A robot has begun a 2-week mission to retrieve melted fuel from the damaged Fukushima nuclear plant

TOKYO — An extendable robot began on Tuesday a two-week mission to retrieve the first sample of melted fuel debris from inside one of three damaged reactors at the Fukushima Daiichi nuclear power plant.

Highly radioactive fuel and other materials in the reactors melted when a massive earthquake and tsunami in 2011 damaged the plant's cooling systems.

The plant's operator, Tokyo Electric Power Company Holdings, has previously used small robots to examine the inside of the reactors, but this is the first time for it to collect a sample of the melted debris in what will mark the start of the most challenging part of the plant's decadeslong decommissioning.

The mission was initially scheduled to begin on Aug. 22 but was suspended when workers noticed that five 1.5-meter (5-foot) pipes to be used to push the robot into the reactor had been arranged in the wrong order, TEPCO said.

The equipment was reassembled in the right order for Tuesday's attempt, the company said.

Once inside the reactor vessel, the robot is operated remotely from a safer location.

The robot, nicknamed “telesco," can extend up to about 22 meters (72 feet), including the pipes pushing it from behind, to reach the melted fuel mound, where it will use tongs to collect a fragment measuring less than 3 grams (0.1 ounce). It is expected to take about two weeks to obtain the fragment.

An estimated 880 tons of fatally radioactive molten fuel remains in the three reactors.

Chief government spokesperson Yoshimasa Hayashi noted that the mission marked the start of the most difficult phase of the Fukushima Daiichi cleanup. “The government will firmly and responsibly tackle the decommissioning until the very end,” he said.

The government and TEPCO have set a 30- to 40-year target for the cleanup, despite criticism it is unrealistic. No specific plans for the full removal of the melted fuel debris or its storage have been decided.