Photographs taken by specially developed miniature drones deep inside the badly damaged reactor at unit 1 of the Fukushima Daiichi NPP show displaced control equipment and misshapen materials. The 12 images were released by plant owner/operator Tokyo Electric Power Company Holdings (Tepco) are of the main structural support (pedestal) in the reactor’s primary containment vessel, which is immediately beneath the reactor core.

The Fukushima Daiichi NPP was destroyed by the 2011 earthquake and tsunami, which damaged the plant’s cooling systems resulting in core meltdowns inside three of the four reactors at the plant. Fuel debris removal was originally scheduled to start within a decade of the 2011 reactor meltdowns, but development of equipment for this has proved problematic.

Some 880 tonnes of highly radioactive melted nuclear fuel is estimated to remain inside the three damaged reactors. Earlier attempts with robots were unable to reach the damaged core areas. However, two days of investigations using tiny drones have now produced new high-definition colour images.

The pictures show objects with various shapes and sizes hanging from various locations in the pedestal. The reveal that parts of the control-rod drive mechanism, and other equipment attached to the core were dislodged. The drones were assisted by snake-shaped robot.

Tepco officials said they were unable to tell from the images whether the suspended material was melted fuel or melted equipment. This would require additional data such as radiation levels and the drones did not carry dosimeters. The drone cameras could not see the bottom of the reactor core, in part because of the lack of light within the containment vessel, officials said. However, information from the probe could help future investigations of the melted debris which to support the development of technologies and robots for its removal.

Tepco noted that, after resolving some issues with the cable of snake-like robot, investigations were carried out on the non-submerged area inside the pedestal using small drones. “We observed the status of inner walls of the pedestal, internal structures of the pedestal and fallen control rod drive mechanism (CRD) housing. Inside the pedestal, we found icicle-like adhesions and clump-like objects in the vicinity of the opening used to replace the CRD, and confirmed that there is no substantial damage to the concrete of the inner walls.” The snake-like robot “relayed radio signals from the location of the CRD replacement rails as planned.”

Tepco, commenting on one of the images noted: “There are clump-like objects on top of the CRD housing that has fallen in the vicinity of the opening used to replace the CRD. Icicle-like objects are seen partly on the clump-like objects. Since they concentrate around the top, it is assumed that these objects migrated downward from above.”

Image (top): A miniature drone designed especially to probe the crippled Fukushima Daiichi nuclear power plant (courtesy of Tepco)

Image (right): The snake-shaped robot shows some of the damage inside the Fukushima Daiichi 1 reactor (courtesy of Tepco)