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Cosmic Rays Might Offer Japan Much Needed Aid in Nuclear Power Plant Cleanup.

On March 11, 2011 a wave almost 100 feet high rose out of a magnitude 9 earthquake centered close to the coast of Japan. As the ground shook in one of the wost earthquakes of the century, the nuclear reactors at the Fukushima Dai-ichi Nuclear Power Plant initiated emergency shutdown: stop fission, turn on backup generators to cool reactors as the heat of radioactive decay gradually abates.

And then the water hit. The generators were destroyed as the tsunami's flood of 40 foot waves washed over the plant. Without the cool water pumped by the generators, heat and pressure inside the reactors continued to build to a series of hydrogen explosions and a nuclear meltdown that damaged three of the plant's six reactors.
Los Alamos National Laboratory Muon Radiography team members on-site
at the Fukushima Dai-ichi Nuclear Power Plant. Courtesy of Los Alamos National Laboratory.

For the past two years, Japan has been trudging through the environmental, health, and safety cleanup but the high radiation levels have prevented anyone from getting close enough to inspect the damage inside the reactors. Now, researchers suggest that cosmic-rays, once used to scour the pyramids of Egypt and search beneath active volcanoes, could expose new information about the damaged reactors.

Scientists on a special team at the Los Alamos National Laboratory have demonstrated that a well-established imaging method called cosmic-ray radiography could be used to take pictures of the damaged reactor, measuring the quantity and location of the melted radioactive fuel and explosion debris.

Cosmic-rays consist of outer-space particles that slam into the oxygen and nitrogen molecules in our atmosphere. These collisions cause a cascade of lighter particles that decay into a resilient particle called the muon. Muons can travel for miles through air and are constantly showering the surface of the Earth. In fact, thousands of muons have passed through you since you started reading (they're basically harmless).  But here's the clincher: muons lose energy and shift direction as the density, depth, and kind of matter they pass through changes.

Using special detectors, scientists can detect these shifts in the muon's path and back-out the different materials through which each muon traveled. In the new research, the team from Los Alamos National Laboratory compared two types of muon detection and identified a method, called muon scattering, that can detect the location and the amount of the radioactive core in the reactor complex.

Materials-- like uranium-- that have a large number of protons have a greater scattering effect on the path of the muons. The muons that pass through these materials of "heavy" atoms, like those in the core of a nuclear reactor, have stronger signatures that reflect their path than muons that pass through other, common, building  materials.  Due to this subtle interaction, the researchers found that the muon detectors can distinguish the radioactive material of the core from the rest of the building, plumbing, and non-radioactive debris.

Cross-section of simulated reactor, nuclear core (red), debris, and detectors.
Source: arXiv:1209.2761v1 [physics.ins-det]
"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 high radiation fields," said Konstantin Borozdin in the Los Alamos National Laboratory press release, "Muon images could be valuable in more effective planning and in executing faster remediation of the reactor complex."

The paper and more information about their results published October 12, 2012 in Physics Review Letters can be found on the arXiv.

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