Friday, Jan. 20, 2012

By MINORU MATSUTANI

One major mystery — and a source of serious concern — at the Fukushima No. 1 nuclear plant is the exact location of the molten fuel from reactors 1, 2 and 3.

Hindered by the dangerously high radiation coming from the melted rods, Tokyo Electric Power Co. can't determine where the fuel came to rest. Tepco does say, however, that computer simulations indicate the fuel should still be inside the reactors' primary containment vessels.

One solution may be found with a Nagoya-based scientist group that is working on capturing images from inside nuclear plant reactors, much like X-ray photos, by using muon cosmic rays.

The scientists say this could help Tepco determine the location of the fuel and accelerate the effort to dismantle the crippled reactors.

"It's technically possible to look inside reactors by using our technology. Japan is in big trouble now and we are ready to help whenever necessary," said Mitsuhiro Nakamura, an assistant professor at Nagoya University's Kobayashi-Maskawa Institute for the Origin of Particles and the Universe.

The group led by Nakamura has been developing the technology to capture images from inside volcanoes. The same method should be able to look into the interiors of the troubled reactors, he said.

Muons, elementary particles like electrons or neutrinos, are naturally abundant in the atmosphere and can penetrate dense matter. As muons pass through solid objects with a high density they are partially absorbed, so the density inside nuclear reactors as well as volcanoes can be determined, Nakamura said.

To take a "muon photo," a special film as well as a shield to block gamma rays would be set about 20 meters from a reactor building, he said. Wait one or two weeks and the film will show a visualization of the density inside the reactors, he said.

Nuclear fuel, a mixture of uranium, plutonium, zirconium and other minerals, has a higher density than iron and the other materials that make up the reactor pressure and containment vessels.

This means the fuel would show up as a lighter color on the film.

What Nakamura's team doesn't know is whether the current radiation level at Fukushima No. 1 is low enough to install the image-capturing equipment, Nakamura said. If the level is too high, it would harm workers and the images wouldn't be precise due to the obscuring effect of gamma rays, he said.

Eventually, the melted fuel has to be removed from the containment and pressure vessels, a process Tepco may not even be able to begin for another 10 years. The ability to peer into the reactors without a human going inside would help speed up the process.

Tepco is currently estimating the conditions inside the containment vessels by analyzing temperature readings. It made a hole in the containment vessel of reactor 2 and inserted an Olympus Corp. endoscopic camera Thursday.

Tepco was believed to also have blown air into the hole so radioactive substances can't escape, he said.

"But we are not sure what we (will be able to) see with the camera. Thus, generally, any options that can visualize inside the containment vessels are worth considering," Tepco spokesman Jun Oshima said. "We are studying all kinds of options."

Tepco, however, has no immediate plan to use muon cosmic rays to look inside the reactors, Oshima said.

Nakamura's team has received no requests from the utility or the government to use the technology at the Fukushima reactors.

If such a request came, it would take several months to obtain the results, Nakamura said.

His team has taken muon photos of the interiors of Mount Asama in Nagano and Gunma prefectures and Mount Showa Shinzan in Hokkaido.

Kanetada Nagamine of the High Energy Accelerator Research Organization is head of another group also studying the use of muon cosmic rays to peer into volcanoes.

Nagamine's team likewise hasn't been asked by Tepco or the government to help out at Fukushima, said a spokesman for the High Energy Accelerator Research Organization.

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