TOKYO (AP) - A tall robot entered a damaged reactor at Japan's Fukushima nuclear power plant on Tuesday, beginning a challenging two-week mission to retrieve the first small amount of molten fuel residue from the bottom.
The robot's journey to the Unit 2 reactor is a crucial first step for what comes next: a daunting, decades-long process to decommission the plant and deal with large amounts of highly radioactive molten fuel inside three reactors damaged by a massive earthquake and tsunami in 2011. Experts hope the robot will help them learn more about the status of the cores and fuel waste.
Below we explain how the robot works, what its mission is, why it is important and what awaits us when the most challenging phase of the reactor clean-up begins.
What is fuel waste?
Nuclear fuel in the reactor cores melted after the magnitude 9.0 earthquake and tsunami in March 2011 caused the cooling systems at the Fukushima Daiichi nuclear power plant to fail. The molten fuel dripped from the cores and mixed with internal reactor materials such as zirconium, stainless steel, electrical cables, broken grids and concrete around the support structure and on the floor of the primary containment vessels.
The reactor meltdowns caused highly radioactive, lava-like material to splatter in all directions, making cleanup much more difficult. The condition of the debris also varies from reactor to reactor.
Tokyo Electric Power Company Holdings, or TEPCO, which operates the plant, estimates there are still 880 tons of molten fuel residue in the three reactors. But some experts say the amount could be higher.
What is the robot's mission?
Workers use five 1.5-meter-long (5-foot-long) tubes connected in sequence to maneuver the robot through an access point in the Unit 2 reactor's primary containment vessel. The robot itself can extend about 6 meters (20 feet) into the vessel. Once inside, it is controlled remotely by operators in another building at the plant because of the deadly high levels of radiation emitted by the molten debris.
The front of the robot, equipped with pincers, a light and a camera, is lowered by a cable to a mountain of molten fuel residue. It then clips and collects a small amount of the residue - less than 3 grams (0.1 ounce). The small amount is intended to minimize radiation hazards.
The robot then returns to the place where it entered the reactor. This round trip takes about two weeks.
The mission takes so long because the robot has to make extremely precise maneuvers to avoid hitting obstacles or getting stuck in passages, something that has happened to previous robots.
TEPCO also limits daily operations to two hours to minimize radiation risk to workers in the reactor building. Eight teams of six members rotate, with each group allowed to stay for a maximum of about 15 minutes.
What do civil servants hope to learn?
Taking samples of the melted fuel waste is "an important first step," said Lake Barrett, who led the cleanup after the 1979 disaster at the U.S. Three Mile Island nuclear plant on behalf of the Nuclear Regulatory Commission and is now a paid consultant for TEPCO's decommissioning of Fukushima.
Although the molten fuel remains cool and stabilized, the aging of the reactors poses potential safety risks. Experts say the molten fuel should be removed as soon as possible and moved to a safer location for long-term storage.
According to the Japan Atomic Energy Agency, understanding the melted fuel residue is essential to determining how best to remove, store and dispose of it.
Experts expect the sample will also provide more clues about exactly how the core meltdown 13 years ago happened, although some of that remains a mystery.
The melted fuel sample is stored in secure canisters and sent to multiple labs for more detailed analysis. If the radiation level exceeds a set limit, the robot takes the sample back into the reactor.
"It's the beginning of a process. It's a long, long road that we have to travel," Barrett said in an online interview. "The goal is to remove the highly radioactive material, put it in specially designed containers ... and store those."
For this mission, the robot's small pincers can only reach the top surface of the rubble. The pace of work is expected to increase in the future as more experience is gained and robots with additional capabilities are developed.
What now?
TEPCO will have to "explore the debris, which is more than a meter (3.3 feet) thick, so you have to go down and see what's inside," Barrett said, noting that at Three Mile Island, the debris at the surface was very different from the material deeper inside. He said multiple samples from different locations will need to be collected and analyzed to better understand the melted debris and develop necessary equipment, such as more powerful robots, for future large-scale removal.
Compared to collecting a small sample for analysis, it will be more challenging to develop and operate robots that can cut up larger pieces of molten debris and put that material into containers for safe storage.
There are also two other damaged reactors, Unit 1 and Unit 3, that are in worse condition and will take longer to treat. TEPCO plans to deploy a set of small drones to Unit 1 later this year for a survey and is developing even smaller "micro" drones for Unit 3, which is filled with a larger amount of water.
Separately, hundreds of spent fuel rods are in unsealed cooling pools on the top floor of both Units 1 and 2. This is a potential safety hazard should another major earthquake occur. The removal of spent fuel rods has been completed in Unit 3.
When will the dismantling be completed?
Removal of the melted fuel was originally scheduled to begin in late 2021, but has been delayed by technical issues, underscoring the difficulty of the process. The government says decommissioning is expected to take 30-40 years, while some experts say it could take as long as 100 years.
Others advocate shutting down the nuclear power plant, as was done at Chernobyl after the 1986 explosion, to reduce radiation levels and risks to workers.
According to Barrett, that won't work at the Fukushima plant on the coast.
"You're in a seismic area, you're in a watery area, and there's a lot of unknowns in those (reactor) buildings," he said. "I don't think you can just bury it and wait."