Workers at the stricken Fukushima Dai-ichi plant have been pumping seawater in a frantic bid to stabilise reactors overheating since a tsunami knocked out the complex's crucial cooling system March 11.

Plant operator Tokyo Electric Power Co is rushing to use freshwater instead because of the corrosive potential of the salt in seawater, Hidehiko Nishiyama of Japan's Nuclear and Industry Safety Agency said at a briefing today.

The switch was the latest tactic in attempts to regain control of the nuclear power plant near the coast, 220 kilometres northeast of Tokyo. Low levels of radiation have been seeping from the plant since a magnitude-9 earthquake and tsunami, forcing residents to evacuate areas within 20 kilometres of the plant.

Elevated radiation have been found in raw milk, sea water and 11 kinds of vegetables, including broccoli, cauliflower and turnips, prompting several countries to halt some food imports from the Fukushima region.

Tap water in several areas of Japan, including Tokyo, also exceeded government standard for infants, who are particularly vulnerable to cancer-causing radioactive iodine.
Yesterday, nuclear safety officials revealed that they suspected a breach in one or more of the plant's units, possibly a crack or hole in the stainless steel chamber around a reactor core containing fuel rods or the concrete wall surrounding a pool where spent fuel rods are stored.

Suspicions were aroused when two workers suffered skin burns after unexpectedly encountering water that was 10,000 times more radioactive than levels normally found in the units, NISA said. Such a breach could mean a much larger release of radioactive contaminants. The most likely consequence would be contamination of the groundwater, experts said.

Following the announcement, government officials urged families in a voluntary evacuation zone between 20 to 30 kilometres from the plant to leave on their own. Radioactivity was rising in some units due to contaminated water, Nishiyama said today.

"It is crucial to figure out how to remove contaminated water while allowing work to continue," he said, acknowledging that the discovery would set back delicate efforts to get the plant's cooling system operating again. Workers have begun pumping radioactive water from one of the units, Masateru Araki, a TEPCO spokesman, said today.