A robot resumes mission to retrieve a piece of melted fuel from inside a damaged Fukushima reactor

An extendable robot on Tuesday resumed its entry into one of three damaged reactors at Fukushima Daiichi nuclear power plant to retrieve a fragment of melted fuel debris, nearly three weeks after its earlier attempt was suspended due to a technical issue.

The collection of a tiny sample of the spent fuel debris from inside of the Unit 2 reactor marks the start of the most challenging part of the decadeslong decommissioning of the plant where three reactors were destroyed in the March 11, 2011, magnitude 9.0 earthquake and tsunami disaster.

The sample-return mission, initially scheduled to begin on Aug. 22, was suspended when workers noticed that a set of five 1.5-meter (5-foot) add-on pipes to push in and maneuver the robot were in the wrong order and could not be corrected within the time limit for their radiation exposure, the plant operator Tokyo Electric Power Company Holdings said.

The pipes were to be used to push the robot inside and pull it back out when it finished. Once inside the 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 its target area to collect a fragment from the surface of the melted fuel mound using a device equipped with tongs that hang from the of the robot.

The mission to obtain the fragment and return with it is to last about two weeks.

The mix-up, which TEPCO called a “basic mistake,” triggered disappointment and raised concerns from officials and local residents. Industry Minister Ken Saito ordered TEPCO President Tomoaki Kobayakawa a thorough investigation of the cause and preventive steps before resuming the mission.

The pipes were brought into the Unit 2 reactor building and pre-arranged at the...