Fuel debris sample extracted from Fukushima Daiichi reactor

Thursday, 7 November 2024

On 19 August, Tepco announced that it planned to remove a few grams of melted fuel debris from the unit on 22 August. The operation was expected to last about two weeks using a telescopic device equipped with a gripper tool. The device can extend up to 22 metres and access the debris through a penetration point in the primary containment vessel (PCV). However, as Tepco was carrying out the operation, workers noticed during the final checking process that the order of the first section of the push pipe was different from the planned order, which meant other sections could not be connected correctly. This led to a delay in the collection of a sample of fuel debris.

As preparations were being made for another attempt, on 17 September it was discovered that cameras on the end of the telescopic device were not functioning correctly. The replacement of these cameras led to a further delay.

On 30 October, Tepco announced that it had managed to grasp a sample of fuel debris using the gripper tool.

_{Unit 2 internal investigation/trial retrieval plan overview (Image: Tepco)}

On 6 November, it confirmed that the dose rate of the sampled fuel debris is less than 24 mSv/h (at a distance of 20cm), which is the criteria for proceeding with debris retrieval, thus the grasped sample was inserted into a transportation box.

On Thursday 7 November, the side hatch of the enclosure (a metal box that contains the telescopic device and the robotic arm) was opened and the transportation box was removed from inside the enclosure. The box was then placed within a DPTE (Double Porte pour Transfert Etanche) container. "The fuel debris trial retrieval work is deemed to have been completed when the transportation box is inserted into the DPTE container," Tepco said.

The company plans to transport the fuel sample within the DPTE off-site to be analysed in detail at off-site analysis facilities. The findings are expected to assist in the full-scale removal of fuel debris.

In Fukushima Daiichi units 1 to 3, the fuel and the metal cladding that formed the outer jacket of the fuel rods melted, then re-solidified as fuel debris. There is an estimated total of 880 tonnes of fuel debris in units 1-3. To reduce the risk from this fuel debris, preparations are under way for retrieving it from the reactors. The current aim is to begin retrieval from unit 2 and to gradually enlarge the scale of the retrieval. The retrieved fuel debris will be stored in the new storage facility that will be constructed within the site.

The removal technique, which is being used for the first time in unit 2, will be gradually extended to unit 3, where a large-scale recovery is expected in the early 2030s.