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.

NISA spokesman Hidehiko Nishiyama said Tepco has been told to make sure there are no other leaks near reactor No. 2 and to strengthen is monitoring of seawater.

Earlier Saturday, NISA said seawater tested near the power plant contained nearly twice the legal limit for radioactive iodine but presented no “immediate” health risk.

According to a March 30 sample taken by the technology ministry, seawater tested about 40 km south of the plant contained 79.4 becquerels per liter of iodine-131, compared with the legal limit of 40 becquerels per liter.

This number shows that the highly contaminated water apparently draining from the plant has spread.

Nevertheless, Nishiyama said the radioactive material has dispersed and gave more assurances that it was not an “immediate” danger to the public.

“As for the high-level number, it is our understanding the water rode the tide toward the south,” Nishiyama said. “We don’t think there are any risks even if people eat the fish . . . but we will continue to observe the situation carefully.”

Despite his assurances, however, he didn’t provide estimates on radioactivity levels that actually would affect human health.

Meanwhile, radiation levels in Tokyo and Chiba, Ibaraki and Miyagi prefectures remain elevated, according to daily readings from the technology ministry. NISA said the numbers were on the decline.

In Tokyo, for example, environmental radioactivity was measured at a harmless 0.098 microsieverts per hour on April 1, compared with the capital’s average range of 0.028 to 0.079 microsieverts per hour.

“Although the levels are higher than past average numbers, you can see that the numbers are roughly declining,” Nishiyama said.

As for Tepco’s latest botched readings for contaminated water at the Fukushima plant, Nishiyama said the utility was reviewing all it past data releases to make sure no other mistakes were made.

Tepco has been scrambling since Friday to review past analyses after finding errors in its tellurium calculations.

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