A piece of plastic padding which clogged up a drain is thought to have caused the breakdown of a decontamination system at Japan's crippled Fukushima nuclear plant, the operator said Sunday.

The Advanced Liquid Processing System (ALPS), designed to remove radioactive material from contaminated water, is expected to play a crucial role in treating huge amounts of toxic water accumulating at the plant.

But it was halted due to a defect only hours after starting operations.

Workers found that a plastic pad, which fixed a ladder in the system, had worked loose and got stuck in a drain, probably causing the defect, said operator Tokyo Electric Power Co.

The ALPS system was switched on early Friday but was stopped late evening the same day when it was found not to be properly flushing fluid used to remove radioactive particles, TEPCO said in a statement.

There are three ALPS systems at the plant, hit by reactor meltdowns sparked by the March 2011 earthquake and tsunami.

The utility 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.

The third system was activated on Friday before the stoppage occurred.

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

The utility says they are now stable but need cooling water daily.

TEPCO has so far disclosed no clear plan for disposing of the huge amounts of stored polluted water.