Both the Washington Post and New York Times are reporting that
prior to the
disaster, Unit 4's fresh fuel core of 548 rods had been off-loaded into the spent
fuel pool. There are a total of 1,149 rods in the pool.
Graphic here in Washington Post -
http://www.washingtonpost.com/wp-srv...ivity/?hpid=z3
Of serious concern is that both papers report that the NRC believes there is no
water left in Unit 4's spent rod pool and that, as the Washington Post reports -
"TEPCO reports that part of the pool's reinforced concrete wall has fallen away,
leaving just a thin stainless-steel liner."
Whilst workers were scrambling to restore power to the plants, those following
these events closely will be aware that most of the effort using water cannons
had gone into Unit 3. Attempts to fill the spent water pool on Unit 4
had been "postponed". At least part of the reason for this was concern
over Unit 3's use of MOX fuel - a uranium/plutonium blend.
Left uncooled, the spent fool rods continue to heat under normal fission decay
and their temperature rises. Over time, once the rods reach a temperature
of around 1800C to 2200C the fuel will melt through the zirconium alloy cladding
and a "zirconium cladding fire" can occur. Such an event results in the airborne
release of radiation.
The information that half of Unit 4's spent reactor pool consists of fresh fuel rods
is of serious concern. When rods are fresh, their concentration levels of the
radioactive isotope Iodine-131 is much higher than when they are depleted.
Iodine-131 has a very short half-life of about eight days. But its intense mode of beta
and gamma decay is such that it poses a significant contribution to the total
health risk from a nuclear accident or nuclear bomb explosion.
The thyroid gland, which is in the neck below the larynx, takes up iodine from the
blood stream and uses it to make thyroid hormone. Unfortunately, the gland
is unable to discriminate between stable and radioactive forms of iodine.
For this reason, Potassium iodide tablets are prescribed to be taken up to
two days before potential radiation exposure. Their job is to flood the thyroid with
stable iodine so it will not take up the harmful Iodine-131. It is these tablets that
were being administered to those Japanese who could potentially be exposed to
harmful levels of radiation, such as those that were living in the exclusion zones.
During the Chernobyl event, unfortunately there was no prior warning of the
radiation release.
The events of the past week gave me reason to re-read Pulitzer Prize winning
author Richard Rhodes' account of the Chernobyl event in the opening chapter
of his book on the arms race, "Arsenals of Folly".
Quote:
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Originally Posted by Richard Rhodes, "Arsenals of Folly"
During the morning of the Pripyat evacuation, the town council sent high school
girls door-to-door dispensing potassium-iodide tables to the towns-people.
The tablets came from civil-defense supplies.
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As Rhodes wrote,
Quote:
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Originally Posted by Richard Rhodes
It was well intended but probably too late. Potassium
iodide administered up to two days before exposure to Iodine 131 protects the
thyroid gland almost completely. But potassium iodide taken more than eight
hours after exposure is only marginally effective, especially if normal dietary
iodine intake is adequate - as it was in the Chernobyl region, where goiter (thyroid
enlargement) is still common because low levels of iodine in the soil leave
locally grown food and milk deficient.
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By contrast, the Japanese population as a whole has one of the highest iodine
consumption intakes in the world, including sources such as seaweed.
In any case, we are all anxiously hoping that those workers who are desperately
trying to stabilize the situation with the Fukushima plant will succeed. However,
they are by no means out of the woods and industry observers quoted by papers
such as the New York Times suggest that this phase of operations still has a
couple of more weeks to play out.
The fact that Reactor 4's spent fuel pool also has fresh rods in it and that the
pool is said to be dry and the concrete damaged, is of a major concern.