A tricky operation to remove a second sample of radioactive debris from inside the stricken Fukushima No. 1 nuclear plant has been completed, the site operator said Wednesday.

Dangerously high radiation levels mean that removing melted fuel and other debris from the plant hit by a huge tsunami in 2011 is seen as the most daunting challenge in the decadeslong decommissioning project.

Around 880 metric tons of hazardous material are inside the power plant — the site of one of history's worst nuclear accidents after the tsunami, which was triggered by a magnitude-9.0 earthquake.