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17 April 2014 12:48 pm ,
Vol. 344 ,
Officials last week revealed that the U.S. contribution to ITER could cost $3.9 billion by 2034—roughly four times the...
An experimental hepatitis B drug that looked safe in animal trials tragically killed five of 15 patients in 1993. Now,...
Using the two high-quality genomes that exist for Neandertals and Denisovans, researchers find clues to gene activity...
A new report from the Intergovernmental Panel on Climate Change (IPCC) concludes that humanity has done little to slow...
Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500...
Three years ago, Jennifer Francis of Rutgers University proposed that a warming Arctic was altering the behavior of the...
- 17 April 2014 12:48 pm , Vol. 344 , #6181
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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.