Around 1.1 tons of highly radioactive water overflowed from a waste container at the Fukushima No. 1 nuclear complex while the experimental ALPS radiation-filtering system was being cleaned, Tokyo Electric Power Co. has reported.

The overflow at the trouble-plagued water treatment system was noticed at about 12:20 p.m. Wednesday, and no one was contaminated, Tepco said. The water was retained by a barrier and inside the building where the Advanced Liquid Processing System is housed, it said.

The water was giving off around 3.8 million becquerels of beta-particle-emitting substances per liter, Tepco said.

The overflow occurred when an absorbent material used by ALPS to remove such substances, including hazardous strontium-90, was being flushed into a waste container.

The apparatus was being cleaned after one of its three lines started having trouble lowering the radiation in the cooling water to the level required.

Tepco developed the ALPS system to drastically reduce radiation in the vast amounts of water it generates by cooling the melted fuel in its three leaky reactors each day.

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