A clay-cement compound will be laid over 73,000 square metres (785,000 square feet) of the floor of the Pacific in front of the Fukushima Daiichi plant on the nation's northeast coast, said Tokyo Electric Power Co. (TEPCO).

The area is equivalent to around 10 football pitches.

"This is meant to prevent further contamination of the ocean... as sample tests have shown a relatively high concentration of radioactive substances in the sea soil in the bay," a company spokeswoman said.

Reactors at the plant went into meltdown after their cooling systems were knocked out by the monster tsunami of March last year, which was generated by a huge undersea earthquake.

Contaminated water from the plant leaked into the sea and radioactive particles concentrated on the seabed. Scientists fear ocean currents could pollute areas further afield.

The cover will be 60 centimetres (24 inches) thick, with 10 centimetres expected to be eaten away by seawater every 50 years, the TEPCO official said.