A long robot entered a damaged reactor at Japan’s Fukushima nuclear power plant on Tuesday, beginning a two-week, high-stakes mission to retrieve for the first time a tiny amount of melted fuel debris from the bottom.
The robot’s trip into the Unit 2 reactor is a crucial initial step for what comes next — a daunting, decades-long process to decommission the plant and deal with large amounts of highly radioactive melted fuel inside three reactors that were damaged by a massive earthquake and tsunami in 2011. Specialists hope the robot will help them learn more about the status of the cores and the fuel debris.
Here is an explanation of how the robot works, its mission, significance and what lies ahead as the most challenging phase of the reactor cleanup begins.
What is the fuel debris?
Nuclear fuel in the reactor cores melted after the magnitude 9.0 earthquake and tsunami in March 2011 caused the Fukushima Daiichi nuclear plant’s cooling systems to fail. The melted fuel dripped down from the cores 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 reactor meltdowns caused the highly radioactive, lava-like material to spatter in all directions, greatly complicating the cleanup. The condition of the debris also differs in each reactor.
Tokyo Electric Power Company Holdings, or TEPCO, which manages the plant, says an estimated 880 tons of molten fuel debris remains in the three reactors, but some experts say the amount could be larger.
What is the robot’s mission?
Workers will use five 1.5-meter-long (5-foot-long) pipes connected in sequence to maneuver the robot through an entry point in the Unit 2 reactor’s primary containment vessel. The robot itself can extend about 6 meters (20 feet) inside the vessel. Once inside, it will be maneuvered remotely by operators at another building at the plant because of the fatally high radiation emitted by the melted debris.
The front of the robot, equipped with tongs, a light and a camera, will be lowered by a cable to a mound of melted fuel debris. It will then snip off and collect a bit of the debris — less than 3 grams (0.1 ounce). The small amount is meant to minimize radiation dangers.
The robot will then back out to the place it entered the reactor, a roundtrip journey that will take about two weeks.
The mission takes that long because the robot must make extremely precise maneuvers to avoid hitting obstacles or getting stuck in passageways. That has happened to earlier robots.
TEPCO is also limiting daily operations to two hours to minimize the radiation risk for workers in the reactor building. Eight six-member teams will take turns, with each group allowed to stay maximum of about 15 minutes.
What do officials hope to learn?
Sampling the melted fuel debris 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 for the Nuclear Regulatory Commission and is now a paid adviser for TEPCO’s Fukushima decommissioning.