The operator of the crippled Fukushima No. 1 nuclear power plant resumed a video survey inside a reactor containment vessel on Wednesday, inserting a second robot after an earlier effort left a similar robot stranded inside.

Tokyo Electric Power Co. began the probe last Friday, in its first attempt since the 2011 meltdown to check the interior of the No. 1 reactor's primary containment vessel and to ascertain the position of the melted-down fuel. The shape-shifting robot produced valuable images and radiation readings but stopped moving after only a few hours and the utility gave up on retrieving it.

On Wednesday morning, Tepco sent another robot equipped with cameras, a dosimeter and a thermometer into the vessel. The robot is expected to go halfway around the container to gather data on radiation levels and temperatures while taking footage of locations that were not covered during last week's survey.

Tepco said the robot used on Friday likely became stuck to the metal-mesh floor. Nonetheless, it was still able to take record some measurements, including radiation levels of close to 10 sieverts per hour — a fatal level for humans.

The company also said it has learned from Friday's troubled probe that the robot can function for a few days under high levels of radiation that are harmful to electronics — much longer than the 10 hours or so initially anticipated.

Fuel inside the Nos. 1 to 3 units is believed to have melted through the reactor pressure vessels and has been accumulating in the outer containers below.

However, its precise condition remains unknown more than four years after a huge earthquake and tsunami triggered the nuclear crisis in March 2011.

The utility eventually plans to inspect the underground area of the containment vessel where fuel debris is believed to be accumulating.