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The science behind how a nuclear meltdown occurs

After the disasters of an earthquake and tsunami, Japan also suffered under the threat of a meltdown in a nuclear reactor. Here's how a meltdown happens.

View of a nuclear reactor

Things at a power plant can go wrong in many places

In a nuclear meltdown, it all comes down to a power plant's reactor. Japan's Fukushima Daiichi plant, which was damaged by Friday's earthquake, employs six boiling water reactors.

When functioning normally, energy production in a boiling water reactor starts with a process known as nuclear fission. That heats up fuel rods in the reactor, which in turn bring water in the reactor to a boil and convert it to steam. Electricity is produced when the steam passes through a turbine. Then, the water is cooled and pumped back into the reactor and the process can begin again.

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.

Fukushima plant before the disaster

Fukushima plant is one of the biggest nuclear stations in the world

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.

Safety mechanisms

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.

Potential breakdown

Fenced-off area near Chernobyl

Chernobyl remains a no-man's land to this day

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 2000 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.

Author: Sybille Golte-Schroeder /srs

Editor: Sean Sinico / Cyrus Farivar

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