An attempt to use an extendable robot to remove a fragment of melted fuel from a wrecked reactor at Japan’s tsunami-hit Fukushima Daiichi nuclear power plant was suspended Thursday due to a technical issue.

The collection of a tiny sample of the debris inside the Unit 2 reactor’s primary containment vessel would start the fuel debris removal phase, 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 work was stopped when workers noticed that five 1.5-meter (5-foot) pipes used to maneuver the robot were placed 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.