The operator of Japan's crippled nuclear power plant said on Tuesday it had found water with 5 million times the legal limit of radioactivity.

Underlining the concern over spreading radiation, the government said it was considering imposing radioactivity restrictions on seafood for the first time in the crisis after a contaminated fish was found in seas well south of the damaged nuclear reactors.

The plant's operator Tokyo Electric Power Co (Tepco) offered token "condolence" money to those affected in the Fukushima region where the plant is based, the local mayors who came to Tokyo to meet Prime Minister Naoto Kan made clear they expected far more help.

"We have borne the risks, co-existed and flourished with Tepco for more than 40 years, and all these years, we have fully trusted the myth that nuclear plants are absolutely safe," said Katsuya Endo, the mayor of Tomioka town.

He was one of eight Fukushima prefecture mayors who went to Kan to demand compensation and support for employment, housing and education for the tens of thousands of people evacuated as a result of the radiation crisis.

In desperation, engineers at the Fukushima Daiichi plant have turned to what are little more than home remedies to stem the flow of contaminated water. On Tuesday, they used "liquid glass" in the hope of plugging cracks in a leaking concrete pit.

"We tried pouring sawdust, newspaper and concrete mixtures into the side of the pit (leading to tunnels outside reactor No.2), but the mixture does not seem to be entering the cracks," said Hidehiko Nishiyama, deputy director-general of Japan's Nuclear and Industrial Safety Agency (NISA).     

"We also still do not know how the highly contaminated water is seeping out of reactor No.2," said Nishiyama.      

Engineers also plan to build two giant polyester "silt curtains" in the sea to block the spread of more contamination from the plant.        Workers are still struggling to restart cooling pumps -- which recycle the water -- in four reactors damaged by last month's 9.0 magnitude earthquake and tsunami. 

Until those are fixed, they must pump in water from outside to prevent overheating and meltdowns. In the process, that creates more contaminated water that has to be pumped out and stored somewhere else or released into the sea.

 There is a total of 60,000 tonnes of highly contaminated water in the plant after workers frantically poured in seawater when fuel rods experienced partial meltdown after the tsunami hit northeast Japan on March 11. - Reuters