Tokyo Electric Power Company (Tepco) said on 19 January that a long telescopic probe has successfully captured images of the fuel inside the primary containment vessel (PCV) of Fukushima Daiichi unit 2, which was severely damaged by core meltdowns in 2011 following the earthquake and tsunami.
The survey used a suspended pan-tilt camera attached to a telescopic guiding pipe which was able to identify deposits and fuel assembly components at the bottom of the pedestal area. The small and radiation-hardened device was developed by Toshiba and the International Research Institute for Nuclear Decommissioning (IRID).
The images showed that at least part of the fuel breached the core, falling to the vessel's floor. They also showed the bottom of the control rod drive, which appeared rusty and coated with unidentified material. Melted fuel had previously been documented only inside Fukushima Daiichi unit 3 during an investigation last July.
In the latest probe, cameras mounted on the robot showed extensive damage caused by the core meltdown, with fuel debris mixed with broken reactor parts. Tepco spokesman Takahiro Kimoto said the debris had apparently fallen from above. “'We believe it is highly likely to be melted fuel or something mixed with it.” He said it would take time to analyse which portions of the rocks were fuel.
Tepco said that robotic exploration inside unit 2, especially the pedestal at the base of the reactor is "essential" to determine the location and condition of the fuel that melted and reformed during the accident. However, it noted that access to the area had been made challenging by deposits from the accident blocking the path designed for robots, as well as high radiation levels damaging electronics.
During the recent inspection at unit 2, the pan-tilt camera attached to a telescopic guiding pipe was introduced into the PCV through a 12cm diameter pipe.
The equipment, which comprised a 13m long guiding pipe, with a 5m long extension, was equipped with a pan-tilt camera, a "bird's eye camera," LED lighting a dosimeter and a thermometer.
After examining the pictures obtained using the device, Tepco said: "The entire bottom of the pedestal was found to be covered with sandy and clay-like deposits. Some fuel assembly components have fallen to the bottom of the pedestal and deposits thought to be fuel debris were identified in the vicinity of these fallen components."
In February 2017, Tepco sent a "scorpion-shaped" robot, also developed by Toshiba and IRID, into the unit 2 PCV, which discovered that part of the grating of the platform inside its pedestal had dropped. However, the robot was unable to reach the area directly under the reactor pressure vessel.
Photo: Image captured from inside the unit 2 containment (Credit: Tepco)