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On 3/15/11 10:24 PM, DW wrote:
First, the situation is clearly (but slowly) stabilising. As each day
passes, the amount of thermal heat (caused by radioactive decay of the
fission products) that remains in the reactor fuel assemblies
decreases exponentially.
http://www.washingtonpost.com/national/latest-nuclear-plant-explosion-in-japan-raises-radiation-fears/2011/03/15/ABwTmha_story.html
Latest nuclear plant explosion in Japan raises radiation fears
By Brian Vastag, Tuesday, March 15, 10:12 PM
New assessments of the explosion at Unit 2 of Japan’s stricken Fukushima
Daiichi nuclear plant Tuesday heightened fears that it will begin
spewing large amounts of radiation.
The explosion probably damaged the main protective shield around the
uranium-filled core inside one of the plant’s six reactors. Such a
breach would be the first at a nuclear power plant since the Chernobyl
catastrophe in the Soviet Union 25 years ago.
The latest explosion — compounded by a fire in a different unit
Wednesday morning — marked yet another setback in the five-day battle to
stabilize the Daiichi facility, which suffered heavy damage to its
cooling systems after Friday’s earthquake and tsunami. Other explosions
occurred earlier at two of the plant’s reactors.
The blast Tuesday t Unit 2 was not outwardly visible, but potentially
more dangerous because it may have created an escape route for
radioactive material bottled up inside the thick steel-and-concrete
reactor tube. Radiation-laced steam is probably building up between that
tube and the building that houses it, experts said, triggering fears
that the pressure would blow apart the structure, emitting radiation
from the core.
“They’re putting water into the core and generating steam, and that
steam has to go somewhere,” said Arnie Gunderson, a nuclear engineer
with 40 years of experience overseeing the Vermont Yankee nuclear
facility, whose reactors are of the same vintage and design as those at
the Fukushima Daiichi plant. “It has to be carrying radiation.”
Nuclear experts have repeatedly stressed that radiation releases on the
scale of Chernobyl are unlikely or even impossible, given the Japanese
plant’s heavier engineering and additional layers of containment.
Still, Tokyo Electric Power Co., which operates the Daiichi facility,
said radiation briefly rose to dangerous levels at the plant Tuesday
morning.
On Wednesday morning, the Japanese government raised the permitted
radiation exposure for plant workers by 2.5 times to allow them to work
longer, according to NHK TV.
Crews noted a drop in pressure inside the reactor and also within a
doughnut-shaped structure below, called a suppression pool. The
simultaneous loss of pressure in those two places indicates serious
damage, nuclear experts said.
The explosion probably happened after the streams of seawater that crews
have been pumping into the reactor faltered. The fuel rods were left
completely exposed to the air for some time, Tepco said in a statement.
Without water, the rods grew white-hot and possibly melted through the
steel-and-concrete tube.
Tepco said a skeleton crew of 50 to 70 employees — far fewer than the
1,400 or more at the plant during normal operations — were working in
shifts to keep seawater flowing to the three reactors now in trouble.
The removal of most of the plant’s workers “is a sign to me that they
have given up trying to prevent a disaster and gone into the mode of
trying to clean up afterward,” Gunderson said.
Also on Tuesday, and again on Wednesday morning, fires temporarily
flared up in Unit 4, causing fear that spent uranium fuel sitting in a
pool above the reactor was burning. Such a conflagration would generate
intense concentrations of cesium-137 and other dangerous radioactive
isotopes.
A spokesperson for the Nuclear Energy Institute, an industry lobbying
group, said that Tepco concluded that the first fire in Unit 4 was not
in the spent fuel pool, “but rather in a corner of the reactor
building’s fourth floor.”
The company briefly considered spraying water into holes in the Unit 4
building — caused by the previous explosions at the site — with
helicopters. The company abandoned that plan, but still may use fire
trucks to shoot water into the building.
Such a measure would be a last-ditch effort to prevent the spent fuel
from burning and to keep cesium-137 and other radioactive isotopes from
being released into the air.
“This is scary,” said Lake Barrett, a nuclear engineer who directed the
cleanup of the Three Mile Island nuclear facility in Pennsylvania. “The
plans in a severe accident are to just get a fire hose in there, get any
kind of water to keep water in the pool above the fuel. ”
With the outer containment building at Unit 2 primed for a possible
explosion, any fire crews would be in grave peril.
During normal plant operations, uranium fuel rods that can no longer
produce enough heat for generating electricity are periodically removed
from a reactor and placed into the spent fuel pools above the reactors.
These rods continue to generate heat and radioactive isotopes for many
years.
Keeping this material covered with water is sufficient to cool it. But
water levels may have dropped dramatically during the crisis, exposing
fuel rods to the air.
Robert Alvarez, an analyst at the Institute for Policy Studies who has
long warned of the dangers of spent fuel pools, said that — unlike the
reactors themselves — the fuel pools typically do not have backup pumps
to maintain water flow. “They were so overwhelmed,” he said of the
workers straining to contain the disaster, that they were unable to
maintain enough water in the pool to prevent boiling.
If the fuel pools are exposed to the air, the radiation doses coming
from them could be life-threatening up to 50 yards, Alvarez said.
Concerns about the dangers of storing used uranium fuel in relatively
poorly shielded pools above reactors increased with the fear of
terrorism after the Sept. 11, 2001, attacks, causing industry experts to
dispute the design. In 2006, the National Research Council issued a
report saying in part that a uranium fuel fire “could result in the
release of large amounts of radioactive material.”
The NRC report also recommended that nuclear power plants build
“redundant and diverse” coolant systems to keep the fuel underwater
during a crisis.
Late Tuesday, Tepco said water levels were “low” in Unit 4’s used fuel
pool. Japanese officials said Wednesday that the water level in Unit 5
was slightly low but that they plan to use a generator to add more.
Satellite photos show steam rising from the facility. The amount of
radioactivity carried by the plume is unknown, but small increases in
radiation — not enough to affect human health — were reported in Tokyo,
about 150 miles to the southwest of the facility, and in other parts of
Japan.
In response, NHK television reported that the Japanese government had
ordered the country’s 47 prefectures to publicly report recorded
radiation levels twice a day.
Also Tuesday, U.S. Energy Secretary Steven Chu announced that his
department was dispatching a team of 34 experts and 17,000 pounds of
equipment to help with the crisis. The team also will help the Obama
administration decide what to tell Americans in Japan and at home about
the crisis, White House spokesman Jay Carney said. In response to
questions about whether Japanese officials were providing complete
information, Carney said American teams on the ground will make
independent assessments of the situation.
When the teams arrive, they will find plenty of work to do. The plant’s
reactor cores take about two weeks to lose half of their intense heat,
Gundersen said, meaning that the battle between the radioactive cores
and Fukushima Daiichi’s badly damaged cooling system will play out for
days or weeks to come.
vast...@washpost.com
Correspondent Akiko Yamamoto in Tokyo contributed to this report.
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