Workers at Japan's crippled Fukushima nuclear plant Thursday spotted a hole in one of the barriers intended to keep radioactive particles contained in the harbour, the operator said.

Tokyo Electric Power set up silt fences in the harbour next to the plant, including one covering each outlet at reactor units 1-4, which were damaged in the 2011 tsunami, and another on the egress of reactors 5-6 which remain intact.

A silt fence is a device to trap sediment before water flows into the sea.

The fence around the undamaged reactors was found to be holed, a TEPCO spokesman said.

"Radiation levels in this area's seawater are very low, and no contaminated water tanks are placed near reactors 5-6," he said.

The monster tsunami swamped the six-reactor nuclear plant on March 11, 2011, knocking out cooling systems and sending units 1-3 into meltdown. Explosions and a fire destroyed the building of unit 4.

Reactors 5-6 are located a short distance from the crippled units.

Radioactive contamination of the sea has emerged as one of the major concerns after the nuclear accident, with TEPCO using thousands of tonnes of water to tame the reactors and keep them cool.

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

Much of that now-contaminated water is being stored in temporary tanks at the plant, and TEPCO has so far revealed no clear plan for its disposal. Some of the tanks have sprung leaks.

Prime Minister Shinzo Abe insisted at a meeting of Olympic chiefs this month that adverse effects from contamination were limited to the 0.3-square-kilometre harbour.

His reassurances, given at the meeting of the International Olympic Committee in Buenos Aires, were seen as key to Tokyo's successful bid to host the 2020 Olympic Games.