High levels of a toxic radioactive isotope have been found in groundwater at Japan's Fukushima nuclear plant, its operator says.
The radioactive isotope tritium has also been detected at elevated levels.
The plant, crippled by the 2011 earthquake and tsunami, has recently seen a series of water leaks and power failures.
The tsunami knocked out cooling systems to the reactors, which melted down.
Water is now being pumped in to the reactors to cool them but this has left Tepco with the problem of how to safely store the contaminated water.
