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Robotic failure: “We don’t know the cause, and the outlook is unclear…” High barrier to internal investigation of high radiation dose at Fukushima Daiichi Nuclear Power Plant Unit 1

January 15, 2022

Due to a robot malfunction, an internal inspection of the Unit 1 reactor at TEPCO’s Fukushima Daiichi Nuclear Power Plant (located in the towns of Okuma and Futaba in Fukushima Prefecture) has not been able to begin. The radiation level inside the containment vessel, where melted nuclear fuel (debris) remains, is too high for people to approach. The work, which requires remote control, has had a series of problems. As the eleventh anniversary of the accident approaches, a high wall continues to block the way. (Kenta Onozawa)

Advance preparations were too lax.
 We didn’t know the cause of the accident. We don’t know the cause, we don’t know the prospects for countermeasures, and we haven’t decided when to resume the investigation.
 At a press conference on March 13, a TEPCO spokesman gave a vague answer. The internal investigation of the Unit 1 reactor, which was delayed for more than two years from the original plan, was supposed to start on the 12th, but it stalled right from the start.
 Of the three reactors that suffered meltdowns, Unit 1 is the only one where no debris has been found. The survey this time has been planned with a lot of effort to make up for the delay, including the use of six different types of robots with multiple functions, and the survey will take about seven months.
 The first underwater robot (25 centimeters in diameter and 111 centimeters in length) will be used to create a survey route. A 30-centimeter-diameter guide ring will be attached to the robot so that subsequent robots can pass through it to prevent cables from getting tangled, which the spokesperson stresses is essential for the survey.

Status of Containment Vessel Survey at Fukushima Daiichi Nuclear Power Plant

However, the preparations in advance were lax. The preliminary tests were limited to confirming the operation of each piece of equipment, and the team was unable to immediately respond to problems with the dosimeters that occurred when they were operated simultaneously.

In the past, there have been cases of “leaving things behind.
 It is expected to take some time to identify the cause of the problem. If similar problems occur with other robots, plans to take images of the inside of the containment vessel, grasp its three-dimensional structure, and collect sand-like deposits in the water will not be able to proceed and may be abandoned.
 We know from previous studies that complex devices are less effective, such as the underwater robot that photographed debris deposits inside the containment vessel of Unit 3 in 2017. The underwater robot that photographed the debris in the containment vessel of the Unit 3 reactor in 2017 was about a quarter of the length and had a simpler structure. It also focused on photographing as its main purpose.
 In the 2006 survey that succeeded in photographing the debris in the Unit 2 reactor, a worker inserted a pipe (13 meters long) with a camera attached to the end, rather than a robot. In the previous year, a camera-equipped pipe was inserted. In the previous year, a camera-equipped robot called a “scorpion” was deployed, but it climbed up on the sediment and could not be retrieved, remaining in the reactor.

Although “human power” can be used to deal with the problem outdoors…
 Remote-controlled operations are always fraught with difficulties, even outside the building where the reactor is located.

TEPCO’s Fukushima Daiichi Nuclear Power Plant, where work to bring the accident under control is underway. From left: Unit 1 and Unit 2 in Okuma Town, Fukushima Prefecture.

 The exhaust stack near the Unit 1 and 2 buildings, which was in danger of collapsing due to the earthquake and was highly contaminated, was cut down to about 60 meters, half the height of the original stack (1 In one case, the saw blade of a cutting device lifted by a large crane got stuck in the cylinders and could not be moved. At that time, a worker climbed up to the cutting device installed at the top of the 110-meter-high cylinder with a crane and cut it with a power tool.
 In late January, they plan to cut the contaminated pipes leading to this exhaust stack. The project was originally supposed to start four months ago, but there was a problem with the remote-controlled cutting device and the crane broke down, delaying the plan.
 Debris collection is planned for Unit 2 by the end of the year. If a problem occurs in the reactor, we cannot rely on human power.

Translated with (free version)

January 15, 2022 - Posted by | Fuk 2022 | , ,

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