Early in the Fukushima Daiichi crisis, the Nuclear Regulatory Commission found itself at odds with Japanese regulators in their assessments of the spent-fuel tank withing fire-damaged unit 4. NRC Chairman Gregory Jaczko said information from agency experts indicated the tank had gone dry, fully exposing spent fuel rods and elevating the risk of widespread radiation release. The tank's condition contributed to NRC's decision to advise Americans to stay 50 miles from the plant, even though Japanese authorities were only evacuating people within 12 miles.
On Wednesday, though, NRC officials reaffirmed what Tokyo Electric Power Co. and the Japanese government maintained at the time – that water likely remained in the tank. The Associated Press quoted NRC executive director for operations Bill Borchardt and other agency employees as saying newly obtained video suggests damage to unit 4 may not have been as serious as once thought.
The reactor in question was down for maintenance at the time of the March 11 earthquake and tsunami that destroyed the plant's power lines and backup generators. But over the ensuing days, the tank storing the unit's spent fuel and fuel assemblies moved temporarily from the reactor core heated up in the absence of a functioning cooling system. Eventually steam and a fire were detected in the unit 4 reactor building, which became severely damaged.