FUKUSHIMA, Japan —
Emergency workers are also funneling water into the complex's most troubled reactors — Units 1, 2 and 3, officials said.
A power company official said holes had to punched in the roofs of the buildings housing Units 5 and 6, as workers tried to prevent dangerous buildups of hydrogen gas — a sign that temperatures continued to rise in those units' fuel storage pools. Firefighters had started pumping water into Unit 5's pool, and the temperature had gone down, but a pump broke, delaying the refilling, the official said.
Meanwhile, Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency said backup power systems at the plant had been improperly protected, leaving them vulnerable to the tsunami that savaged the northeastern coast.
The failure of Fukushima's backup power systems, which were supposed to keep cooling systems going in the aftermath of the massive 9.0-magnitude earthquake, let uranium fuel overheat and were a "main cause" of the crisis, Nishiyama said.
"I cannot say whether it was a human error, but we should examine the case closely," he told reporters.
A spokesman for Tokyo Electric Power Co., which owns and runs the plants, said that while the generators themselves were not directly exposed to the waves, some electrical support equipment was outside. The complex was protected against tsunamis of up to 5 meters (16 feet), he said. Media reports say the tsunami was at least 6 meters (20 feet) high when it struck Fukushima.
Spokesman Motoyasu Tamaki also acknowledged that the complex was old, and might not have been as well-equipped as newer facilities.
Plant operators also said they would reconnect four of the plant's six reactor units to a power grid Saturday. Although a replacement power line reached the complex Friday, workers had to methodically work through badly damaged and deeply complex electrical systems to make the final linkups without setting off a spark and potentially an explosion.