Tokyo Electric Power Company (Tepco) said it is installing a muon detection system at the unit. The system comprises two measuring devices. The first was installed on the north side of the reactor building yesterday, while the second was scheduled to be put in place on the west side of the building today. Once the system is installed, it will be ready for testing, the company said.

Muons are high-energy subatomic particles that are created when cosmic rays enter Earth's upper atmosphere. These particles naturally and harmlessly strike the Earth's surface at a rate of some 10,000 muons per square meter per minute. Muon tracking devices detect and track these particles as they pass through objects. Subtle changes in the trajectory of the muons as they penetrate materials and change in direction correlate with material density. Nuclear materials such as uranium and plutonium are very dense and are therefore relatively easy to identify.

The 3-D image produced by the detectors should give a clear picture of the condition and location of the fuel in the cores of the three damaged reactor at Fukushima Daiichi. The detector system currently being installed at Fukushima Daiichi was developed by Japan's High Energy Accelerator Research Organization (KEK). The system uses the so-called permeation method to measure the muon data. The analysis of the data is scheduled to be completed by the end of March.

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Tepco intends to install a second muon detection system featuring different technology at unit 2 in the coming months.

http://www.world-nuclear-news.org/RS-Looking-inside-Fukushima-Daiichi-unit-1-1002154.html