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.
The next backup measure for cooling the fuel rods was a battery system, but the batteries lasted only a few hours. Later, technicians brought in mobile generators and also attempted to inject seawater into the nuclear reactors, which makes them permanently unusable but could help prevent a complete meltdown. While the nuclear technicians searched for better cooling options, the water levels continued to decrease, exposing the tops of the fuel rods.
Pressure also began building in some of the reactors. So far, at least three explosions have occurred in Reactors 1, 2, and 3. The explosions happened when the fuel rods began to melt and release gases that reacted with the surrounding steam, producing hydrogen. To release some pressure and prevent explosions, technicians vented some of the reactors, which also released some radioactive material into the environment.
Also, a fire ignited at Reactor 4, thought to be caused by a large pile of spent fuel rods in a pond. Spent fuel rods need to be kept fully submerged in water for cooling, but the lack of water has left some of the rods partially exposed.
Thank you for registering with Physics World If you'd like to change your details at any time, please visit My account. 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.
The technique involves using granular carbonate materials rather than water and has been demonstrated in both small and large-scale testbeds using molten lead oxide. The developers are now working toward a commercial application of the system.
The molten mass could also react chemically with surrounding materials such as concrete to create hydrogen gas that can build up and cause an explosion. The standard technique for dealing with corium is to try and cool it with water. However, this approach typically works too slowly, allowing the disaster to continue evolving and letting radioactive contaminants escape into the surrounding area.
The water also provides a source of explosive hydrogen. Looking for a better method to cool and contain corium, Louie and colleagues turned to granular carbonate minerals like calcite and dolomite, which they say could be injected into the heart of reactors in the event of a meltdown. They then combined this with both a sample of granular calcite and, for comparison, grains of silicon dioxide sand. A follow-up experiment, run on a kilogram-scale, also showed that carbonate granules could be successfully applied to contain the molten material.
Oliver Alderman of Materials Development Inc. 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. 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.
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