Insight:
After disaster, the deadliest part of Japan's nuclear clean-up
The
operator of Japan's crippled Fukushima nuclear plant is preparing to
remove 400 tons of highly irradiated spent fuel from a damaged
reactor building, a dangerous operation that has never been attempted
before on this scale.
14
August, 2013
Containing
radiation equivalent to 14,000 times the amount released in the
atomic bomb attack on Hiroshima 68 years ago, more than 1,300 used
fuel rod assemblies packed tightly together need to be removed from a
building that is vulnerable to collapse, should another large
earthquake hit the area.
Tokyo
Electric Power Co (Tepco) is already in a losing battle to stop
radioactive water overflowing from another part of the facility, and
experts question whether it will be able to pull off the removal of
all the assemblies successfully.
"They
are going to have difficulty in removing a significant number of the
rods," said Arnie Gundersen, a veteran U.S. nuclear engineer and
director of Fairewinds Energy Education, who used to build fuel
assemblies.
The
operation, beginning this November at the plant's Reactor No. 4, is
fraught with danger, including the possibility of a large release of
radiation if a fuel assembly breaks, gets stuck or gets too close to
an adjacent bundle, said Gundersen and other nuclear experts.
That
could lead to a worse disaster than the March 2011 nuclear crisis at
the Fukushima plant, the world's most serious since Chernobyl in
1986.
No
one knows how bad it can get, but independent consultants Mycle
Schneider and Antony Froggatt said recently in their World Nuclear
Industry Status Report 2013: "Full release from the Unit-4 spent
fuel pool, without any containment or control, could cause by far the
most serious radiological disaster to date."
Tepco
has already removed two unused fuel assemblies from the pool in a
test operation last year, but these rods are less dangerous than the
spent bundles. Extracting spent fuel is a normal part of operations
at a nuclear plant, but safely plucking them from a badly damaged
reactor is unprecedented.
"To
jump to the conclusion that it is going to work just fine for the
rest of them is quite a leap of logic," said Gundersen.
The
utility says it recognizes the operation will be difficult but
believes it can carry it out safely.
Nonetheless,
Tepco inspires little confidence. Sharply criticized for failing to
protect the Fukushima plant against natural disasters, its handling
of the crisis since then has also been lambasted.
Last
week, Prime Minister Shinzo Abe ordered the government to take a more
active role in controlling the overflow of radioactive water being
flushed over the melted reactors in Units 1, 2 and 3 at the plant.
GIANT
FRAME
The
fuel assemblies are in the cooling pool of the No. 4 reactor, and
Tepco has erected a giant steel frame over the top of the building
after removing debris left behind by an explosion that rocked the
unit during the 2011 disaster.
The
structure will house the cranes that will carry out the delicate task
of extracting fuel assemblies that may be damaged by the quake, the
explosion or corrosion from salt water that was poured into the pool
when fresh supplies ran out during the crisis.
The
process will begin in November and Tepco expects to take about a year
removing the assemblies, spokesman Yoshikazu Nagai told Reuters by
e-mail. It's just one installment in the decommissioning process for
the plant forecast to take about 40 years and cost $11 billion.
Each
fuel rod assembly weighs about 300 kilograms (660 pounds) and is 4.5
meters (15 feet) long. There are 1,331 of the spent fuel assemblies
and a further 202 unused assemblies are also stored in the pool,
Nagai said.
Almost
550 assemblies had been removed from the reactor core just before the
quake and tsunami set off the crisis. These are the most dangerous
because they have only been cooling in the pool for two and a half
years.
"The
No. 4 unit was not operating at the time of the accident, so its fuel
had been moved to the pool from the reactor, and if you calculate the
amount of cesium 137 in the pool, the amount is equivalent to 14,000
Hiroshima atomic bombs," said Hiroaki Koide, assistant professor
at Kyoto University Research Reactor Institute.
Spent
fuel rods also contain plutonium, one of the most toxic substances in
the universe, that gets formed during the later stages of a reactor
core's operation.
INADVERTENT
CRITICALITY
"There
is a risk of an inadvertent criticality if the bundles are distorted
and get too close to each other," Gundersen said.
He
was referring to an atomic chain reaction that left unchecked could
result in a large release of radiation and heat that the fuel pool
cooling system isn't designed to absorb.
"The
problem with a fuel pool criticality is that you can't stop it. There
are no control rods to control it," Gundersen said. "The
spent fuel pool cooling system is designed only to remove decay heat,
not heat from an ongoing nuclear reaction."
The
rods are also vulnerable to fire should they be exposed to air,
Gundersen said.
The
fuel assemblies are situated in a 10 meter by 12 meter concrete pool,
the base of which is 18 meters above ground level. The fuel rods are
covered by 7 meters of water, Nagai said.
The
pool was exposed to the air after an explosion a few days after the
quake and tsunami blew off the roof. The cranes and equipment
normally used to extract used fuel from the reactor's core were also
destroyed.
Tepco
has shored up the building, which may have tilted and was bulging
after the explosion, a source of global concern that has been raised
in the U.S. Congress.
The
utility says the building can withstand shaking similar to the quake
in 2011 and carries out regular structural checks, but the company
has a credibility problem. Last month, it admitted that contaminated
water was leaking into the Pacific Ocean after months of denial.
The
fuel assemblies have to be first pulled from the racks they are
stored in, then inserted into a heavy steel chamber. This operation
takes place under water before the chamber, which shields the
radiation pulsating from the rods, can be removed from the pool and
lowered to ground level.
The
chamber is then transported to the plant's common storage pool in an
undamaged building where the assemblies will be stored.
Tepco
confirmed the Reactor No. 4 fuel pool contains debris during an
investigation into the chamber earlier this month.
Removing
the rods from the pool is a delicate task normally assisted by
computers, according to Toshio Kimura, a former Tepco technician, who
worked at Fukushima Daiichi for 11 years.
"Previously
it was a computer-controlled process that memorized the exact
locations of the rods down to the millimeter and now they don't have
that. It has to be done manually so there is a high risk that they
will drop and break one of the fuel rods," Kimura said.
Under
normal circumstances, the operation to remove all the fuel would take
about 100 days. Tepco initially planned to take two years before
reducing the schedule to one year in recognition of the urgency. But
that may be an optimistic estimate.
"I
think it'll probably be longer than they think and they're probably
going to run into some issues," said Murray Jennex, an associate
professor at San Diego State University who is an expert on nuclear
containment and worked at the San Onofre nuclear plant in California.
"I
don't know if anyone has looked into the experience of Chernobyl,
building a concrete sarcophagus, but they don't seem to last well
with all that contamination."
Corrosion
from the salt water will have also weakened the building and
equipment, he said.
And
if an another strong earthquake strikes before the fuel is fully
removed that topples the building or punctures the pool and allow the
water to drain, a spent fuel fire releasing more radiation than
during the initial disaster is possible, threatening about Tokyo 200
kilometers (125 miles) away.
When
asked what was the worst possible scenario, Tepco is planning for,
Nagai said: "We are now considering risks and countermeasures."
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