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Commencement of Reactor Interior Survey Using ‘Muon Transmission Method’ (February 12, 2015)


A survey of the interior of Unit 1 at the Fukushima Daiichi Nuclear Power Station (NPS) using the ‘muon transmission method’ began on February 12, 2015.
In order for the decommissioning of the damaged reactors at the Fukushima Daiichi NPS to proceed, molten fuel debris inside the reactors must first be retrieved. However, radiation levels are extremely high, preventing workers from entering the reactor building and directly observing the condition and location of the fuel in the cores.


Muons are subatomic particles that are created when cosmic rays pass through the Earth’s upper atmosphere. The muon detection system being installed is designed to give a clear picture of the fuel in the cores of the reactors by measuring the trajectory and number of muons after they pass through objects; the location and condition of highly-dense nuclear materials can be readily identified using this technology.

Verification tests of the system will first be run to determine if the muon transmission method is functioning properly and able to obtain information despite the various obstacles present and highly radioactive environment, and if it gives a better understanding of conditions inside the reactor. It will take approximately one month for the collection of data and obtainment of results. Analysis of the data is expected to allow for an estimate of the location and amount of fuel debris remaining inside the reactor to be made.

The muon detection system has been placed on the first floor of Unit 1; the tests currently being run will therefore be unable to verify conditions at the bottom of the containment vessel (basement level), where most of the melted fuel is thought to have settled. Evaluation will therefore also incorporate the results of investigations that are to be carried out inside the containment vessel by remotely controlled robots which will soon be put into service. The vast amount of data that will be gathered will firstly be comprehensively analyzed and assessed; planning of the next step of debris retrieval – a major challenge in the decommissioning process – will then take place.

The muon detection system now installed at Unit 1 was developed in partnership with the High Energy Accelerator Research Organization (KEK). Measurements using a second detection system that uses the muon diffusion method at Unit 2 will be taken by IRID member Toshiba and commence around October.

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Installation of the Muon Detection System (Date: February 9, 2015)

*Video of the installation of the system can be viewed on the TEPCO website here: