Ute Kehse Japan Report: Fight against the meltdown

What might happen at worst

Meltdown. The term used in reactor technology has become a metaphor for the ultimate catastrophe for the collapse of governments, financial markets and even entire cultures. For days the word thrilled by the news. All speculate whether the meltdown in the four reactor blocks of derelict Japanese nuclear power plant in Fukushima I already taking place or may be prevented.
But the potential consequences are unclear. The molten reactor core could cause an explosion, the radioactive element widely distributed in the environment? Or it burns through the pressure vessel, penetrates into the soil and contaminates the ground water?

Experience of the reactor technician Höllenlava have the 1979 accident at the Three Mile Iceland reactor in Harrisburg and in 1986 made the catastrophe of Chernobyl.

Also in Three Mile Iceland failed to cool. The fuel rods are heated, their zirconium shell burst. In addition, spurred by the chemical reaction of zirconium with oxygen, the fuel began to melt inside and sink to the bottom (see illustration). About half of the reactor core, about 19 tons, dissolved itself and formed a paste made of uranium, zirconium and oxygen. Later samples showed that temperatures must have reached in the melt 2600-2850 degrees Celsius. Despite these extreme temperatures of the reactor vessel remained intact, the meltdown was captured. Probably had a solid crust formed early on the ground, which prevented the melting of steel.

When the catastrophe of Chernobyl, however, there was complete meltdown. The explosion of the overheated reactor burned for ten days, because the core contained flammable graphite as a neutron brake. It is distributed enormous quantities of radioactive material into the environment. Five percent of reactor core may have been blown into the air, indicates the World Nuclear Association. The remainder of the core melted, broke through the reactor vessel and spread within a few days in the basement of the plant. There, the brownish mass solidified into a kind of glass, were formed stalactites, stalagmites and other bizarre formations. One of these is known as "elephant". Until the concrete floor of the plant before the meltdown but not penetrated. Due to the continued bombardment with radiation initially rock-hard mass over the years has gradually become brittle.

In Fukushima, the situation is far less serious than had been present at Chernobyl, the verdict of the British science magazine New Scientist. Currently it looks like this: The reactor cores of reactors 1 to 3 are damaged and exposed to half, it is now on the side of the Society for Plant and Reactor Safety. Even in the two cooling ponds of reactors 3 and 4, where disused fuel storage, the water level seems low. Both the three reactor cores and the space between the steel shell of the reactor and the containment cooling but still with sea water.

That it has already come to the overheating of the core, shows the release of the isotope iodine-131 and cesium-137. These two radioactive substances arising from the fission of uranium, so they come from inside the fuel rods. The hydrogen explosions are a sign that the temperatures of 1,200 degrees Celsius must have at least reached - in this heat begins the oxidation of zirconium, results from the hydrogen.

If the cooling experiments ultimately fail and melt the reactor cores or old fuel in the cooling pond of still solid start, then no one knows exactly what will happen. "The physical processes in the late phase of severe accidents are very complicated and not fully understood for a long time," it says in a lecture in 2005 by four Russian scientists from the Institute for Nuclear Safety in Moscow. Chemical reactions or contact between the nearly 3,000 degrees hot core melt and water could trigger such as a steam explosion, arrive at the radioactive gases and aerosols into the environment - which would be bad, but release probably not as much radioactivity would like the massive fires in the Chernobyl disaster means that on the website of Science magazine. Meets the melt to concrete, could also be held violent chemical reactions in which decomposes the concrete and radioactive gases or aerosols are released. Within a few days, the meltdown could eat through a meter-thick layer of concrete if the temperature is still higher than 1,100 degrees Celsius. In the ground they would probably get no more than a few feet.

This morning, reports the International Atomic Energy Agency (IAEA), Japanese authorities that the damage in the reactors 2 and 3 review in Fukushima on the International Nuclear Event Scale Rating (INES) as a "serious accident" (Level 5). In the same category fall the fire in the Windscale reprocessing plant (Sellafield) in 1957 in the UK and the accident at Three Mile Iceland. The loss of cooling water in the cooling pond of Reactor 4 steps the Japanese authorities as a "serious incident / near-miss" a (level3).