The operator of the wrecked Fukushima nuclear plant says the first robot sent inside one of melted reactors stalled before finishing its work but it collected data that indicates a path to send robots deeper into the reactor is intact.

The Tokyo Electric Power Co. said Monday the robot stalled after completing two-thirds of Friday's planned mission inside the Unit 1 containment vessel. TEPCO said the robot sufficiently collected temperature, radiation levels and images from parts of the platform just below the reactor core's bottom. The data will be used to improve future damage assessments, which are crucial in the decommissioning of the plant damaged by a 2011 earthquake and tsunami.

A second robot mission scheduled Monday was postponed as engineers investigated the cause of the robot's stall.