The discovery of apparent icicle-shaped melted nuclear fuel within a reactor at the stricken Fukushima No. 1 nuclear power plant could be an important first step toward decommissioning the facility.
Images taken July 21 by a submersible robot that was remotely controlled to a position directly under the pressure vessel of the No. 3 reactor showed what appeared to be fuel debris that had melted through the vessel and later solidified, hanging like an icicle from the bottom of the pressure vessel.
Another pile of solidified material had also accumulated like lava on a structure below the vessel. The material was orange and gray.
The images provided the first confirmation of sizable amounts of solidified material although robots have been sent into three reactors at the No. 1 plant. In other "expeditions," high radiation levels crippled the robot activity and prevented further study.
Operator Tokyo Electric Power Co. said the manner in which the solidified material was found within the No. 3 reactor suggested the material is fuel debris.
"It clearly appears to be something that solidified after melting out of the pressure vessel," said one official. "We believe the material emerged after nuclear fuel mixed with structural matter within the pressure vessel."
Past analysis of the No. 3 reactor led to the assumption that almost all the nuclear fuel had dropped through the pressure vessel after burning a hole in the bottom and dripping down. The latest robot survey confirms that is what likely happened.
TEPCO officials plan to deploy the submersible robot July 22 to an even greater depth within the containment vessel that holds the pressure vessel. It aims to ascertain the amount of fuel debris that has spread at the bottom of the containment vessel.
Images taken by the robot over the two days of study will be analyzed, along with other data, to gain a firmer understanding of what the material is.
That could prove important in deciding how to proceed with decommissioning.