The company said the leak forced the system to stop for one hour and 40 minutes, but the pool's temperature did not rise.
Tokyo Electric said 7 tons of water had leaked from the No.6 reactor.
The temperature fell to minus 8 degrees Celsius on Sunday morning near the damaged plant.
Ruptured pipes caused 3 water leaks on the previous day.
Tokyo Electric official Junichi Matsumoto admitted that the utility failed to take sufficient steps to prevent frozen pipes. He said it will take quick action to protect the pipes from the cold weather.