Cosmic rays employed to spot melted fuel in crippled Fukushima No. 1 reactors


Where is the melted fuel in the stricken reactors at Fukushima No. 1? This remains a question four years after the earthquake and tsunami of March 11, 2011, triggered the three meltdowns at the Tokyo Electric Power Co. power station.

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 remote-control devices have been deployed 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,” a team member said. “We believe we can produce results by the end of March.”

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

The same approach has been used 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 are partly absorbed by materials with extremely high densities, such as uranium. Therefore, such high-density materials is projected 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.

“I never imagined I’d be involved in a nuclear accident,” Takasaki said. Calling the nuclear disaster a “national crisis,” he stressed that he believes everyone with the knowledge and skills should do everything they can to help.

As part of the project, three 1-sq.-meter muon detectors were put 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, only two units are being used in the experiment.

As detector units can’t be 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.