  We have investigated the status of the nuclear fuel assemblies in Unit-1
reactor of the Fukushima Daiichi Nuclear Power plant by the method called
Cosmic Muon Radiography. In this study, muon tracking detectors were placed
outside of the reactor building. We succeeded in identifying the inner
structure of the reactor complex such as the reactor containment vessel,
pressure vessel, and other structures of the reactor building, through the
concrete wall of the reactor building. We found that a large amount of fuel
assemblies was missing in the original fuel loading zone inside the pressure
vessel. It can be naturally interpreted that most of the nuclear fuel was melt
and dropped down to the bottom of the pressure vessel or even below.
