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5 December 2013 11:26 am ,
Vol. 342 ,
An animal rights group known as the Nonhuman Rights Project filed lawsuits in three New York courts this week in an...
Researchers have been hot on the trail of the elusive Denisovans, a type of ancient human known only by their DNA and...
Thousands of scientists in the Russian Academy of Sciences (RAS) are about to lose their jobs as a result of the...
Dyslexia, a learning disability that hinders reading, hasn't been associated with deficits in vision, hearing, or...
Exotic, elusive, and dangerous, snakes have fascinated humankind for millennia. They can be hard to find, yet their...
Researchers have sequenced and analyzed the first two snake genomes, which represent two evolutionary extremes. The...
Snake venoms are remarkably complex mixtures that can stun or kill prey within minutes. But more and more researchers...
At age 30, Dutch biologist Freek Vonk has built up a respectable career as a snake scientist. But in his home country,...
- 5 December 2013 11:26 am , Vol. 342 , #6163
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Quake Question #9: Why Didn't Reactors Melt Down Immediately?
17 March 2011 6:17 pm
Readers ask: By what mechanism is a reactor shut down (to replace spent fuel, for instance)? Did that mechanism fail after the quake? If not, why are the cores experiencing meltdowns?
Science answers: A reactor is shut down by inserting control rods between the fuel rods. The control rods are made of a neutron-absorbing material, so that they slow the rate of fission reactions. In a boiling water reactor like the ones at Fukushima, the fuel rods remain hot even after the reactor is shut down because of spontaneous fissions from the nuclear fuel and fission products. Hence the reactor needs to have a supply of cooling water even when shut down.
Following the earthquake, automatic systems shut down those reactors that were still operating, inserting control upward from below the core in a boiling water reactor. But the loss of electricity from the power grid meant that the water pumps stopped working. When the backup generators, powered by diesel fuel, where knocked out by the tsunami, there was no system left to replenish the cooling water. The heat from fuel rods continued to boil away the cooling water until eventually the core was exposed to the air.
As the temperature rose around the core, the zircaloy cladding on the fuel rods began to react with the steam, oxidizing and releasing hydrogen. Nuclear plant workers, concerned about the buildup of pressure in the containment vessels, vented some of the gas inside. It was the hydrogen in this escaping gas that exploded, destroying the buildings around reactors 1, 3, and 4.