TOKYO — The stricken Fukushima Daiichi nuclear plant this week experienced its worst power failure since the disaster there in 2011, and though the plant’s operator said all electricity was restored by early Wednesday, the problem underlined its continuing vulnerability.

This week’s partial blackout, which started Monday, halted crucial cooling systems for as long as about 30 hours at four pools where used fuel rods are stored. The company that operates Fukushima Daiichi, the Tokyo Electric Power Company, said the plant had not been in danger because the fuel rods were never close to overheating, a state that could have led to a new, catastrophic release of radioactive materials.

The company said that temperatures in the fuel pools would have remained at safe levels for at least four days.

But the cutoff of the vital systems appeared to support fears by some experts and critics that the plant remains dangerous in part because some vital safety systems were makeshift fixes devised at the height of the nuclear crisis.