A spokeswoman said Wednesday that one of four valves connecting the center and left main fuel tanks was open. That led to fuel flowing between those tanks to a surge tank near the wing tip and then out a vent, spilling about 40 gallons.
She says that valve was deactivated and the flight was cleared to depart later Tuesday. A more detailed inspection was scheduled in Tokyo.
The leak Tuesday was the second problem in two days for Japan Airlines at Boston Logan. A fire broke out in a battery pack in the belly of a different Boeing 787 on Monday, after passengers had exited.