Investigation of the Status of Unit 2 Nuclear Reactor of the Fukushima Daiichi by the Cosmic Muon Radiography

TL;DR

This study used cosmic muon radiography with a downsized detector to investigate the internal structure and debris of Fukushima Daiichi's Unit 2 reactor, revealing melted fuel debris at the bottom of the pressure vessel.

Contribution

It demonstrates improved muon radiography techniques with a smaller detector to effectively image reactor internals and identify melted fuel debris.

Findings

Identified reactor internal structures through concrete walls.

Detected heavy material at the bottom of the pressure vessel.

Confirmed presence of melted nuclear fuel debris.

Abstract

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.