TOKYO -- Three hundred tons of highly contaminated water have leaked from a storage tank at the ravaged Fukushima Daiichi Nuclear Power Plant on Japan's Pacific Coast, its operator said on Tuesday, raising further concerns over the site's safety and prompting regulators to declare a radiological release incident for the first time since disaster struck there in 2011.
Workers raced to place sandbags around the leak at the site to stem the spread of the water, a task made more urgent by a forecast of heavy rain for the Fukushima region later in the day. A spokesman at Tokyo Electric Power, the plant's operator, acknowledged that much of the contaminated water had seeped into the soil and could eventually reach the ocean, adding to the tons of radioactive fluids that have already leaked into the sea from the troubled plant.
The leaked water contains levels of radioactive cesium and strontium many hundreds of times higher than legal safety limits, Tokyo Electric said. Exposure to either element is known to increase the risk of cancer.
The company said it had not determined the source of the leak.
"We must prevent the contaminated water from dispersing further due to rain and are piling up more sandbags," said Masayuki Ono, a spokesman for the operator, also known as Tepco. But he also said much of the water has been absorbed into the soil, and workers would need to try to remove some of the soil using shovel cars and other heavy machinery.
Tepco has acknowledged in recent weeks that leaks of radioactive runoff at the site, about 150 miles north of Tokyo, are at crisis levels. The runoff comes from cooling water that workers are pumping into the damaged cores of the site's three most damaged reactors, as well as from groundwater pouring into the breached basements of those reactors.
Some of that runoff has been seeping into the ocean since the accident at the site in 2011, triggered by a powerful earthquake and a 14-meter tsunami. To reduce the leaks, Tepco has started pumping out some of the contaminated water and storing it in almost 1,000 large tanks it has built on the debris-strewn site.
Tepco hopes to start cleansing the water using an elaborate filtering system and start releasing low-level contaminated water into the ocean. Those plans have been delayed by technical problems and protests from local fishermen.
Desperate for options, Japan's nuclear regulator has suggested surrounding the plant with a huge underground ice wall to stem any leaks. That plan has its own drawbacks, however, and would require huge amounts of electricity almost indefinitely.
The latest leak comes from one of the site's 1,000 tanks, about 500 yards inland, Tepco said. Workers discovered puddles of radioactive water near the tank on Monday. Further checks revealed that the 1,000-ton capacity vessel, thought to be nearly full, only contained 700 tons, with the remainder having almost certainly leaked out.
There had been concerns raised among some experts over the durability of the tanks. Mr. Ono said that Tepco had assumed the tanks would last at least five years, but the latest leak comes less than two years after the company started installing the storage vessels at the site to deal with the growing amounts of runoff.
"It is going to be very difficult and dangerous for Tepco to keep on storing all this water," said Hiroshi Miyano, an expert in nuclear system design at Hosei University in Tokyo. He said, for example, that another strong earthquake or tsunami could destroy the tanks and lead to a huge spill.
At some point, Tepco will have no choice but to start releasing some of the water into the ocean after cleaning it, Dr. Miyano said. The continued mishaps at the site have heightened public scrutiny of Tepco and made it more difficult to build public consensus around any release of water, he said.
"That just makes the problem worse, with no viable solution," he said.
The Nuclear Regulation Authority described the leak as a Level 1 incident, the lowest level, on a global scale that rates radiological releases. This was the first time that Japan had declared a radiological event since earthquake and tsunami in March 2011, which was rated at Level 7, the highest on that scale and on par with the 1986 accident at Chernobyl.
In a statement, the regulator ordered Tepco to do its utmost to identify the exact source of the leak, to step up radiation monitoring at the site and to remove contaminated soil. Tepco said it would do its best to comply.
Makiko Inoue contributed reporting from Tokyo.
This article originally appeared in The New York Times.