Hunting for Melted Fuel at Fukushima

Tuesday, March 10, 2015 @ 03:03 PM gHale


It has been a while since the Fukushima disaster has been in the news. On many fronts that may be a good thing, as it seems the area is now under a bit more control.

Inching forward, officials in Japan want to determine the location of the melted fuel in the stricken reactors at Fukushima No. 1. To this day, the location remains a mystery four years after the earthquake and tsunami of March 11, 2011 that triggered the three meltdowns at the Tokyo Electric Power Co. (TEPCO) power station.

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High levels of radioactivity are blocking efforts to pinpoint the melted fuel in each of the three reactors. Still, TEPCO and the government hope to begin removing the fuel in the first half of fiscal 2020.

While officials deployed remote-control devices on a trial basis to study the surfaces in and around the reactors, particle physicists are now proceeding with a project to use cosmic rays to “see through” the reactors.

In February, a team that included researchers from the High Energy Accelerator Research Organization, known as KEK, launched an experiment at Fukushima No. 1 to look for “shadows” of the fuel debris.

“We are gathering data, and it’s going well,” one team member said. “We believe we can produce results by the end of March.”

A method focusing on particles known as muons, created when cosmic rays hit the Earth’s atmosphere, is seeing action in the project, led by Fumihiko Takasaki, a professor emeritus at KEK.

Along those same lines, researchers used the same approach to understand volcanic magma activity and to search for secret chambers in pyramids.

Muons rain down onto Earth at a pace of one per second per every square meter. They pass through iron and concrete but end up partly absorbed by materials with extremely high densities, such as uranium. Therefore, such high-density materials project onto muon detectors, similar to X-ray images.

KEK came up with the idea of using muons to “see” into the crippled reactors soon after the nuclear crisis started.

In 2012, the institute tested muon detectors at Japan Atomic Power Co.’s idled Tokai No. 2 plant in Ibaraki Prefecture and located spent fuel stored in a pool with an accuracy of 0.5 to 1.0 meter.

As part of the project, three 1-sq.-meter muon detectors ended up placed in a 10-cm-thick steel box to avoid the effects of high-level radiation and placed last July at reactor No. 1 at the TEPCO plant.

Each detector unit also has a device to automatically control the temperature and transmit data.

Three detector units at different locations can create three-dimensional images. But due to operational restrictions, they are only using two units in the experiment.

As detector units can’t end up placed underground at reactor 1, any melted fuel in the underground part of the reactor will go undetected.

“If we can learn at least whether nuclear fuel remains in the reactor’s inner pressure vessel, that will be an important discovery,” Takasaki said.



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