[Peace-discuss] Japanese warning

Sun Mar 13 23:10:07 CDT 2011

Another evaluation…  It will take some time to get a clearer picture of what happened and its health consequences.  

Perspectives of a nuclear engineer on the Japanese nuclear power plant accidents

by AJ Nosek on Saturday, March 12, 2011 at 2:49pm
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. 

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. 

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. 

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. 

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. 

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. 

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. 

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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.chambana.net/pipermail/peace-discuss/attachments/20110313/59487913/attachment.html>