The operator of Japan’s ruined Fukushima nuclear power plant began removing radioactive fuel rods on Monday at one of three reactors that melted down after an earthquake and a tsunami in 2011, a major milestone in the long-delayed cleanup effort.

Thousands of former residents have been barred from the area around the plant for years as crews carried out a large-scale radioactive waste cleanup in the aftermath of the worst nuclear disaster since Chernobyl. The process of removing the fuel rods from a storage pool had been delayed since 2014 amid technical mishaps and high radiation levels.

The plant operator, Tokyo Electric Power, said in a statement that workers on Monday morning began removing the first of 566 spent and unspent fuel rods stored in a pool at the plant’s third reactor. A radiation-hardened robot had first located the melted uranium fuel inside the reactor in 2017.

“Thanks to their training, the work has been going smoothly,” Tomohiko Isogai, the director of the nuclear plant, was quoted as saying by the Japanese broadcaster NHK, referring to workers involved in the fuel cleanup. He added that plant officials were “very sorry” over the delays in the process.