Radioactivity rises in sea near Japan nuclear plant

The announcement came after a 5.9-magnitude earthquake jolted Japan yesterday morning, hours after the country’s nuclear safety agency ordered plant operators to beef up their quake preparedness systems to prevent a recurrence of the nuclear crisis.

There were no reports of damage from the earthquake, and there was no risk of a tsunami similar to the one that struck the Fukushima Daiichi plant March 11 after the 9.0 magnitude earthquake, causing Japan’s worst-ever nuclear plant disaster.

Since the tsunami knocked out the plant’s cooling systems, workers have been spraying massive amounts of water on the overheated reactors. Some of that water, contaminated with radiation, leaked into the Pacific. Plant officials said they plugged that leak on April 5 and radiation levels in the sea dropped.

But the government said yesterday that radioactivity in the sea water has risen again in recent days.

The level of radioactive iodine-131 rose to 6,500 times the legal limit, according to samples taken Friday, up from 1,100 times the limit in samples taken the day before. Levels of cesium-134 and cesium-137 rose nearly fourfold.

The increased levels are still far below those recorded earlier this month before the initial leak was plugged. Health officials said they do not pose an immediate health risk.

The new rise in radioactivity could have been caused by Friday’s installation of steel panels intended to contain radiation. The installation might have temporarily stirred up stagnant waste in the area, said Hidehiko Nishiyama of the Nuclear and Industrial Safety Agency.

However, the increase in iodine-131, which has a relatively short eight-day half life, could signal the possibility of a new leak, he said.

“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 yesterday 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 the utility, 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 yesterday, and the report could not be immediately confirmed.

Following 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 Tokyo Electric, 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 wave that swamped Fukushima Daiichi last month knocked out emergency generators meant to power cooling systems.

Since then, explosions, fires, and other malfunctions have compounded efforts by Tokyo Electric to repair the plant and stem radiation leaks.

The company said yesterday it had moved power sources for some of the reactors at the stricken plant to higher ground by Friday evening in order to avoid another disastrous failure in the event of a tsunami.

Goshi Hosono, an adviser to the prime minister and member of the nuclear crisis management task force, said that the damaged reactors were much more stable than they had been earlier in the crisis and that Tokyo Electric was preparing to unveil a plan for restoring cooling capacity to the ailing reactors “soon.’’