On Saturday Tepco estimated about 120 tons of radioactive water with an estimated 710 billion becquerels of radioactivity had already escaped the No. 2 underground tank, the Japan Times reported.

Tepco said it is working to transfer the water from the leaking tanks to other tanks onsite, but warned some 47 tons of the highly irradiated water may also leak out before the task is completed.

The tank in question held processed water that had been used to cool down the plant’s stricken reactors. Able to hold about 14,000 tons of water, the tank, which Tepco began using to store contaminated water in February, almost reached full capacity last month.

“It is the largest amount of radioactive substances that has been leaked” since the crippled facility’s cold shutdown was declared in December 2011, Tepco official Masayuki Ono told the paper.

The utility believes the radioactive water may have leaked out through a joint in the tank’s seepage control sheets. Although much of the cesium had been removed, the water was still tainted with other dangerous radioactive substances.

The tank measures around 60 meters long, 53 meters wide and 6 meters deep and is covered by three layers of waterproof sheeting.

As much as 3 liters of radioactive water may have also escaped the No. 3 tank, according to Bloomberg.

Leaks the latest setback

The leaks are the latest stumble in efforts to stabilize the plant after the March 2011 earthquake and tsunami that caused the worst nuclear crisis in 25 years.

“This is unfortunately a situation that will continue for the next couple of years,” said Nils Bøhmer, Bellona’s general manager and nuclear physicist, who last month visited Japan with other Bellona staff. “The facilities at Fukushima Daiichi are in such bad shape and so damaged that I expect to keep seeing similar events for a very long time.”

The weekend also saw a three-hour shut-down of coolant to the plant’s No 3 spent nuclear fuel pond, the second such shut down to have occurred in weeks.  Late last month fresh water to spent nuclear fuel storage pond Nos 1, 3 and 4, as well as the plant’s common pond, ceased for several hours after a rat apparently short-circuited a switch board.

The shut down to pond No 3 on Friday took place when workers were trying to secure a switchboard from intrusion by rodents.

Japan’s Nuclear Regulation Authority yesterday identified the possibility of the second leak. Tepco plans to transfer some of the contaminated water from the No. 3 tank to determine whether the leak stops, Tepco said on its website.

Tepco has built almost 1,000 tanks to hold water tainted in the process of cooling the reactors. The utility has had several radioactive water leaks at the Fukushima Daiichi plant, raising concerns about radiation effects on health and the environment.

Tepco found the first leak from the No 2 underground tank on the night of April 5, it said in a release the next day. The utility estimates it will complete transferring the remaining water stored in the No. 2 tank to other underground tanks by April 11, statements on Tepco’s website said.

About 276,000 tons of highly radioactive water is stored in tanks at the Fukushima Dai-ichi plant, according to Tepco’s latest data.