Investigation Of The Status Of Unit 2 Nuclear Reactor Of The Fukushima Daiichi By Cosmic Muon Radiography

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 it possible for us to locate it closer to the reactor and to investigate the lower part of the fuel loading zone in particular. 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.