The vessel remained intact, but some radiation did escape from the plant into the surrounding environment. The Chernobyl accident was far more devastating; it rates as a 7, or a "major accident," on the INES scale. In Ukraine, then part of the Soviet Union, a power surge caused an explosion in one of the plant's reactors, releasing huge doses of radioactive fallout into the air.
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. Go Paperless with Digital.
Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Create Account See Subscription Options. By some estimates, there may be as many as half a million spent fuel rods that are still radioactive and could catch fire if not kept cool. Officials have implemented a km mile -radius evacuation zone, and have advised people to stay indoors.
The US has told its citizens living in the area to stay at least 50 miles away from the power plant. Fortunately, westerly winds have so far blown much of the radioactive material out to sea. Explore further. More from General Engineering. Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page.
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Share Twit Share Email. This illustration of a nuclear reactor shows water entering the core and surrounding the fuel rods vertical red bars. When the water level decreases, the fuel rods begin to heat up and face the risk of melting. Image from video below. Citation : How does a nuclear meltdown work? This document is subject to copyright.
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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.
Increasing temperatures inside the reactor were producing steam, which caused pressure in the reactor to go up. To prevent an explosion, engineers released some of the slightly radioactive steam through a valve. Since that measure was only partially successful at lowering the reactor's pressure, officials began to fill the damaged reactor with sea water.
Similar plans were underway for Fukushima plant's other reactors, as engineers had lost the capacity to control their pressure, too. A meltdown has not occurred at the Fukushima power plant or any other of Japan's 55 nuclear power stations. In a complete nuclear meltdown, the fuel rods' contents - uranium and fission by-products such as cesium - can be exposed and sink to the bottom of the reactor. This, in turn, can lead to uncontrolled reactions and raise the reactor's temperature and pressure even further.
In the case of Chernobyl, an experiment gone awry led to a feedback loop of these chemical reactions. That then led to a rupture in the reactor's fuel rods, which exploded, blowing the heavy sealing cap off of the building. The fuel rods melted at a temperature of degrees Celsius, and without an effective containment structure, radioactive material and radiation were spewed into the atmosphere, and spread, via wind, to the surrounding area and onward to the rest of Europe.
Japan confirmed there was an explosion at one of its nuclear plants on Saturday following a major earthquake.
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