Fukushima: complete meltdown in reactor 1 bevestigd

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The Tokyo Electric Power Company (TEPCO) has announced that its muon tomography scanning efforts at Fukushima have borne fruit, and confirmed that nuclear plant’s Reactor #1 suffered a complete meltdown following the earthquake and tsunami that struck Japan on March 11, 2011.

Thus far, the muon tomography scans haven’t revealed anything that scientists and cleanup crews working at Fukushima didn’t expect. But that doesn’t make the work any less important. The only way to safely clean the site and dispose of the highly radioactive slag that’s now believed to fill the bottom of the Pressure Containment Vessel, or PCV, is to first map out what melted within the core and where the flow went afterwards.

Muon tomography was used to scan the damaged reactor because muons can penetrate materials that absorb other imaging wavelengths, like X-rays, in their tracks. Muons have also been used to image buildings and structures like the Great Pyramid in a search for secret chambers, and to examine volcano magma chambers for evidence of imminent eruptions. Superman’s X-ray vision is actually more like muon vision, except for that whole can’t-see-through-lead restriction.

What today’s findings confirm is that nuclear fuel rods inside the reactor underwent complete meltdown. The image below shows a before-and-after shot of what a reactor looks like in normal operation and then after partial meltdown has begun. Note that the water level inside the Reactor Pressure Vessel (RPV) has dropped and the rods are melting as a result. This began to happen in Reactor #1 within hours of the tsunami. Subsequent analysis over the past few years has confirmed that there seemed to be very little nuclear fuel remaining inside the RPV.

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What happened to the fuel rods is more than an academic question. Reactor #1 contained an estimated 125 tons of uranium dioxide, zirconium, steel, boron carbide, and inconel, and finding out where the corium flowed is critical. TEPCO has announced that unlike Chernobyl, which is slowly being sealed inside a layer of concrete, they intend to scrap reactor Daiichi 1, 2, 3, and 4. This makes it particularly critical to understand where the corium is in order to facilitate its eventual removal. The scrapping process is a long one — it’ll take an estimated 30-40 years to finish, and the company won’t start removing reactor fuel until ten years after the accident.