TOKYO (AFP) - A system to decontaminate radioactive water at the crippled Fukushima nuclear power plant has been halted due to a defect only hours after it started operations, the plant's operator said on Saturday.

The Advanced Liquid Processing System (Alps) is designed to remove radioactive materials contained in contaminated water and is expected to play a crucial role in the utility's fight against the toxic water accumulating at the plant, Tokyo Electric Power Company (Tepco) said.

The equipment was started in the early hours of Friday but was stopped at 10.37pm on the same day when it was found not to be properly flushing fluid used to remove radioactive particles, Tepco said in a press release.

"We are in the process of investigating the cause of the incident," the statement said.

There are three Alps systems at the plant, which was left in a meltdown crisis by the March 2011 earthquake and tsunami.

The utility initially started trial operations of two of the three systems in March but halted them in June after corrosion in one was found to be causing water leakage. A third system was activated on Friday before the stoppage.

Tepco has poured thousands of tonnes of water onto the plant's reactors and continues to douse them to keep them cool.

The utility says they are now stable but need more water every day to prevent them running out of control again.

Highly polluted water from the plant is contaminating hundreds of tonnes of groundwater daily and is also leaking from temporary storage tanks, making its way into the sea.

Tepco has so far revealed no clear plan for disposal of the stored polluted water.