Tokyo Electric Power Co, which operates the stricken Fukushima nuclear power plant, has said a second underground storage pool is leaking radioactive water.

The announcement has concerned critics of the storage strategy that has, since the 2011 earthquake and tsunami ravaged northeastern Japan, sought to contain toxic and radioactive materials.

The Japan Times quotes Masayuki Ono, a Tepco official as saying: “It is the largest amount of radioactive substances that has been leaked” since the crippled facility’s cold shutdown was declared in December 2011.

Japan’s Jiji press agency says that a first pool, No. 2, was found to have leaked 120 tons of highly radioactive water on Friday. The size of the leak at the second pool, No. 3, was confirmed at 3 liters late Sunday.

Masayuki Ono, a senior Tepco official, said at a news conference Sunday that it is difficult for the plant to store all of the radioactive water in the temporary tanks.