A small drone flies into a damaged Fukushima reactor for the first time to study its melted fuel

TOKYO (AP) — A drone small enough to fit in one’s hand flew inside one of the damaged reactors at the wrecked Fukushima Daiichi nuclear power plant Wednesday in hopes it can examine some of the molten fuel debris in areas where earlier robots failed to reach.

Tokyo Electric Power Company Holdings also began releasing the fourth batch of the plant’s treated and diluted radioactive wastewater into the sea Wednesday. The government and TEPCO, the plant’s operator, say the water is safe and the process is being monitored by the International Atomic Energy Agency, but the discharges have faced strong opposition by fishing groups, as well as a Chinese ban on Japanese seafood.

A magnitude 9.0 quake and tsunami in March 2011 destroyed the plant’s power supply and cooling systems, causing three reactors to melt down. The government and TEPCO plan to remove the massive amount of fatally radioactive melted nuclear fuel that remains inside each reactor — a daunting decommissioning process that’s already been delayed for years and mired by technical hurdles and a lack of data.

To help fill in that data, a fleet of four drones were set to fly one at a time into the hardest-hit No. 1 reactor’s primary containment vessel.

TEPCO has sent a number of probes — including a crawling robot and an underwater vehicle — inside each of the three reactors, but got hindered by debris, high radiation and inability to navigate them through the rubble, though they were able to gather some data in recent years.

In 2015, the first robot to go inside the reactor got stuck on a grate. Some helpful information came from the mission, but the crawling robot had to be abandoned.

Wednesday’s drone flight comes after months of preparations and training that