About 300 tons of highly radioactive water had leaked from a tank at the damaged Fukushima No. 1 nuclear power plant as of Tuesday afternoon, Tokyo Electric Power Co. said.

Tepco claims none of the water, which had been used to cool its stricken reactors and is highly radioactive, has flowed directly into the Pacific, apparently contained by a 30-cm-high waterproof concrete barrier surrounding dozens of tanks, including the leaking No. 5 tank. But some of the water may have been absorbed into the ground, joining with already tainted groundwater.

Where the leak is occurring in the tank, which is bolted together and has sealed seams, remains elusive, however, even after workers finished pumping water from inside the barrier Tuesday afternoon.

Rain was forecast across much of the Tohoku region, including Fukushima, Tuesday afternoon.

The pump is powerful enough to keep water from flowing over the 30-cm-high barrier fence, even if rainfall is heavy, Tepco executive and spokesman Masayuki Ono said at a news conference, adding: “We apologize again for causing anxiety among the public.”

The Nuclear Regulation Authority released a preliminary assessment of a level 1 incident on the eight-notch international severity scale for nuclear accidents.

The amount of beta rays being emitted by radioactive materials in the leaked water, including strontium, was 80 million becquerels per liter, Tepco said.

At 9:50 a.m. Monday, Tepco workers on patrol found a pool of at least 120 liters of highly contaminated water thought to have escaped from concrete barrier’s drain valves. The valves had been opened to drain rainwater.

The radiation level measured around 50 cm above the toxic water stood at about 100 millisieverts per hour, Tepco said.

Exposure to 100 millisieverts increases the incidence of death by cancer by 0.5 percent, according to the International Commission on Radiological Protection. It is also the legal upper limit for a nuclear worker over five years.

On Tuesday, Tepco said the water level in tank No. 5 had dropped by 3 meters, meaning about 300 tons of contaminated water had been lost. From Monday to Tuesday, about 10 tons were lost, indicating this amount may have leaked every day over the past 30 days, a senior Tepco official told The Japan Times.

“So far, we had four similar (tank) leakage cases. The problem this time is that we didn’t detect it for as long as 30 days,” the official said.

All five leaks were in temporary water tanks made of steel plates bolted together with waterproof packing to seal the seams. In contrast, welded steel tanks are more watertight.

The temporary tanks are supposed to be replaced or repaired every five years.

Tepco has set up more than 1,000 huge above-ground water tanks to hold the ever-increasing amounts of highly contaminated coolant water. It must inject water to cool the melted nuclear fuel inside the damaged reactors.

Of those tanks, 350 are temporary, according to Tepco.

Ono stressed that the cause of the leak has not been determined, adding that Tepco has used temporary tanks of this type for about two years.

It started using the first tanks of this type on Dec. 12, 2011.

A temporary tank can be set up in just over a week. Although Tepco acknowledges welded steel tanks are more robust, it plans to set up more temporary tanks to contain the contaminated water at the plant.