The pumps at AREVA's portion of the water treatment system at Fukushima I Nuclear Power Plant shut down automatically when too much water went through them. TEPCO has been conducting the test run after the Kurion's problem earlier.
AREVA's subsystem comes after Kurion's (cesium absorption) which in turn comes after Toshiba's (oil separation). After AREVA's subsystem, the water goes to Hitachi's for desalination.
It was the 6th time since the test run started on June 10 that the system had stopped, as the following Yomiuri article says.
From Yomiuri Shinbun (1:06PM JST 6/21/2011):
TEPCO announced on June 21 that the water treatment system for highly contaminated water at Fukushima I Nuclear Power Plant had an emergency shut down at 7:20AM. TEPCO was running the test run in order to resume the full-scale operation.
The problem was at AREVA's subsystem for decontamination. Two pumps for the chemical injection line that connects to the main line for the contaminated water were shut down automatically. TEPCO adjusted the pumps, and the test run resumed in the afternoon. It was the 6th time since the test run started on June 10 that the system had stopped because of troubles and malfunctions.
The pumps feed water to the chemical injection line for dilution. They seem to have stopped when too much water went through. TEPCO will adjust the amount of water. The water treatment system is a combination of multiple subsystems, and AREVA's subsystem mixes chemicals and silica to remove cesium and strontium.