A long robot entered the damaged reactor at Japan’s Fukushima nuclear power plant on Tuesday (Sept. 10, 2024), beginning a high-stakes two-week mission to remove a small amount of melted fuel debris from below for the first time.
The robot’s visit to the Unit 2 reactor is a crucial initial step for what’s next — a difficult, decades-long process of shutting down the plant and dealing with the vast quantities of highly radioactive melted fuel inside the three reactors, which were damaged by a massive earthquake and tsunami in 2011. Experts hope the robot will help them learn more about the state of the core and fuel debris.
Here’s how the robot works, its mission, its importance and what happens next before the most challenging phase of cleaning up the reactor begins.
Nuclear fuel in the reactor core melted when the Fukushima Daiichi nuclear plant’s cooling system failed after the magnitude 9.0 earthquake and tsunami of March 2011. The molten fuel dripped down the core and mixed with internal reactor materials such as zirconium, stainless steel, electrical cables, broken grates and concrete around the supporting structure and at the bottom of the primary containment vessels.
The meltdown of the reactor caused highly radioactive, lava-like material to spread in all directions, greatly complicating the clean-up work. The state of the debris in each reactor also varies.
Tokyo Electric Power Company Holdings (TEPCO), which manages the plant, says an estimated 880 tonnes of melted fuel remains in the three reactors, but some experts say the amount could be higher.
Workers will use five 1.5-meter-long pipes connected in sequence to operate the robot through an entry point into the Unit 2 reactor’s primary containment vessel. The robot itself can extend about 6 meters inside the vessel. Once inside, it will be operated remotely by operators in another building of the plant because of the lethally high radiation emitted from the molten debris.
The front of the robot will be equipped with tongs, a light and a camera and lowered via cables to the pile of melted fuel debris. It will then chop up and collect the debris — less than 3 grams (0.1 ounces) in size. This small quantity is meant to minimize radiation hazards.
The robot will then return to the reactor where it came from, a round-trip trip that will take about two weeks.
The mission takes so long because the robot has to make very precise maneuvers to avoid hitting obstacles or getting stuck in the paths. This has happened with the previous robot as well.
TEPCO is limiting daily operations to two hours a day to minimise radiation exposure for staff working in the reactor building. Eight six-member teams will work in rotation, with each group allowed to stay for a maximum of 15 minutes.
Sampling the melted fuel debris is “an important first step,” said Lake Barrett, who led clean-up operations for the Nuclear Regulatory Commission after the 1979 disaster at the US Three Mile Island nuclear plant and is now a salaried consultant for TEPCO’s Fukushima decommissioning.
Experts say that although the melted fuel debris has been cooled and stabilized, the ageing reactors pose safety risks, and the melted fuel needs to be removed as quickly as possible and moved to a safe location for long-term storage.
According to the Japan Atomic Energy Agency, it is important to understand melted fuel debris to determine how to remove, store and dispose of it.
Experts hope the pattern will provide more clues about how the meltdown 13 years ago actually happened, some of which remains a mystery.
Samples of the melted fuel will be placed in secure containers and sent to several laboratories for more detailed analysis. If the radiation level exceeds a set limit, the robot will take the samples back to the reactor.
“This is the beginning of a process. There’s a long road ahead,” Mr. Barrett said in an online interview. “The goal is to remove the highly radioactive material, put it into engineered canisters … and put them into storage.”
The robot’s tiny forceps for this mission can only reach the top surface of the debris. The pace of work is expected to increase in the future as more experience is gained and robots with additional capabilities are developed.
Barrett said TEPCO would have to “go down into the debris pile, which is more than a meter (3.3 feet) thick, so you have to go down and see what’s inside,” adding that at Three Mile Island, the debris on the surface was very different from the material deep inside. He said multiple samples from different locations must be collected and analyzed to better understand the melted debris and develop the necessary equipment, such as robust robots, for future large-scale removal.
Compared to collecting a small sample for analysis, it will be a much more difficult challenge to develop and operate robots that can shred large pieces of melted debris and place that material in bins for safe storage.
There are also two other damaged reactors, Unit 1 and Unit 3, which are in worse condition and will take even longer to fix. TEPCO plans to deploy a set of small drones to Unit 1 for inspection later this year and is developing even smaller “micro” drones for Unit 3, which is filled with a large amount of water.
In addition, hundreds of spent fuel rods remain in open cooling pools on the upper floors of both Units 1 and 2. This is a potential safety risk if another major earthquake occurs. Removal of spent fuel rods in Unit 3 has been completed.
Work to remove the melted fuel was initially planned to begin in late 2021 but was delayed due to technical issues, underlining the difficulty of the process. The government says decommissioning is expected to take 30-40 years, while some experts say it could take up to 100 years.
Others push to decommission the plant, as was done at Chernobyl after the 1986 explosion, to reduce radiation levels and minimize risks to people working at the plant.
“It wouldn’t work at the onshore Fukushima plant,” says Mr Barrett.
“You’re in a high seismic zone, you’re in a high water zone, and there are a lot of unknowns in those (reactor) buildings,” he said. “I don’t think you can just bury it and wait it out.”
Published – September 10, 2024 10:58 am IST