Sorry, the Google translation sucks so bad, here's an ad-hoc translation of the article:
What's a "temporary" or "partial" core meltdown?
by Werner Eckert, March 28, 2011, 1:13 PM (roughly translated)
Within the reactor there are several hundred fuel elements that contain bundles of about 60 fuel rods each. A fuel rod is only a few cm thick and consists of a hull of zirconium alloy. Within, there are the individual fuel tablets, 1 by 1 cm each. The fuel rod isn't entirely full, to give gases and volatile stuffs room during operation. Just such materials have been found around Fukushima. That's why the statement that the fuel rods must be damaged. And now it's becoming clear -- in the light of the sheer amount of fission products, that this is not just little damage, but that the reactor core is or has been molten at least partially.
Because of insufficient cooling, the core is heating up even when it's already "switched off". The waste products that occur during normal operation keep radiating even then. At around 900 °C, the fuel begins to oxidize and to burst the fuel rod hulls, the Society for Factory, Power Plant and Reactor Safety writes. What exactly happens then can't even be overlooked by experts then: Because of the so-called decay heat the fuel elements keep heating up. Even the oxidation generates warmth. The longer cooling is absent, the more this is building up.
Even when exactly what melts is subject to various estimates. This is because the various materials react differently in mixtures and alloys. From 1200 °C, the control rods could melt, from 1750 °C the metal of the fuel rod hulls. Uranium oxide -- the main fuel -- melts only from 2800 °C upward, but experiments have shown that really even at far lower temperatures around 2250 °C, a hefty core destruction begins.
By that, large amounts of strongly radiating materials are set free that have been enclosed in the fuel rod before. Everything that's becoming liquid now sinks down and forms a soup. This at least, the partial meltdown, has long already happened in Fukushima, company and government officials now admit. The question is: Did the cooling from outside suffice to stop the activity, perhaps to re-harden the molten material to a lump? Then the core meltdown would've been stopped. It would've been a partial meltdown. Or is it still "souping"? And then there's the question: How tight are the barriers to the outside, the reactor pressure vessel and the containment vessel?
The fact that so many radiating materials have come to the outside and can be found in water, speaks for the assumption that there are gaps and holes. And it can even be that the mass is still liquid, is still heating up and then melts through the casing. It doesn't seem to be clear whether the chain reaction could resume then -- it's apparently improbable but thinkable. Anyway, a highly heated and completely liquid core could eat through steel and concrete even without that. Probably, because of gravity, simply downwards.
Then the catastrophe could take on completely new dimensions: When the glowing mass gets in contact with ground water, there could be steam explosions, even hydrogen explosions are possible. In effect, large amounts of radiating material would be spread over a large distance. Basically, this is the scenario of a so-called "dirty" bomb. But a huge one. Even if "only" radiating materials would reach the ground water, that would mean an enormous burden for a long time.
A partial meltdown has been assumed by experts already two weeks ago. New is only the evaluation by the Japanese government. It is a merciless race between cooling and decay heat. And it will last for at least weeks.
Post scriptum: This blog isn't about telling news to the expert. It is an attempt to summarize the available information for everyone by using journalistic means.
---------- Post added at 02:37 AM ---------- Previous post was at 01:40 AM ----------
A heute.de report with some interesting information (German)
A worker who has been exposed to 880 mSv of radiation has made some photos inside the plant and told that they were/are sleeping in the irradiated conference room and eating food from cans.
German companies and institutes are offering to help out with robots, but so far help has not been requested yet (earlier, Japan had asked for availability of such robots).
---------- Post added at 02:43 AM ---------- Previous post was at 02:37 AM ----------
Another report, text (German)
People are leaving evacuation centers and returning home to the exclusion zone, b/c they probably didn't receive appropriate help (supplies, food etc.), despite warnings from the government that that would be very dangerous. Probably that's why the government needs to send out more buses now to try to convince them to come back out of the zone. There's the risk of cancer, radiation burns and death, depending on how close they are to the plant.
---------- Post added at 03:01 AM ---------- Previous post was at 02:43 AM ----------
p.s. when before, there have been 400,000 evacuees from the zones, and now there are only 100,000, does that mean that 300,000 have returned to the exclusion zone?? Does the Japanese government want to see reports of 300,000 dead, sick or injured people from radiation risks? Why did the Japanese government not ensure proper supplies to the evacuation centers?? Why didn't the JRC at least??
The JRC reports only about one activity of shipment of food to evacuation centers ... normally, you'd expect exhausting coverage about the progress that is being made on a daily basis.
In the images they showed on television, people were apparently getting food for the first time in ages ... no wonder many have already left the evacuation centers.
To ensure the trust in the Japanese government, measures should have been taken to ensure that people are healthy and well-fed under those circumstances.
But the general message seemed to have been "if you're a Japanese citizen, and a disaster happens, you're essentially out of luck".
I'm sure they're trying to do what they think is possible, but that could perhaps be far more than what they think they can do.
I've always thought in the highest terms possible about the Japanese people, but now I'm very disappointed, that they do not manage to help their own people.
Perhaps the government and the relief organizations think that people are able to help themselves, but if that means scouring the wreckage for food (as has been reported many times now on Japanese TV), or starving in an evacuation center, then I have to wonder what they are doing, and if they even want to help their people at all.
It also makes me wonder what would happen if we would have a similar disaster in our own country, would it be just as disorganized, chaotic, and ineffective?
What do people pay taxes and disaster relief aid for anyway, if something like that is possible, that you're essential f**ked when something really happens. That's not the purpose of such a society.
---------- Post added at 03:58 AM ---------- Previous post was at 03:01 AM ----------
I just read that the disaster relief section of the EU has sent 220 tons of supplies to Japan and 15 million euros to the Red Cross and prepares or plans to help set up additional evacuation centers in case of further problems with the power plant. Perhaps the EU can do something there.
A US expert said that the UN should help Japan with the reactor problem (not only b/c it can have global impact).
There's help being offered all around the world, Japan only needs to accept it. This could speed up disaster relief and perhaps even bring a solution to the reactor problem. We're all one, Japan doesn't need to do this on its own.
---------- Post added at 05:27 AM ---------- Previous post was at 03:58 AM ----------
BBC article