<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Another evaluation… It will take some time to get a clearer picture of what happened and its health consequences. <div><br></div><div><div class="uiHeader uiHeaderBottomBorder mbm"><div class="clearfix uiHeaderTop"><div><h2 class="uiHeaderTitle">Perspectives of a nuclear engineer on the Japanese nuclear power plant accidents</h2></div></div><div class="clearfix"><div class="mbs mbs uiHeaderSubTitle lfloat fsm fwn fcg">by <a href="http://www.facebook.com/ajnosek">AJ Nosek</a> on Saturday, March 12, 2011 at 2:49pm</div><div class="uiHeaderSubActions rfloat"></div></div></div><div class="mbl notesBlogText clearfix"><div><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><div> <br class="webkit-block-placeholder"></div><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><div> <br class="webkit-block-placeholder"></div><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><div> <br class="webkit-block-placeholder"></div><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><div> <br class="webkit-block-placeholder"></div><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><div> <br class="webkit-block-placeholder"></div><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><div> <br class="webkit-block-placeholder"></div><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><div> <br class="webkit-block-placeholder"></div><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></div></div><br></div></body></html>