Inside the reactor there are several hundred fuel elements, each with around 60 fuel rods bundled together. A fuel rod is only a few cm thick and consists of a shell made of a zirconium alloy. In there are the individual fuel tablets, about once a centimeter in size. The fuel rod is not completely full, so that gases and volatile substances generated during operation have room. Exactly such substances have been proven around Fukushima. Hence the statement: fuel rods must be damaged. And now it is becoming clear – in view of the amount of fission products – that this is not just minor damage, but that the reactor core is or was at least partially melted.
The consequences of a core meltdown can hardly be assessed
Due to the lack of cooling, the core heats up even when it is actually "switched off". The waste materials that are produced during operation continue to radiate even then. At about 900 degrees, the fuels then begin to oxidize and the fuel rod cladding to burst, writes the Society for Plant and Reactor Safety. What exactly happens then cannot be overlooked, even for experts: the fuel elements themselves continue to heat up due to the so-called decay heat. Oxidation also generates heat. The longer the cooling fails, the more it rocks. Even when exactly what melts, there are different assessments. This has to do with the fact that the various substances in mixtures and alloys react differently. The control rods could melt from 1200 degrees, the metal of the fuel rod cladding from 1750 degrees. The uranium oxide – the main fuel – only melts at 2800 degrees. But experiments have shown that violent core destruction actually begins at significantly lower temperatures of 2250 degrees.
This releases large amounts of highly radiating substances that were previously enclosed in the fuel rod. Everything that becomes liquid now sinks to the bottom, forms a soup. At least that – the partial meltdown – has long taken place in Fukushima, operators and authorities are now admitting. The question is: was the external cooling sufficient to stop the process, perhaps to make the molten material hard again in a lump? Then the meltdown would have stopped. It would have been a temporary melt ”Or is it still“ supping ”? And then the question arises: how tight are the barriers to the outside, the reactor pressure vessels and the containment?
Merciless race between cooling and decay heat
The fact that so many radiant substances have now come out and can be found in the water suggests that there are gaps and holes. And it may well be that the mass is still liquid, heats up more and more and then melts through the coating. It is not entirely clear whether the chain reaction can get going again – it is unlikely, but it is conceivable. In any case, a highly heated and completely liquid core could melt through steel and concrete. Probably – because of gravity – just down.
Then the catastrophe can take on new dimensions: if the embers come into contact with the groundwater, steam explosions can occur, and hydrogen explosions are also conceivable. As a result, large amounts of the radiating material would continue to be scattered. Basically, this is a so-called "dirty" bomb scenario. But a huge one. Even if "only" radioactive substances get into the groundwater, this would result in enormous pollution for a long time.
Experts have been assuming a partial melt for two weeks. Only the evaluation by the Japanese government is new. It is a merciless race between cooling and decay heat. And that will continue for at least weeks.
Questions about Fukushima
The SWR environment editors Werner Eckert and Axel Weib answered numerous questions about Fukushima in the blog. this site has now combined these texts, which were originally written for the blog, into a dossier.