In time, that ultra-hot zirconium metal starts to melt. Exposed parts of the fuel rods eventually become liquid, sink down into the coolant and solidify. A full meltdown would involve all of the fuel in that core melting and a mass of molten material falling and settling at the bottom of the reactor vessel. If the vessel is ruptured, the material could flow into the larger containment building surrounding it. That containment is shielded by protective layers of steel and concrete.
Meltdown can also occur in the pools containing spent fuel rods. Overheating of the spent fuel pools could cause the water containing and cooling the rods to evaporate. Without coolant, the fuel rods become highly vulnerable to catching fire and spontaneously combusting, releasing dangerous levels of radiation into the atmosphere.
Since spent fuel is less radioactive than fuel in the reactor core, these pools are easier to control, said Peter Caracappa, a professor and radiation safety officer at Rensselaer Polytechnic Institute. Most of the problems with the backup generators were caused by the tsunami flooding them. But Makhijani suspects that unseen damage from the earthquake may be adding another challenge. Yesterday, they had trouble releasing a valve.
If you have a question on science or technology for Just Ask , send an e-mail to science newshour. Support Provided By: Learn more. Sign in Register. Enter e-mail address Show Enter password Remember me. Sign in to Unlock all the content on the site Manage which e-mail newsletters you want to receive Read about the big breakthroughs and innovations across 13 scientific topics Explore the key issues and trends within the global scientific community.
Enter e-mail address This e-mail address will be used to create your account. Reset your password. Please enter the e-mail address you used to register to reset your password Enter e-mail address.
Registration complete. Nuclear power Research update How to stop a nuclear meltdown by leavening the reactor core like a loaf of bread 11 Oct Cooling corium: Sandia National Laboratories researchers Jessica Kruichak and William Chavez pour a granulated material onto hot lead oxide.
Courtesy: Randy Montoya A new way of cooling and containing the radioactive, lava-like mass that forms in the core of a nuclear reactor during a catastrophic meltdown has been developed by researchers in the US. Want to read more? Register to unlock all the content on the site. Can a reactor melt down once the nuclear reaction is stopped? Even after the control rods have done their job and arrested the fission reaction the fuel rods retain a great deal of heat.
What is more, the uranium atoms that have already split in two produce radioactive by-products that themselves give off a great deal of heat. So the reactor core continues to produce heat in the absence of fissioning. If the rest of the reactor is operating normally, pumps will continue to circulate coolant usually water to carry away the reactor core's heat.
In Japan the March 11 earthquake and tsunami caused blackouts that cut off the externally sourced AC power for the reactors' cooling system. According to published reports, backup diesel generators at the power plant failed shortly thereafter, leaving the reactors uncooled and in serious danger of overheating. Without a steady coolant supply, a hot reactor core will continuously boil off the water surrounding it until the fuel is no longer immersed.
If fuel rods remain uncovered, they may begin to melt, and hot, radioactive fuel can pool at the bottom of the vessel containing the reactor. In a worst-case meltdown scenario the puddle of hot fuel could melt through the steel containment vessel and through subsequent barriers meant to contain the nuclear material, exposing massive quantities of radioactivity to the outside world. How can a meltdown be averted? The Japanese plant's operators have made a number of attempts to cool the reactors, including pumping seawater into the reactor core to replenish the dwindling cooling fluid.
The Tokyo Electric Power Company has also injected boric acid , an absorber of neutrons, into the reactors. How does this incident compare with Chernobyl or Three Mile Island? At present, three of the reactors at Fukushima Daiichi station are seriously crippled. Units 1 and 3 have experienced explosions that destroyed exterior walls, apparently from buildups of hydrogen gas produced by the zirconium in the fuel rods reacting with coolant water at extremely high temperatures—but the interior containment vessels there thus far seem to be intact.
A third explosion was reported March 15 at reactor No. Pressure in the suppression pool—a doughnut-shaped water vessel below the reactor—dropped after the explosion, indicating that the containment vessel had been compromised. In reactor Nos.
And a fire at a pool storing spent fuel rods at dormant reactor No. At that Pennsylvania nuclear station in a cooling malfunction combined with worker error led to a partial meltdown—about half of the reactor core melted and formed a radioactive puddle at the bottom of the steel pressure vessel. The vessel remained intact, but some radiation did escape from the plant into the surrounding environment.
0コメント