It may take a century
An elongated robot entered the damaged reactor at Japan’s Fukushima nuclear power plant on Tuesday and began a high-risk two-week mission to retrieve small amounts of melted fuel fragments from the bottom for the first time.
The entry of robots into the Unit 2 reactor was a critical first step in what happened next-a daunting, decades-long process to decommission nuclear power plants and dispose of large amounts of highly radioactive molten fuel in three reactors damaged by a massive explosion. The 2011 earthquake and tsunami. Experts hope the robot will help them learn more about the state of the reactor core and fuel debris.
The following explains how the robot works, its mission, significance, and prospects once the most challenging phase of reactor cleanup begins.
What is fuel debris?
A 9.0-magnitude earthquake and tsunami in March 2011 melted nuclear fuel in the reactor core, causing a failure in the cooling system at the Fukushima Daiichi nuclear power plant. Molten fuel drips from the core and mixes with internal reactor materials such as zirconium, stainless steel, cables, broken grids and concrete around the support structures and at the bottom of the main containment vessel.
The reactor meltdown caused highly radioactive lava-like material to splash in all directions, complicating the cleanup. The debris condition of each reactor is also different.
Tokyo Electric Power Company Holdings (TEPCO), which manages the plant, said an estimated 880 tons of molten fuel debris remained in the three reactors, but some experts said the amount could be much higher.
What is the mission of robots?
Crews will use five sequentially connected 1.5-meter-long (5-foot-long) pipes to maneuver the robot through the entry point to the No. 2 reactor main containment vessel. The robot itself can extend about 6 meters (20 feet) inside the container. Once inside, operators will manipulate it remotely from another factory building because the melted debris emits deadly high radiation.
Equipped with pliers, lights and a camera, the robot will be lowered through cables onto a pile of melted fuel fragments. It then cuts and collects some of the fragments-less than 3 grams (0.1 ounces). Small amounts are used to minimize radiation risks.
The robot will then return to where it entered the reactor, a round-trip that takes approximately two weeks.
The task takes so long because the robot must maneuver extremely precise to avoid hitting obstacles or getting stuck in channels. This also happened with early robots.
Tokyo Electric Power has also limited daily operating hours to two hours to minimize radiation risks to workers in the reactor building. Eight six-person groups will take turns, and each group can stay for up to approximately 15 minutes.
What do officials hope to learn?
Lake Barrett said sampling melted fuel fragments is an “important first step.” After the Three Mile Island nuclear power plant disaster in the United States in 1979, Barrett led the cleanup of the Nuclear Regulatory Commission and is now a paid consultant for the decommissioning of the Fukushima nuclear power plant by Tepco.
Experts said that although the melted fuel fragments have remained cool and stable, the aging of the reactor poses a potential safety risk that requires the melted fuel to be removed as soon as possible and transferred to a safer place for long-term storage.
Japan’s atomic energy agency said understanding melted fuel fragments is crucial to determining how best to remove, store and dispose of it.
Experts expect the sample will also provide more clues about how the disaster happened 13 years ago, some of which remain a mystery.
Samples of the melted fuel will be kept in safe cans and sent to multiple laboratories for more detailed analysis. If the radiation level exceeds a set limit, the robot will bring the sample back to the reactor.
“This is the beginning of a process. The road ahead is a long, long way,”Barrett said in an online interview. “The goal is to remove the highly radioactive material, put it into engineering tanks… and store it.”
For this mission, the robot’s small pliers can only reach the upper surface of the debris. As more experience is accumulated and robots with additional functions are developed, it is expected that the pace of future work will accelerate.
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Original text:https://techxplore.com/news/2024-09-robot-fuel-fukushima-nuclear-century.html
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