As much as 120 tonnes of radioactive water may have leaked from a storage tank at Japan's crippled Fukushima nuclear plant, contaminating the surrounding ground.
The Tokyo Electric Power Company says it has yet to discover the cause of the leak, detected on one of seven tanks that store water used to cool the plant's reactors.
The power company says it lost the ability to cool radioactive fuel rods in one of the plant's reactors for about three hours.
It was the second failure of the system to circulate sea water to cool spent fuel rods at the plant in the past three weeks.
The facility was the site of the worst nuclear disaster since Chernobyl in March 2011 when a magnitude 9.0 earthquake triggered a tsunami that destroyed back-up generators and disabled its cooling system.
Three of the reactors melted down.
The storage tanks, pits excavated at the site in the wake of the disaster, are lined with water proof sheets meant to keep the contaminated water from leaking into the soil
Work to decommission the plant is expected to take decades to complete.