via Daily Kos / March 19, 2015 / “Japan’s Fukushima Daiichi Nuclear Power Station, destroyed 4 years ago in explosions and meltdowns triggered by an earthquake and tsunami, won’t be truly safe until engineers can remove the reactors’ nuclear fuel.But first, they have to find it.muon-scan

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.

The image at right is from a TEPCO handout (in Japanese). As expected, the Fukushima scans revealed no fuel in the reactor vessel.

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 re-contamination of rice fields downwind during the cleanup of the unit-3 refueling floor. 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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