Japan plans to reinvestigate the 2011 Fukushima nuclear disaster that forced 160,000 people to flee.
The Nuclear Regulation Authority (NRA) will look into how radiation leaked from damaged reactor containment vessels and other factors.
It will also look at cooling systems set up to keep the melted fuel in the reactors from overheating.
Authorities said radiation levels gradually fall enough in reactor buildings to allow closer examination.
In 2011, three reactors of the power plant melted after it was struck by an earthquake and tsunami.
The disaster, which was termed as the world's worst nuclear disaster since Chernobyl in 1986, led to the eventual shutdown of all Japan's reactors.
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