A leak was found on a joint of PVC pipes that transport the treated water.
In earlier Yomiuri article, 4 kilometer long PVC pipes feed the treated water to the reactors, and TEPCO people were worried about the leak all along.
I guess they were too preoccupied with Kurion's zeolite vessels (which require frequent change and system downtime) and didn't have time to test the 4 kilometer long network of PVC pipes.
From Yomiuri Shinbun (1:31AM JST 6/28/2011):
東京電力は２７日午後４時２０分、福島第一原子力発電所の原子炉から漏れ出した高濃度汚染水を浄化し、冷却水として再利用する「循環注水冷却」を開始した。
TEPCO started the cooling of the reactors using the water treated by the contaminated water treatment system at 4:20PM JST on June 27.しかし、注水用配管の継ぎ目に水漏れが見つかり、約１時間半後に原子炉への注水を停止した。２８日早朝から原因調査と対策を進める。
However, a water leak from the pipe joint of a water feeding pipe was discovered, and the water injection into the reactors were stopped after one and a half hour. TEPCO will investigate in the morning of June 28.
東電によると、建屋地下などにたまった汚染水を浄化する処理システムは、稼働を続けている。原子炉への注水は、従来のタンクからの配管に切り替えて継続しており、問題ないという。
According to TEPCO, the contaminated water treatment system itself continues to operate. Cooling of the reactors is unaffected, as they are using the water from the storage tanks as before.
水漏れがあった配管はポリ塩化ビニール製で直径が約１０センチ。原子炉への注水ポンプに真水を送る配管で、流量調整に向かった作業員が漏出を見つけた。東電は、漏れた水は処理済みのため、作業への影響はないとしている。
A leak was found on a 10-centimeter diameter PVC pipe. It is a pipe that feeds fresh water to the pump that pumps water into the reactors. A worker who went to adjust the flow rate found the leak. TEPCO says the leaked water was treated, and there is no effect on the work at the plant.
To clarify, this "fresh water" was made "fresh" by going through the 4-step contaminated water treatment system.