  We have investigated the status of the nuclear debris in the Unit-2 Nuclear
Reactor of the Fukushima Daiichi Nuclear Power plant by the method called
Cosmic Muon Radiography. In this measurement, the muon detector was placed
outside of the reactor building as was the case of the measurement for the
Unit-1 Reactor. Compared to the previous measurements, the detector was
down-sized, which made us possible to locate it closer to the reactor and to
investigate especially the lower part of the fuel loading zone. We identified
the inner structures of the reactor such as the containment vessel, pressure
vessel and other objects through the thick concrete wall of the reactor
building. Furthermore, the observation showed existence of heavy material at
the bottom of the pressure vessel, which can be interpreted as the debris of
melted nuclear fuel dropped from the loading zone.
