Fuel debris removal attempt halted at Fukushima Daiichi

Tokyo Electric Power Company (Tepco) has suspended the first attempt to extract fuel debris from within the primary containment vessel of unit 2 at the damaged Fukushima Daiichi nuclear power plant after an error in the setup of the debris retrieval device was identified.The push pipes to be used in the fuel debris extraction operation (Image: Tepco)

On 19 August, Tepco announced that it planned to remove a few grams of melted fuel debris from unit 2 on August 22. 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). The removal technique, which is being used for the first time in unit 2, will then be gradually extended to unit 3, where a large-scale recovery is expected in the early 2030s.

"Unit 2 was selected as the first block for the recovery of the fuel debris because we take into account the situation in terms of safety, reliability, speed and progress in the removal of the used fuel elements," Tepco said.

Workers at the plant today began inserting the guide pipe of the telescopic debris retrieval device into the PCV penetration point. The guide pipe was inserted to the front of the isolation valve and the first of five 1.5-metre-long push pipes to be used was prepared to be connected. The pipes are used to push the device inside the PCV and pull it back out when the operation is completed.

However, it was then noticed during the final checking process that the order of the first push pipe was different from the planned order. The workers had in fact prepared the second push pipe for insertion instead of the first one. It was confirmed the push pipe that should have been in the first position was in the fourth position. The incorrect order of the pipes meant they could not be connected correctly.

"Today's work will be limited to just before the isolation valve," Tepco said. "We are currently investigating the cause of the matter. We will confirm and organise the necessary work and procedures going forward. We will provide further information about the future actions."

"It is better to carry on with the work safely and steadily rather than rushing," Tepco President Tomoaki Kobayakawa was quoted as saying by the Kyodo news agency.

A pre-investigation of the area directly below the pressure vessel - known as the pedestal - was carried out in January 2017 at Fukushima Daiichi 2 using a remotely operated camera on a telescopic probe. Photos taken during that investigation showed a black mass and deposits near a grating in the pedestal area, possibly melted nuclear fuel.

The following month, Tepco sent a "scorpion-shaped" robot into the PCV of unit 2. Although the robot was unable to reach the part of the vessel directly under the reactor pressure vessel, the company said the information it gathered would help it determine how to decommission the unit.

In January 2018, an internal investigation of the PCV of unit 2 using a suspended pan-tilt camera attached to a telescopic guiding pipe identified deposits and fuel assembly components at the bottom of the pedestal area.

The utility carried out a survey in February 2019 of the debris - much of it resembling pebbles - that has accumulated in the bottom of the unit's PCV using a remotely-operated probe. The probe was able to pick up debris in five of the six areas surveyed. No samples were removed from the containment vessel during the survey.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. 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. Fuel debris removal attempt halted at Fukushima Daiichi : Waste & Recycling - World Nuclear News

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