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Cosmic Rays To Reveal the Melted Nuclear Fuel In Fukushima's Reactors
the_newsbeagle writes: Muons, produced when cosmic rays collide with molecules in the atmosphere, are streaming through your body as you read this. The particles pass through most matter unimpeded, however they can interact with heavy elements like uranium and plutonium. That's why engineers at Japan's Fukushima Daiichi power plant are using muon detectors to look for the melted nuclear fuel inside the plant's three melted-down reactors. By determining where muons are being diverted from their paths, the detectors create images of the blobs of fuel. That's necessary because nobody knows exactly where the radioactive gloop ended up during the meltdowns.
Muon shadowgraphs of the Moon, a signature of the Moon's cosmic ray shadow on the upper atmosphere, are a common way of testing neutrino detectors buried under a km or more of rock. (Muons from the atmosphere tend to be the major source of confusion for such detectors; that's why they frequently do best looking down, as muons can't go through 12,000 km of rock.)
Oh, and archeologists have used muons to look through the Great Pyramid.
The industry term for the mixed, melted contents of a reactor core is "corium". It's a mix of fuel rod assemblies ( fuel and fission products, additives, moderators, salts, and cladding), fuel rods (zirconium), and containment vessel (stainless steel), all compounded with reactor water and whatever additives were in it. In a theoretical worst case, you get to add in some concrete from the floor of the reactor building, too.
In short, about half the periodic table.
Not really.
In reality, under a complete loss of coolant scenario, the fast erosion phase of the concrete basement lasts for about an hour and progresses into about one meter depth, then slows to several centimeters per hour, and stops completely when the corium melt cools below the decomposition temperature of concrete (about 1100 C). Complete melt-through can occur in several days, even through several meters of concrete; the corium then penetrates several meters into the underlying soil, spreads around, cools, and solidifies.