Fukushima Daiichi nuclear power plant. File photo: Toru Hanai

Tokyo - The operator of Japan's destroyed Fukushima nuclear plant said on Tuesday the company's efforts to freeze radioactive water in the facility had hit a glitch and may take longer than expected.

Tokyo Electric Power had planned to freeze some of the 11,000 tonnes of toxic water pooled in trenches below two of the reactor buildings at the plant and began construction of the project in April, but said the water has not frozen over yet.

The utility is also working on a project to build a 1.4-kilometre underground wall of ice around four reactor buildings at Fukushima to prevent groundwater from flowing in, and radioactive water from seeping towards the Pacific Ocean.

The giant ice wall employs the same technology as the trench project and involves the same contractor, Kajima Corp.

A Tokyo Electric (Tepco) spokesman said fluctuating water levels in the tunnel under the No. 2 reactor building were preventing the water from freezing over.

In the three years since a massive earthquake and tsunami set off nuclear meltdowns at Fukushima, Tepco has been fighting a constant battle to pump out, treat and store hundreds of thousands of tonnes of contaminated water.

Tepco flushes huge amounts of water over the reactors to keep them cool, but that water mixes with groundwater that seeps into basements, requiring more pumping, treatment and storage.