The radioactive water purifying unit at the Fukushima No. 1 nuclear power plant operated at 53 percent of capacity during the seven-day period through July 19.
The radioactive water purifying unit at the Fukushima No. 1 nuclear power plant operated at 53 percent of capacity during the seven-day period through July 19.
The utilization rate, down 20 percentage points from the previous week, fell short of Tokyo Electric Power Co.'s 70-percent target, the utility said July 20.
The cause of the slowdown is under investigation. Sludge and rust sticking to pipes may be obstructing water flow, TEPCO said.
TEPCO said it treated 4,510 tons of highly radioactive water during the latest week, down 1,620 tons from the preceding week.
The plant was halted temporarily for repairs after water leaked from pipes at a treatment unit manufactured by France's Areva SA.
About 95,000 tons of radioactive water is currently housed in reactor and turbine buildings for the No. 1 through No. 4 reactors.
An additional 22,000 tons have been transferred from the buildings to a central waste treatment plant, making up a total of 117,000 tons currently at the plant.
Another 100,000 tons are expected to be added to the total, meaning about 200,000 tons will have to be treated. TEPCO said there was no change in the plan to treat the 200,000 tons by the end of the year.