Japan's Fukushima plant could spill more radioactive water into the sea unless a glitch in the cleanup system is fixed.
Tokyo Electric Power Co (Tepco) has been pumping huge amounts of water to cool three reactors at the nuclear plant after its cooling systems were knocked out by the March 11 earthquake and tsunami.
Around 110,000 tonnes of highly radioactive water is stored at the power plant, which is running out of space to keep it.
Tepco teamed up with French nuclear group Areva , U.S. firm Kurion and other companies to test-run a system in which radioactive water is decontaminated and re-used to cool the reactors.
However in the plant's latest setback Tepco said water had leaked from a facility used to absorb cesium.
It hopes to replace equipment and start the decontamination process by the end of the week as planned, but if the treatment system does not work the plant could run out of space for the radioactive water by the beginning of next week.
The contaminated water could then spill into the sea, Tepco said.
The start of Japan's month-long rainy season has also added to the risk of water buildup.
In early April, the utility dumped about 10,000 tonnes of water with low-level radioactivity into the ocean, prompting criticism from neighbours China and South Korea.
Tepco will also have to deal with highly radioactive sludge that will be left over from the decontamination process, and it is unclear where they can store it long-term.
It aims to complete initial steps to limit the release of further radiation from the Fukushima plant and to shut down its three unstable reactors by January next year.
Tepco wants to bring the reactors to a state of "cold shutdown", where the uranium at the core is no longer capable of boiling off the water used as a coolant.
Officials will then be able to start cleaning up the site and eventually remove the fuel - a process that could take more than a decade.