The latest robot attempting to find the 600 tons of nuclear fuel and debris that melted down six years ago in the wrecked Fukushima No. 1 nuclear power plant met its end in less than a day.

The scorpion-shape machine, built by Toshiba Corp., entered the No. 2 reactor core Thursday and stopped 3 meters (10 feet) short of a grate that would have provided a view of where fuel residue is suspected to have gathered. Two previous robots aborted similar missions after one got stuck in a gap and another was abandoned after finding no fuel during six days of searching.

After spending most of the time since the 2011 disaster containing radiation and limiting ground water contamination, scientists still don't have all the information they need for a cleanup that the government estimates will take four decades and cost ¥8 trillion. It is not yet known if the fuel melted into or through the containment vessel's concrete floor, and determining the fuel's radioactivity and location is crucial to inventing the technology needed to remove it.