On March 31, the Tokyo Electric Power Co. (TEPCO) released still images taken inside the pedestal (the foundation beneath the reactor pressure vessel, or RPV) looking upward at the bottom of the RPV at Unit 1 of the Fukushima Daiichi.
The images were obtained using an underwater remotely-operated vehicle (ROV), a submersible boat-shaped device—one of six such devices capable of performing six distinct functions—used for access and investigation.
The images allowed identification of a structure believed to be related to a control rod drive mechanism (CRDM). This was the first insertion of a camera inside the unit’s pedestal.
Since the end of FY21 (i.e., March 31, 2022), underwater ROVs have conducted a series of investigations on the RPV, toward the removal of fuel debris from Unit 1 at Fukushima Daiichi, and deposits approximately 80-100cm thick had already been confirmed around the pedestal openings.
This time, in the final phase of the investigative process scheduled to the end of FY22 (i.e., March 31, 2023), the small ROV-A2 was sent into the interior of the pedestal to obtain images, starting on March 28. Two days later it confirmed that concrete had melted down along nearly half the circumference of the cylindrical pedestal at the inner part of the foundation, and that a bar structure was partially exposed.
It had been believed that the melting of fuel had been more severe at Unit 1 than at Units 2 and 3, where similar melting occurred, and that was confirmed by the results of this investigation.
As for the soundness of the pedestal, TEPCO said that based on earthquake-proofing evaluations done in the past by the International Research Institute for Decommissioning (IRID), “there is no serious risk due to partial damage of the pedestal,” although it added that it would continue its investigations and evaluations based on the data obtained so far.