In the latest crisis to strike the Fukushima-Daiichi nuclear power plant in Japan, operator Tokyo Electric Power (TEPCO) has discovered that 300 tons (nearly 72,000 gallons) of highly radioactive water has leaked from a holding tank into the ground over the past month.

The development comes on top of TEPCO's admission last month that an estimated 300 tons of radioactive groundwater, which picks up small amounts of contamination when it flows through the damaged reactor buildings, has been leaking into the Pacific Ocean every day. (See related story: "Fukushima's Radioactive Water Leak: What You Should Know.")

The new storage tank leak presents a different and potentially more serious problem than the ongoing groundwater flow leaks. The water from the leaking tank is so heavily contaminated with strontium-90, cesium-137, and other radioactive substances that a person standing less than two feet away would receive, in an hour's time, a radiation dose equivalent to five times the acceptable exposure for nuclear workers, Reuters reported. Within ten hours, the exposed person would develop radiation sickness, with symptoms such as nausea and a drop in white blood cells.

A More Hazardous Leak

The latest leak comes from one of the massive array of 1,000 above-ground storage tanks built inside the plant by TEPCO, which store water that deliberately has been pumped into the damaged reactors in an effort to cool the nuclear fuel inside and prevent a meltdown. Such water is heavily contaminated and dangerous compared with the larger radioactive groundwater flow problem, which scientists say does not pose an immediate health hazard to humans (though it has made some types of fish from the area unsafe for consumption).

The Japanese government's Nuclear Regulation Authority is calling the leak a "serious accident" and wants to raise the official threat level from 1 to 3 on the International Nuclear and Radiological Event Scale—the highest level since the level 7 rating given when the March 2011 earthquake and tsunami severely damaged the facility. (See related "Pictures: The Nuclear Cleanup Struggle at Fukushima.")

While about two-thirds of Fukushima's storage tanks are welded steel vessels, the leaking tank is one of about 350 improvised temporary tanks that TEPCO has employed to augment its capacity. The temporary tanks are made of steel plates bolted together with plastic packing materials to seal the seams, and apparently are more vulnerable to leaks. A TEPCO official told The Japan Times, an English-language daily, that there have been four previous leaks in the temporary tanks. Unlike the previous ones, this leak somehow went undetected by plant workers for as long as a month. During that time, it leaked an estimated ten tons (about 2,400 gallons) of highly radioactive water per day. (See related photos: "A Rare Look Inside Fukushima Daiichi.")

TEPCO hasn't yet found the precise leakage spot or spots on the faulty tank, which according to Reuters is located just 550 yards from the ocean. But the company said that workers have pumped all of the water from inside a small concrete containment area where the leaking tank is located. In the event of rain, they plan to continue running the pump, which they say is powerful enough to keep rainwater from flowing out of the containment.

"We apologize again for creating anxiety among the public," TEPCO executive Masayuki Ono told reporters on Tuesday.

TEPCO said on Wednesday that tests of seawater from a ditch near the leaking tank didn't show any significant increase in the amount of cesium-137 and other radioactive materials, suggesting that the highly radioactive water isn't directly reaching the ocean. However, the possibility remains that the contaminated water might be mixing into groundwater that flows through the plant site into the ocean. In mid-July, levels of radioactive cesium-137 and cesium-134 from monitoring wells inside the plant unexpectedly surged nearly 15-fold, a phenomenon that scientists have been unable to explain. (See related story: "One Year After Fukushima, Japan Faces Shortages of Energy, Trust.")

Ken Buesseler, a senior scientist as the Woods Hole Oceanographic Institution in Massachusetts who has studied radiation leakage from the Fukushima plant, said he is concerned about the lack of data on levels of strontium-90 in the waters off Fukushima. He said that the groundwater now leaking into the Pacific—including, possibly, some contamination from leaking tanks—might now have much higher levels of that particular substance. Strontium-90 has potentially greater health risks than cesium isotopes because it becomes concentrated in the bones of fish and humans, he said.

'No Time to Waste'

The new problem further escalates the dilemma faced by TEPCO, which already has been struggling to find a way to deal with massive amounts of water contaminated with various radioactive substances at the site. When the company belatedly revealed last month the daily leakage of radioactive groundwater into the Pacific Ocean, a problem that outside scientists have long suspected, public confidence in TEPCO's ability to manage the cleanup threatened to erode further.

The development prompted Japanese government officials to step in and take a more direct role: The government announced last week that it is considering spending 50 billion yen ($410 million) to finance construction of a frozen soil barrier—also known as an ice wall—in an effort to block the groundwater from the plant from reaching the ocean. (See related story: "Can an Ice Wall Stop Radioactive Water Leaks From Fukushima?") That technology has long been used in the mining and construction fields, and reportedly performed well in containing radioactive water in a U.S. government test project in the early 1990s, but has never been used on a large scale at a nuclear power plant.

"This leak is very serious," said Dr. Janette Sherman, an Alexandria, Virginia-based physician who specializes in radioactive and toxic exposure. Dr. Sherman, who edited an in-depth study of health effects on cleanup workers in the 1986 Chernobyl nuclear disaster in the former Soviet Union, said she is concerned that the cleanup crew at Fukushima Daiichi may face long-term health risks. She also raised the prospect of the radiation's as-yet unknown effects on fish and other marine life in the Pacific.

Buesseler said he was concerned that the high level of radiation from the leaking tank might just be a harbinger of what is to come if more of the other temporary tanks begin to fail. But he's even more worried by revelations of leaks and other problems at the plant, which lately have been coming with dismaying frequency. "There is still a lot of contamination at Fukushima—in the land, in the buildings, and now from these tanks," Buesseler said. "Every bit of news that we've been getting is that the [radioactivity] numbers are going up."

"I'm becoming less confident that [TEPCO] can contain the problem," he said.

Japan's Nuclear Regulation Authority shares Buesseler's concern, warning that the latest leakage problem might be beyond TEPCO's ability to cope. "We should assume that what has happened once could happen again, and prepare for more," watchdog chairman Shunichi Tanaka told a news conference, BBC News reported. "We are in a situation where there is no time to waste."