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Not for some. <br>
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On 3/13/11 11:10 PM, Brussel Morton K. wrote:
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cite="mid:E5EDFF13-EF94-49E7-AD2C-615B64948C51@comcast.net"
type="cite">Another evaluation… It will take some time to get a
clearer picture of what happened and its health consequences.
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<h2 class="uiHeaderTitle">Perspectives of a nuclear
engineer on the Japanese nuclear power plant accidents</h2>
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<div class="mbs mbs uiHeaderSubTitle lfloat fsm fwn fcg">by
<a moz-do-not-send="true"
href="http://www.facebook.com/ajnosek">AJ Nosek</a> on
Saturday, March 12, 2011 at 2:49pm</div>
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<p>I am a nuclear engineer and I study radionuclide releases
from severe accidents where I work, including scenarios
very close to this one. This is what I can understand
from the news at this point in time, but first let me
explain what the issue is. </p>
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<p>After a reactor is shut down, it is still hot. Think of
it as a stove: after you turn it off, you can still burn
yourself. The difference is that a reactor still produces
some residual power. Therefore the goal in accident
mitigation is to shut down the reactor core and keep it
cooled. Keeping the core covered with water (or even just
a good fraction of the core), will avert significant core
damage. Keeping the core covered requires power to run
valves, pumps, and sensors. </p>
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<p>The accident at the five reactors in question is
classified as a “station blackout” (SBO). SBO is an event
in which you lose all AC power, and is very rare as it
requires an initiating event that knocks out power from
both offsite power sources and both backup diesel
generators. </p>
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<p>In such an event, Boiling Water Reactors (BWR) have a
special pump that is powered by steam instead of
electricity, known as the Reactor Core Isolation Cooling
(RCIC) pump. However, in order to properly run the pump,
you still need indicators and the ability to manipulate
valves. As a stop gap, this power can be supplied with
batteries for 8 hours, and potentially much longer
depending how fast operators can shed unnecessary loads,
find other power sources, or restore AC power. A small
generator with gas and should be able to keep the
batteries charged indefinitely. </p>
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<p>Many (if not all) the five reactors have been using this
system. The steam from the reactor vessel goes through a
safety relief valve and into the suppression pool (inside
containment), which has a high decontamination factor. At
some point, however, if heat is not being removed from the
suppression pool/containment, containment pressure will
increase and the steam inside will have to be vented
through an additional filtered system. This means a very
small amount of radiation will be released, as this steam
has been through the reactor, but fuel has not been
damaged, fission products will not be released, and it
itself is not a significant event. </p>
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<p>The Fukushima Daiichi Unit-1, a small Boiling Water
Reactor (BWR) with a Mark-I containment, is having
problems maintaining the water level in the reactor core.
The battle has centered on restoring power, and keeping
the core covered with water. The reactor core has
certainly had some amount of damage, and this is confirmed
by the hydrogen buildup. Hydrogen is produced from
zirconium (a structural metal in the core) and steam at
about 1700 degrees. The hydrogen must have been released
from the vessel and from the containment, as the
detonation occurred outside, in the reactor building.
This is consistent with the KyodoNews report that stated
that (at one point) a third of the active core had become
uncovered, and also consistent with reports of cesium
which indicate there has been at least some level of core
damage. </p>
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<p>This has been a long, protracted accident. The many
hours that have past since the reactor was operating, and
that means the residual power has dropped significantly.
Without decay heat, significantly smaller fraction
radioactive material will escape into the atmosphere.
More time also means more decay of most radioactive
material and evacuation. Whatever the final outcome, this
accident is much less significant than Chernobyl. </p>
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<p>At this time, reports say the reactor has been covered
with seawater and radiation levels are dropping. The
doses from the plant are about 100 mrem/hr. (A nuclear
plant worker dose annual dose limit is 5000 mrem, so a
worker could stand there for 2 days before reaching his
limit.) This is a good sign because this is not a
significant amount of release from a severe accident.</p>
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