Daily Kos, updated March 23, 2015: ” So begins an in-depth article in the March 6th edition of the Journal Science entitled Muons probe Fukushima’s ruins. In February of this year two muon detectors from the High Energy Accelerator Research Organization [KEK] in Tsukuba, Japan were installed outside the Fukushima Daiichi unit-1 ruins at reactor vessel height for the purpose of finding that ‘missing’ reactor fuel. Muons – particles somewhat akin to electrons – are generated as cosmic rays slam into atoms in the upper atmosphere and rain down on Earth to the tune of 10,000 or so per square meter every minute. They pass right through solid objects, but a few do get absorbed or deflected by massive objects, so these can generate an X-ray like image of the reactor vessel and what’s in it despite the thick concrete and steel of the containment vessel and assorted ruins/debris all around.
Now the plan is to go ahead and insert the new shape-changing robot in April to see if there is enough left of the control rod drive rail to get that robot onto the containment catwalk, where it should be able to circle inside the containment itself to collect more data about the location of the corium (melted fuel). Hopefully it’s still in the containment drywell, not having melted through the base pad into the lower level basement or ground below. If it exited the drywell it may have melted through the downcomer vents and into the torus in the first level basement, and some may have found its way into drains and drainpipes as one of the flows at Chernobyl did to produce the corium formation known as the “elephant’s foot.”
The geology at Chernobyl is quite different from Fukushima, having been built atop a solid granite bedrock rather than rock and gravel fill. The Chernobyl “elephant’s foot” formation exits a large drainpipe in what was the basement of that plant and melted 3 meters (~9 feet) into the granite. If the Fukushima unit-1 corium made it to the ground underneath the plant it is likely to have spread much further through the fill and be much more difficult to retrieve, even as the ‘underground river’ of groundwater that runs beneath the facility picks up contamination and takes it on out to the ocean. Decommissioning requires that all nuclear fuel – in whatever state – be removed from direct contact with the environment and safely isolated.
The unit-1 muon scans apparently also found some evidence that some fuel fragments may have been relocated from the reactor vessel to the spent fuel pool and refueling floor, though the precise nature of this evidence and how the fuel managed to get to these locations is not explained. If there is corium/fuel debris in these locations it will make cleanup in preparation for defueling the SFP more complicated, especially in light of the recontamination of rice fields downwind during the cleanup of the unit-3 refueling floor in 2013. Now that the glorified ‘tent’ over unit-1 has been removed, cleanup of that mess is scheduled to start sometime in the next week.
According to the article in Science, TEPCO thinks some fuel may remain in the cores at units 2 and 3. Muon scanning of unit-2 is scheduled to begin in May of this year. ”
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The Japan Times, posted March 22, 2015: ” Confirmation this week that all the fuel inside one of the Fukushima No. 1 plant’s broken reactors has long since melted leaves its operator with the tricky task of eventually scooping it all out, experts say.
Tokyo Electric Power Co. said Thursday it had performed a sophisticated scan of the plant’s No. 1 reactor core, giving the most detailed picture so far of what is going on in the high-radiation environment.
Nuclear experts said Friday that the test showed the unit’s fuel rods had melted beyond recognition.
“The results reaffirmed our previous understanding that a considerable amount of fuel had melted inside the nuclear pressure vessels,” said Hiroshi Miyano, a visiting professor at Hosei University in Tokyo.
“But there has been no evidence that the fuel has melted through the nuclear containment buildings and reached the outer environment,” Miyano said.
However, the scan — based on tomography imaging that made use of elementary particles called muons — did not look at the bottom part of the reactor, where the molten fuel would have pooled. So some experts suggested that it was not possible to tell whether the fuel had indeed been contained.
The fuel rods are installed [in] the reactor’s pressure vessel, which is in turn enclosed by the primary containment vessel. These rods generate the heat used to drive steam turbines and produce electricity but must be submerged at all times to avoid melting.
“Eventually, Tepco is aiming to scoop out the melted fuel little by little, rather than burying it in concrete” as was done at Chernobyl in the former Soviet Union, Miyano said.
Muons, which continually shower the earth from space, penetrate solid objects to a greater depth than x-rays. The rate at which they pass through a material indicates its density and helps scientists to identify it.
Since muons move more slowly through relatively dense plutonium and uranium fuel than through the reactor vessel itself, mapping their trajectory can reveal exactly where the fuel is — or isn’t.
The data from this test should help Tepco’s effort to decommission the plant, which lost all power in March 2011 after the Pacific coast of Tohoku was swamped by huge tsunami. The blackout triggered a triple core meltdown.
The decommissioning process at Fukushima is expected to take three or four decades.
Experts say the latest results and the operator’s assessment of them were in line with earlier expectations.
“We presume that despite the meltdown, the fuel is still in the containment vessel,” said Tomohisa Ito, a spokesman for the International Research Institute for Nuclear Decommissioning, a special research unit involved in dismantling the troubled plant.
“But we still need to directly check the situation one day using remote-controlled robots,” he said.
Last month the International Atomic Energy Agency said Japan had made “significant progress” in its cleanup efforts but warned the situation “remains very complex” due to the growing amounts of contaminated water being generated by the process.
While the quake and tsunami that triggered the man-made nuclear crisis killed almost 19,000 people, mostly by drowning, no one is officially recorded as having died as a direct result of the meltdowns at Fukushima, though indirect deaths related to the disaster continue to climb.
However, tens of thousands of people remain displaced because of radioactive contamination around the plant, and scientists warn that some settlements may have to be abandoned forever. ”