An apparent miscalculation amid a typhoon caused a storage tank to overflow at the wrecked Fukushima No. 1 power plant, releasing about 430 liters of radioactive water into the Pacific Ocean, Tokyo Electric Power Co. said Thursday.

Authorities are still groping for a solution to the water crisis at the crippled nuclear plant, which is rapidly running out of storage space and facing a growing risk of flooding from typhoons.

Around 8 p.m. Wednesday, plant workers checking a group of five flange-type tanks in a storage sector called B-South discovered water leaking from the cover of a tank that had apparently been overfilled. Because the group of interconnected tanks was built on a slope, the water was coming out of the one farthest downhill — and landing outside the flood containment barrier encircling it.

From there, it apparently drained into the rainwater diversion ditch that leads to the sea.

The water contained strontium and tritium and was emitting beta-ray radiation of 580,000 becquerels per liter, according to Tepco, which was apparently trying to drain rainwater from the flood containment area into the tanks during the typhoon.

Tepco usually releases rainwater that accumulates in the flood containment areas after confirming radiation levels are within the government-set limits. But the typhoon did not give Tepco much time to conduct radiation surveys, prompting it to use the five tanks as a temporary storage measure.

The five tanks, all connected with pipes, were built along a moderate slope. Tepco pumped rainwater into the tanks until the water gauge of the high tank read 98.6 percent full. Tepco said it thought there was enough room left in the low tank to account for the slope’s effect on its water level but miscalculated.

During the previous typhoon in September, about 1,400 tons of rainwater accumulated within the flood-prevention fences guarding the more than 1,000 water tanks at the plant.

The tank that leaked this time has a capacity of about 450 tons but doesn’t have a watertight lid.

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