Tokyo Electric Power Company, or TEPCO, says it discovered a drop in the amount of water injected into a reactor at the Fukushima Daiichi nuclear plant.
TEPCO says water was moving at a rate of 3.8-cubic-meters per hour to cool down nuclear fuel in reactor number 1 on Sunday morning.
But an alarm warned that the rate had decreased to 3-cubic-meters per hour.
This was third time this month that such trouble had occurred, causing TEPCO to replace the pump in order to stabilize water injection.
Reactor number 2 shares the same pump but has not seen similar trouble. TEPCO says some deposits in the plumbing might have been behind the unstable flow of water.
TEPCO says a decrease in water injection of this level should not trigger a sudden temperature rise in the reactor.