On Wednesday, Tepco, the operator of Japan's Fukushima Daiichi Nuclear Power Plant, announced that it had removed the rest of the spent fuel from the stricken facility. Four hundred tons of spent uranium fuel has been removed from a reactor building that was damaged in an earthquake and ensuing tsunami in March 2011.

It is the first of four sets of used rods that still need to be removed in a cleanup process that will take decades. The remaining 180 fuel assemblies that still need to be removed are considered less dangerous.

The cleanup has been plagued with delays and hiccups such as the leakage of radioactive water.

Once the spent fuel has been removed, Tepco can address the most difficult task of extracting the three reactor cores that reacted during the disaster - a task that has never before been done, according to the Reuters news agency.

The nuclear disaster was the world's worst since Chernobyl, where an explosion and fire released huge amounts of radioactive waste into the environment in 1986.

sb/mkg (Reuters, dpa)