How does reactor meltdown

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. Smoke from the fire temporarily increased radiation levels around the reactor, so preventing future fires is very important.

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.

When lava has low viscosity, it can flow very easily as demonstrated by stalactites hanging from valves and tubes in the destroyed reactor core. Corium lava flowing out a safety valve within the Chernobyl plant. Four hundred miners were brought to Chernobyl to dig a tunnel underneath. It was feared that the radioactive lava would burn through the containment structure and contaminate the groundwater. Only later it was discovered that the lava flow stopped after 3 meters 9 feet.

About eight months after the incident and with the help of a remotely operated camera, the solidified lava was discovered in the ruins of the reactor building. In , radioactivity levels were low enough to visit the reactor's basement and took some photographs.

The photos are blurry due to radiation damage. The U. About 20 percent of U. What fuels a nuclear reactor? Most nuclear reactors use uranium fuel that has been "enriched" in uranium , an isotope of uranium that fissions readily. Isotopes are variants of elements with different atomic masses. Uranium is much more common in nature than uranium but does not fission well, so fuel manufacturers boost the uranium content to a few percent, which is enough to maintain a continuous fission reaction and generate electricity.

Enriched uranium is manufactured into fuel rods that are encased in metal cladding made of alloys such as zirconium. Reactor No. How do you turn off a nuclear reaction?

Sustained nuclear fission reactions rely on the passing of neutrons from one atom to another—the neutrons released in one atom's fissioning trigger the fissioning of the next atom. The way to cut off a fission chain reaction, then, is to intercept the neutrons. Nuclear reactors utilize control rods made from elements such as cadmium, boron or hafnium, all of which are efficient neutron absorbers. When the reactor malfunctions or when operators need to shut off the reactor for any other reason technicians can remotely plunge control rods into the reactor core to soak up neutrons and shut down the nuclear reaction.

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.