Tokyo Electric Power Company Holdings (Tepco) plans to resume work to remove a sample of highly radioactive material from unit 2 of the damaged Fukushima Daiichi NPP after having suspended the effort over a technical problem.
Tepco on 17 September had suspended attempts for a second time to collect melted nuclear fuel from unit 2 after two cameras on the robotic device ceased to function. Four days before, Tepco began a two-week operation to retrieve the first sample of melted fuel debris.
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 subsequently reassembled in the right order.
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.
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. The 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 It will be withdrawn to a separate area next to the vessel, where it will be inspected using cameras installed in that space.
Tepco said of the replacement of the faulty cameras has now been completed. “After completing the final check of the camera replacement procedures, we commenced to replace the cameras on 16 October. And between 17and 18 October we conducted cable conduction tests for the end jig monitoring camera and the camera on the end of the arm and replaced them. There were no problems with cable conductivity, and we have confirmed that the camera footage from both of the new cameras is being sent properly to the remote operations room.”
Tepco added: “Going forward we plan to confirm function checks for the telescopic device and replace the atmosphere inside the enclosure with nitrogen, and confirm airtightness. As soon as the entire camera replacement procedure has been completed and we have concluded our check of procedures aimed at recommencing fuel debris retrieval, we will announce when we plan to recommence fuel debris retrieval work.
