Restoring the power to all six units at the tsunami-damaged complex is key, because it will, in theory, power up the maze of motors, valves and switches that help deliver cooling water to the overheated reactor cores and spent fuel pools that are leaking radiation.

Ideally, officials believe it should only take a day to get the Fukushima Dai-ichi nuclear under control once the cooling system is up and running. In reality, the effort to end the crisis is likely to take weeks.

Late Monday night, the deputy director general of Japan's nuclear safety body suggested to reporters why there is so much uncertainty about when the job will be finished.

"We have experienced a very huge disaster that has caused very large damage at a nuclear power generation plant on a scale that we had not expected," said Hidehiko Nishiyama of the Nuclear and Industrial Safety Agency.