painstakingly removing the fuel rods in storage
pools, finding and extracting the melted fuel within the broken
reactors, demolishing the buildings and decontaminating the soil.
"This is a much more challenging job," Barrett said during a recent
visit to Japan. "Much more complex, more difficult to do."
Also, water must continuously be channeled into the pools and
reactor cores to keep the fuel cool. Tons of contaminated water
leaks out of the reactors into their basements, some of it into the
ground.
Uncertainty runs high as Japan has never decommissioned a full-size
commercial reactor, even one that hasn't had an accident. TEPCO has
earmarked about 1 trillion yen ($10 billion) for the
decommissioning, and says it will agree to Prime Minister Shinzo
Abe's request to set aside another 1 trillion yen to fight water
leaks.
The government itself has contributed or promised 145 billion yen,
and is expected to step up its involvement in the years to come,
following criticism over its lack of support and growing concern
that the technical and funding challenges are beyond TEPCO's
capabilities.
TEPCO began removing fuel rods Monday from a storage pool at Unit 4,
whose building was severely damaged but didn't have a meltdown
because the fuel had been removed from the core for maintenance. In
an underwater operation, 22 of the 1,533 sets of fuel rods in a pool
on the building's top floor were transferred to a cask that will be
used to move them to safer storage. By 2018, the utility hopes to
remove all 3,100 fuel assemblies from storage pools at the four
damaged units.
After that would come the real challenge: removing melted or
partially melted fuel from the three reactors that had meltdowns,
and figuring out how to treat and store it so it won't heat up and
start a nuclear reaction again.
"This is an unprecedented task that nobody in the world has
achieved. We still face challenges that must be overcome," said
Hajimu Yamana, a Kyoto University nuclear engineer who heads a
government-affiliated agency that is overseeing technological
research and development for the cleanup.
Closing the holes and cracks in the containment vessels is the
biggest hurdle in the decommissioning process, experts say. Every
opening must be found and sealed to establish a closed cooling
system. Then, under the current plan, the next step would be to fill
the reactor vessels with water and examine the melted fuel.
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Because of still fatally high radiation levels, the work will have
to rely on remote-controlled robots for years. Scientists are
developing robots to spot leaks, monitor radiation levels and carry
out decontamination. They are also developing robots that can detect
holes and fill them with clay.
Among them is a camera-loaded swimming robot that can go underwater
to spot holes and cracks, and another one that can go into ducts and
pipes.
Computer simulations show the melted fuel in Unit 1, whose core
damage was the most extensive, has breached the bottom of the
primary containment vessel and even partially eaten into its
concrete foundation, coming within about 30 centimeters (one foot)
of leaking into the ground.
"We just can't be sure until we actually see the inside of the
reactors," Yamana said. "We still need to develop a number of robots
and other technology."
Three Mile Island needed only a few robots, mainly for
remote-controlled monitoring, sampling and handling debris, as the
melted fuel remained in the core. Manned entry was possible a little
more than a year after the accident.
Some experts say Japan's current decommissioning plan is too
ambitious. They counsel waiting until contamination levels come
down, and even contemplate building a shell around the reactors for
the time being, as at Chernobyl.
"I doubt if Fukushima Dai-ichi's full decommissioning is possible.
Its contamination is so widespread," said Masashi Goto, a nuclear
engineer who designed the Unit 3 reactor and now teaches at Meiji
University in Tokyo. "We should not rush the process, because it
means more exposure to workers. Instead, we should wait and perhaps
even keep it in a cement enclosure."
Others say the Chernobyl solution wouldn't be effective, noting that
the reactor was a different type without massive water leaks.
Developing expertise during the operation is also important to
Japan, which has dozens of reactors that face eventual retirement
and is considering turning decommissioning into a viable business at
home, and possibly in a growing global market.
"If you just put concrete over this, groundwater still will be
flowing and things like that, and you have an uncontrolled
situation," Barrett said. "I just don't see that as a plausible
option."
Only a small test reactor had been successfully scrapped in Japan,
with five others now being decommissioned — two experimental and
three commercial. The furthest along is Tokai Power Station's No. 1
reactor, which is 15 years into a planned 22-year process.
Japan also has to worry about future natural disasters.
"There will be many more earthquakes and typhoons," Goto said. "I
hope these plans won't fail, but we might just have to pray."
[Associated
Press; MARI YAMAGUCHI]
Copyright 2013 The Associated
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