Water filters expected to last a month accumulated excessive radiation levels after only five hours this weekend at Japan's severely damaged Fukushima Daiichi nuclear plant. After recovering from delays last week, Tokyo Electric Power Co. again suspended operation of the new water filtration system Saturday.

At issue are cartridges that use chemical processes to separate radioactive isotopes like cesium from water accumulating in plant basements. The Asahi Shimbun newspaper quoted TEPCO officials as saying they planned to replace cartridges when their radiation levels reached 4 millisieverts per hour, which was expected to take about a month. On Saturday, though, workers took readings of 3.9 millisieverts per hour and 4.7 millisieverts per hour near cartridge intake valves just hours after full-scale operation of the system began. TEPCO said it was surprised that radiation levels rose so quickly, and that contaminated oil and other residues not removed in an earlier treatment step might be responsible.

The utility has not yet identified its next step in dealing with irradiated water filling the plant's lower levels. High doses present in the equipment now make repairs to the filtration system engineered by Areva, Veolia and Kurion challenging.

Bloomberg quoted a TEPCO spokesman Sunday as saying plans to use treated water to cool reactors would be delayed, but that the utility still expects it will be able to bring units to a cooler, more stable state by mid-July.