Tokyo Electric Power Company Holdings (Tepco) has again suspended attempts to collect melted nuclear fuel at unit 2 of the Fukushima Daiichi NPP after two cameras on the robotic device ceased to function. “On 17 September, as a preparation for grasping fuel debris, conditions inside the primary containment vessel were being checked and a functions check of the telescopic device was performed. As a result, it was detected that footage from the cameras on the end of the telescopic device … was not being sent properly to the monitors in the remote operations room for some reason,” Tepco said.
“In order to investigate the cause of this incident, we have been inspecting all camera cables, connection terminals, and the outside of the video converters by checking their exterior, signals and measuring insulation resistance,” Tepco noted. “Going forward, we will check the video signals coming from the camera on the end of the arm and end jig monitoring camera, by using measuring instruments.”
On 13 September, Tepco begun a two-week operation to retrieve the first sample of melted fuel debris from inside the damaged reactor at unit 2.
Three reactors at the NPP suffered a reactor meltdown after the March 2011 earthquake and tsunami. The debris is a mixture of molten nuclear fuel and parts of the reactor structure. Removal of the debris has proved challenging, requiring the development of a special robotic arm to extract radioactive fuel, metal cladding and other structures from the reactor.
The operation was initially scheduled to begin on 22 August but was suspended when workers noticed that five 1.5-metre pipes to be used to push the robot into the reactor had been arranged in the wrong order, Tepco said. The equipment has since been reassembled in the right order.
Tepco had originally planned to begin retrieval of the debris in 2021 but delayed the start of procedure pending development of the technology and regulatory approvals. After three postponements, Tepco decided to use the fishing rod-type device, which had been used in previous internal inspections.
About 880 tonnes of fuel debris remain in the three reactors that suffered a meltdown, according to estimates by the International Research Institute for Nuclear Decommissioning. This initial demonstration aims to retrieve less than 3 grams of the material, which will be analysed for its composition and hardness, providing essential data for the full-scale removal process. If a sample of melted fuel is retrieved Tepco plans to analyse its structure and characteristics at a research facility in Ibaraki Prefecture.
The robotic device was positioned inside the reactor’s containment vessel, where the fuel debris has collected at the bottom. It will be withdrawn to a separate area next to the vessel, where it will be inspected using cameras installed in that space. All the work will be conducted from a remote control room situated 400 metres from the site due to the high levels of radiation. If the two cameras on the device need to be replaced, the entire instrument will have to be removed to another site, causing further delays in the project.
According to Asahi Shumbun a government official speculated that the cameras were damaged due to the high humidity within the vessel, where conditions resemble a sauna because water is continuously being pumped in to cool the debris. However, Tepco is not convinced. “The cameras and cables are waterproof, but we’ll investigate that possibility nonetheless,” a Tepco spokesperson said.
Tepco has launched a special portal detailing the fuel removal process, including the initial abortive attempt, the reorganisation of the five push pipes and the camera failures. It includes photos and videos as well as explanatory diagrams.
Researched and written by Judith Perera
