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.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.
Fukushima’s before and after
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.
The expected position of Reactor #1’s corium
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. Maybe.
Did Fukushima suffer a melt-through at Reactor #1?
After first denying that a melt-through had occurred, TEPCO later changed its tune and said that it most likely had, at least at Reactor #1. This means that molten corium flowed completely through the RPV and into the PCV before being stopped by the several meters of concrete within the base. This wasn’t an entirely settled question, however, since radiation measurements and water testing have not found the isotope levels that would be expected if the majority of the corium were in direct contact with the concrete layer beneath the PCV. One alternate theory is that the seawater that was pumped into Reactor #1 after the disaster may have cooled the corium before it finished burning through the reactor pressure vessel.
Muon tomography scan of Reactor #1. The corium, if present in the RPV, should be visible.
Scans like the above appear to support TEPCO’s position that melt-through occurred, but the organization’s trustworthiness and understanding of the conditions at Fukushima Daiichi have been called into question multiple times since the accident. Conditions at the facility have been repeatedly misrepresented (or were simply inaccurate), and the company ignored multiple safety reports and warnings that the plant was vulnerable to a tsunami in the first place.
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.
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