The earthquake and tsunami that devastated Japan in March 2011 also seriously damaged the Fukushima Daiichi nuclear reactor complex, leading to a critical meltdown. But Japanese nuclear scientists have been unable to gather vital data about the locations of damaged nuclear cores because of the high radiation levels, hampering cleanup efforts.


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In a new paper in the journal Physical Review Letters, scientists at Los Alamos National Laboratory have confirmed that they can use cosmic rays to pinpoint the location of nuclear material inside the reactor buildings.

The technique is called muon radiography, a scattering method developed after the 9/11 terrorist attacks that can be used to produce images of objects the particles strike, similar to how x-rays work, only with less damage to the objects they contact. It’s also used to detect potentially smuggled nuclear materials.

Cosmic rays are highly energetic particles that originate from outer space, originally believed to be a form of electromagnetic radiation.

Scientists began to suspect the existence of cosmic rays in 1909, when Theodor Wulf used an electrometer to demonstrate that there were higher radiation levels at the top of the Eiffel Tower in Paris than at its base. He surmised that the source of this extra radiation could not come from Earth; it had to come from space.

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Three years later, in 1912, Victor Hess took a historic balloon ride with three Wulf electrometers, rising to an altitude of 5300 meters. Like Wulf, he found higher radiation as he rose, four times that back on the ground at the flight’s peak.

Of course, much of that might have come from the sun, which is why Hess made his ascent during a near-total eclipse, when the moon was blocking much of the sun’s visible light. (The sun does give off low-energy cosmic rays when solar flares erupt.)

His conclusion: “The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Today we call that radiation cosmic rays.

Muons are secondary particles that result when cosmic rays collide with the Earth’s upper atmosphere, first identified as distinct particles in 1936 and confirmed as such a year later.The Earth is showered with an abundance of muon particles every second.

Muons are ideal for remotely detecting highly radioactive materials such as uranium because their scattering angle increases with atomic number, and most reactor core materials have a high atomic number. So they show up more clearly in images produced from muon scattering data, compared to surrounding objects.

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Using a computer simulation of the Fukushima reactor building run over a six-week time frame, the Los Alamos researchers found that standard transmission methods for locating nuclear material that may have been damaged as a result of the Fukushima meltdown produced blurred images. In contrast, muon radiography produced much higher resolution images that clearly showed the location of the simulated missing core materials.

“We now have a concept by which the Japanese can gather crucial data about what is going on inside their damaged reactor cores with minimal human exposure to the high radiation fields that exist in proximity to the reactor buildings,” lead author Konstantin Borozdin said via press release. “Muon images could be valuable in more effectively planning and executing faster remediation of the reactor complex.”

That’s good news for Fukushima, and the nuclear industry in general.

Images: (top) Los Alamos Muon Radiography team members in front of the damaged Fukushima Daiichi reactor. Credit: DOE/Los Alamos National Laboratory. (bottom) Victor Hess after landing his historic balloon flight. Public domain. Source: American Physical Society.