Source: Le Monde
Mar 142011
sbkrivit
Mar 142011
Several people have asked me about the worst-case scenario from the Japanese nuclear power plant incidents.
Many news reports referred to the incidents, initially caused by a 10-meter-high tsunami, as failures or accidents. I don’t know whether these labels are accurate.
It will be days, weeks or even months before we have clear analyses of the success or failure of the engineering plans for these 40-year-old plants. Certainly, if the designers planned for a tsunami of this magnitude, then engineering or operations failures have occurred.
Did the designers plan for this contingency? Probably not. Based on the contingencies they considered, they compared the costs versus the risks of their engineering choices. Civil engineers and land use planners had to use a similar decisionmaking process. Despite their best efforts, more than 10,000 people have died.
So what is the worst-case scenario? Nobody knows. Our points of reference are the Three Mile Island and Chernobyl incidents.
At Three Mile Island, no one died.
Chernobyl, on the other hand, was a disaster waiting to happen, and the design and operations failures that occurred there are well-understood. No operating nuclear reactor in the world is vulnerable to the technical and operational failures that were responsible for the Chernobyl disaster.
One possible worst-case scenario from the Japanese nuclear incidents is that misinformed fearmongers will take advantage of the public’s lack of understanding and fear of nuclear technology. Misguided policymakers could interfere with a highly reliable, dependable energy technology that provides 45.7 percent of the world’s emission-free electricity.
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Mar 132011
What is likely to be going on right now at Fukushima Nuclear Power Plant?
I contacted one of the authors for the forthcoming Wiley Encyclopedia of Nuclear Energy to seek a concise assessment of the situation.
Normally, in this type of reactor, water is constantly flowing to remove the heat from the core. It is this heat which produces steam and eventually turns turbines to make electricity. When the earthquake and tsunami hit, the normal flow of water stopped.
The nuclear fission process was halted by a procedure known as scramming. The scram was triggered automatically by the quake without human intervention. The procedure involves inserting control rods into the core. As soon as this happened, the reactor went “subcritical.” The chain reaction, which normally keeps the reactor self-sustaining, slows down but it takes a while to come close to a full stop because of the delayed neutrons.
Even though the reactor scrammed, the core continues to emit some neutrons that cause new fissions, which are the source of the heating. So-called “delayed neutrons,” from the decay of fission products previously produced, keep a certain level of fission reactions going on for a while, thus generating more heat.
The Japanese plant workers are in a race against time. They have sustained failures to their primary and secondary cooling systems and have now implemented a third system to cool the reactor by using ocean water. They have to continue to cool the reactor long enough until the heat released by fissions and decay of fission products drop below a level which can cause structural damage.
News reports suggest the operators need another 24 to 36 hours of cooling to keep the reactor under control and bring the fission reactions to a nominal level.
This article from CNN is useful for additional information.
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Mar 132011
CNN: “The powerful earthquake that unleashed a devastating tsunami Friday appears to have moved the main island of Japan by 8 feet (2.4 meters) and shifted the Earth on its axis … The quake was the most powerful to hit the island nation in recorded history.”
Washington Post: “Japan earthquake and tsunami death toll expected to exceed 10,000.”
CNN: “Edano said doctors were examining nine people who tested positive for high radiation levels on their skin and clothing.”
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Mar 132011
Source: IAEA
International Nuclear and Radiological Event Scale (pdf)
“The INES Scale is a worldwide tool for communicating to the public in a consistent way the safety significance of nuclear and radiological events.
“Events are classified on the scale at seven levels: Levels 1-3 are called “incidents” and Levels 4-7 “accidents.” The scale is designed so that the severity of an event is about 10 times greater for each increase in level on the scale.”
Chernobyl – 1986- Category 7
Three Mile Island, USA – 1979 – Category 5
Fukushima I-1, Japan – 2011 – Category 4
