Japanese nuclear workers are still trying to contain the disaster at the earthquake-hit Fukushima Daiichi plant.
Engineers are using "liquid glass" in the hope of plugging cracks in a leaking concrete pit, after a failed attempt to pour sawdust, newspaper and concrete mixtures into the side of the pit.
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.
"We still do not know how the highly contaminated water is seeping out of Reactor No.2," said Hidehiko Nishiyama, deputy director-general of Japan's Nuclear and Industrial Safety Agency (NISA).
Workers are struggling to restart cooling pumps, which recycle the water, in four damaged reactors.
They must pump in water from outside to prevent overheating and meltdowns until they are fixed, but this creates more contaminated water that has to be pumped out and stored somewhere else or released into the sea.
Tokyo Electric Power Co (TEPCO) has been forced to start releasing 11,500 tonnes of low-level radioactive seawater after it ran out of storage capacity for more highly contaminated water.
Radioactive iodine of up to 4,800 times the legal limit has been recorded in the sea near the plant, while caesium has been found at levels above safety limits in tiny "kounago" fish in waters off Ibaraki Prefecture, south of Fukushima.
Japanese officials have also said they are considering asking Russia to lend a floating radiation treatment plant used to decommission Russian submarines.
The "Suzuran", one of the world's largest liquid radioactive waste treatment plants, treats radioactive liquid with chemicals and stores it in a cement form.
TEPCO said it would also build tanks to hold contaminated seawater, was towing a floating tank which will arrive next week, and was negotiating the purchase of three more.
Engineers also plan to build two giant "silt curtains" made of polyester fabric in the sea to block the spread of more contamination from the plant.
The disaster has also caused power blackouts and cuts to supply chains, threatening the country's economic growth and the operations of global firms from semiconductor makers to shipbuilders.
Fujimoto said TEPCO wants to avoid having to impose rolling power blackouts in summer, which analysts say could cause the biggest economic damage to Japan.
TEPCO has started to pay "condolence money" to local governments to aid people evacuated from around its stricken plant or affected by the radiation crisis.
The debt-laden plant operator has seen its shares lose more than 80 per cent of their value since the earthquake and faces a huge bill for the damage caused by its crippled reactors, while the government could also be faced with several claims for civil damages.