Debris and snapped air ducts are strewn about the interior of the No. 3 reactor building of the Fukushima No. 1 nuclear power plant on Dec. 12. (Provided by the Nuclear Regulation Authority)

The Nuclear Regulation Authority on Dec. 26 released its first video of the destroyed interior of the No. 3 reactor building at the crippled Fukushima No. 1 nuclear power plant.

NRA members, clad in protective suits, entered the reactor building on Dec. 12. The team's mission was to gather data on the cause and location of the March 14, 2011, hydrogen explosion that followed a meltdown at the reactor and inspect damage to the facility.

The members' 15-minute video shows the building's interior unchanged since the explosion. Chunks of concrete and what appear to be twisted pieces of snapped metal air ducts are strewn everywhere on the floor. Mangled ceiling beams hang precariously above.

The blast blew off the building's roof above the fifth floor. The extensive damage and debris prevented the team from going higher than the third level.

As more than eight years have passed since the accident, the NRA judged that the facility was safe to enter for brief periods.

The NRA team found a radiation level of 50 millisieverts per hour on the third floor. Remaining there for only one or two minutes would expose a person to more than the limit of 1 millisievert per year recommended as safe by the government.

During the first days following the March 11, 2011, accident at the Fukushima No. 1 nuclear plant, workers at the No. 3 reactor repeatedly attempted to release vapor that contained radioactive substances through a vent to prevent the growing inner pressure from damaging the containment vessel.

But hydrogen flowed backward through a pipe into the reactor building, and is believed to have triggered the hydrogen explosion three days later.