A huge floating structure to hold radioactive water was today berthed at the quay of Japan's disaster-hit Fukushima Daiichi nuclear power plant.

The plant's operator, Tokyo Electric Power Company (TEPCO), said the pontoon-type structure would be used to contain part of nearly 90,000 tonnes of contaminated water which has been stored at the facility.

The so-called "mega-float" -- 136 metres (450 feet) long, 46 metres wide and three metres high -- can hold 10,000 tonnes of water, TEPCO spokesman Hajime Motojuku said.

It was provided by the city of Shizuoka, which had used it as floating park for anglers, he said.

"The mega-float is one of the options to contain radioactive water from the plant. We have also built tanks to store waste water."

A massive 9.0-magnitude earthquake and monster tsunami knocked out cooling systems at the Fukushima plant on the Pacific coast on March 11.

In a stop-gap measure to contain the emergency at the plant, workers have been pouring massive amounts of water onto reactors where fuel rods are reported to have melted, and topped up pools for spent fuel rods.