Tokyo Electric Power Co. said Saturday that a cracked storage pit at the Fukushima No. 1 power plant was the source of a radioactive water leak contaminating the ocean and that it is attempting to fill it with concrete.

According to the utility and the Nuclear and Industrial Safety Agency, the square, concrete-covered pit is situated near an intake used to pump seawater into reactor No. 2.

Although the pit is small, it contains highly contaminated water with a radioactivity exceeding 1,000 millisieverts per hour that is leaking into the ocean from a 20-cm crack, Tepco said.

The pit, which is 1.2 meters x 1.9 meters and 2 meters deep, is usually used to store cables. But it is also connected directly to the reactor building through a cable trench, raising the possibility that the source of the contaminated water is the reactor itself, a NISA official said.

The cable trench is different from the pipe trench at No. 2, where water with the same level of radioactivity was discovered Monday. Although the two trenches are connected, no water has been found in the cable trench because it is at a higher elevation, the official said.

How much water has leaked and for how long were not known as of Saturday afternoon.