March 17, report. The heat converts the surrounding water into steam, which turns turbines and generates electricity. But if you remove the water, you also remove the most important cooling element in a nuclear reactor and open up the possibility for nuclear meltdown. A handful of nuclear meltdowns of varying degrees of severity have occurred since the s, when researchers began building and testing nuclear reactors. The most serious instance happened in in Chernobyl, Ukraine.
Plagued by design flaws and operator errors, the plant experienced fires, explosions, and radiation leakage. As a result, 30 people died of acute radiation syndrome, and thousands of cases of fatal cancers and birth defects have been reported in the following years.
Today, limited access is allowed inside a km mile exclusion zone surrounding the area. In , a minor cooling system malfunction led to a series of events that caused a partial meltdown that damaged one of the reactors.
However, very little radiation was released into the environment due to the surrounding primary containment vessel. Although the accident caused public concern, no deaths or adverse health effects have been officially attributed to the meltdown. In Japan, the current nuclear crisis at the Fukushima Daiichi power plant lies somewhere in between Three Mile Island and Chernobyl , according to recent news reports.
Although employees at the plant have been risking their lives to try to keep the reactors cool, the chance of a serious meltdown seems to be increasing. Inside the core of a nuclear reactor are thousands of long, thin fuel rods made of zirconium alloy that contain uranium. When a reactor is turned on, the uranium nuclei undergo nuclear fission , splitting into lighter nuclei and producing heat and neutrons. The neutrons can create a self-sustaining chain reaction by causing nearby uranium nuclei to split, too.
Fresh water flows around the fuel rods, keeping the fuel rods from overheating and also producing steam for a turbine. Even when the reactor is turned off so nuclear reactions no longer occur, the fuel rods remain extremely radioactive and hot and need to be cooled by water for an extended period of time. Without enough water, the fuel rods get so hot that they melt. If they begin to melt the nuclear reactor core and the steel containment vessel, and release radiation into the environment, nuclear meltdown occurs.
When the earthquake struck Japan, three of the six reactors Reactors 4, 5, and 6 at the Fukushima power plant were already off for routine inspections. Earthquake tremors triggered the automatic shutdown of the other three reactors, Reactors 1, 2, and 3 along with eight other nuclear reactors at other power plants. To stop the chain reaction, control rods that absorb neutrons were inserted in between the fuel rods.
But the fuel rods are still hot, since radioactive byproducts of past fission reactions continue to produce heat. As a backup measure, diesel generators turned on to spray the fuel rods with coolant. For Teachers. NewsHour Shop. About Feedback Funders Support Jobs. Close Menu. Email Address Subscribe.
Yes Not now. By — Jenny Marder Jenny Marder. Leave your feedback. Share on Facebook Share on Twitter. Mechanics of a Nuclear Meltdown Explained Science Mar 15, PM EST After a powerful explosion on Tuesday, Japanese workers are still struggling to regain control of an earthquake and tsunami-damaged nuclear power plant amid worsening fears of a full meltdown.
Two plant workers died within hours, according to the U. Nuclear Regulatory Commission; 28 more died in the following months from radiation poisoning. The fallout from Chernobyl was widespread, and the health effects of the disaster are difficult to quantify.
A report from the United Nations Scientific Committee on the Effects of Atomic Radiation found that 6, individuals who were under the age of 18 in Ukraine, Belarus or Russia at the time of the disaster had by contracted thyroid cancer, "a substantial fraction" of whom likely contracted the disease due to radiation exposure.
John Matson is a former reporter and editor for Scientific American who has written extensively about astronomy and physics. Follow John Matson on Twitter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.
See Subscription Options. Discover World-Changing Science. Get smart. Sign up for our email newsletter. Sign Up. Support science journalism.
It's the nuclear fission - how fuel rods are heated up - that is important. During the process, the rods, which are made of enriched uranium, undergo radioactive decay and release large amounts of energy. That's how water in the reactor is brought to a boil. The process is regulated with so-called control rods located between the fuel rods.
The control rods can be used to regulate the water temperature by absorbing neutrons floating in the water. To shut down a power plant, engineers activate the control rods to cut off the process of nuclear fission inside the fuel rods. This stops the nuclear reaction from continuing, but the fuel rods are still extremely hot.
As a way to cool them down, the entire apparatus is submerged in water. It takes electrical power to maintain the water flow, so if there is a power failure, the nuclear plant's situation becomes critical. Without maintaining the water flow, temperature and pressure in the reactor will continually rise.
At the Fukushima Daiichi plant, a power failure after Friday's earthquake disrupted safety circuits at one of the station's reactors.
Diesel-powered generators at the site also failed. Electric batteries were the only resource left to keep the water-cooling process going, although those had a limited lifespan. In other words, plant operators could not replace the water - which was quickly heating up and turning into steam - quickly enough.
If the process goes unabated, the fuel rods' protective covering can be corrupted or even destroyed, which can then release radioactive gases and hydrogen into the outside environment - a likely cause of the Saturday explosion.
0コメント