A probe by nuclear regulators into the causes of the Fukushima crisis has been hampered by the coronavirus pandemic, with staff dispatches from Tokyo postponed to protect the 4,000 on-site decommissioning workers.

The Nuclear Regulation Authority had resumed its investigation in October, deeming radiation levels in parts of the Fukushima No. 1 power plant low enough to visit, nearly a decade since the triple core meltdown.

After repeated visits, NRA officials succeeded last December in filming scattered debris and a damaged ceiling on the third floor of the No. 3 reactor building, which was gutted by a hydrogen explosion during the crisis following the March 2011 mega-quake and tsunami.