"We want to determine the origin and contain the leak, but I must admit that tracking it down is difficult," he said.
Authorities have insisted the radioactivity will dissipate and poses no immediate threat to sea creatures or people who might eat them. Most experts agree.
Regardless, plant workers on Saturday began dumping sandbags filled with zeolite, a mineral that absorbs radioactive cesium, into the sea to combat the radiation leaks.
Meanwhile, the newspaper Asahi Shimbun reported, without citing its sources, that a secret plan to dismantle Tokyo Electric Power Co., which runs the radiation-leaking Fukushima plant, was circulating within the government. The proposal calls for putting TEPCO, the world's largest private electricity company, under close government supervision before putting it into bankruptcy and thoroughly restructuring its assets. Most government offices were closed Saturday, and the report could not be immediately confirmed.
In the wake of the nuclear crisis, the government ordered 13 nuclear plant operators to check and improve outside power links to avoid earthquake-related outages that could cause safety systems to fail as they did at the Fukushima plant, Nishiyama told reporters late Friday. The operators, including TEPCO, are to report back by May 16.
Power outages during a strong aftershock on April 7 drove home the need to ensure that plants are able to continue to operate crucial cooling systems and other equipment despite earthquakes, tsunamis and other disasters, Nishiyama said.
Utility companies were ordered to reinforce the quake resistance of power lines connected to each reactor or to rebuild them. They also must store all electrical equipment in watertight structures. Earlier, the nuclear agency ordered plant operators to store at least two emergency backup generators per reactor and to install fire pumps and power supply vehicles as further precautions.
The massive 46-foot (14-meter) wave that swamped Fukushima Dai-ichi last month knocked out emergency generators meant to power cooling systems. Since then, explosions, fires and other malfunctions have compounded efforts by TEPCO to repair the plant and stem radiation leaks.