5. Location of molten fuel in damaged reactors
NHK has learned a survey conducted by the operator of the damaged Fukushima Daiichi nuclear power plant shows most of the molten fuel at one of the reactors may have fallen through the bottom of the reactor and reached its containment vessel.
Nuclear fuel inside the No.1, No.2 and No.3 reactors melted down in the 2011 nuclear accident. Experts believe it mixed with reactor parts to form fuel debris and lodged inside the reactor pressure vessels or their containment vessels. The pressure vessels are inside larger containment vessels.
The plant's operator, Tokyo Electric Power Company, has been taking X-ray like pictures of the No.3 reactor since May, using elementary particles called muon.
The particles, generated by cosmic rays, can penetrate the reactor's structures and project images of the fuel debris.
Sources say the survey shows it is highly likely that little fuel is left inside the reactor's pressure vessel, and most of the molten fuel fell to its containment vessel.
This is the first time information helping identify the status of fuel inside the No.3 reactor has been found.
A muon investigation done on the No.2 reactor last year found it very probable most of the molten fuel is at the bottom of the reactor pressure vessel.
The company plans a robotic survey of the No.3 reactor on Friday. It hopes to unveil clearer information about the fuel debris through the probe.