Earlier in the night came reports that pressure has risen in the hardest hit unit reactor 3, said Nuclear safety authorities.
The technicians were considering lowering the pressure by opening a hole in the reactor shell.
According to Japanese Prime Minister Naoto Kan improved situation in Fukushima "slowly", according to Kyodo news agency.
The power plant's operator says now that it has succeeded in connecting electric cables to all the six units, including units 3 and 4, which is considered to be the hardest-hit units.
But before the cooling system is in operation must be verified with a number of other mechanisms inside the plant.