Incurious TEPCO's conclusion for now is that the white pieces floating in the buffer tank and caught by the strainer are the shavings of plastic pipes and they are the cause of the decreased water flow into the reactors.
From Yomiuri Shinbun (9/6/2012):
Decrease in the amount of water injected [into the reactor] caused by shavings from the pipes?
TEPCO announced on September 6 that it was likely that the shavings from the polyethylene pipes were clogging up the valves and pipes and causing the amounts of water injected into the reactors at Fukushima I Nuclear Power Plant to fall below the necessary amounts for cooling.
TEPCO had hinted at the possibility of metal rust clogging the pipes. However, the company thought it important that the problem started to happen after August 30, when the 2.7-kilometer pipes that transport the treated water to the cooling tanks were switched from polyvinyl chloride pipes [probably Kanaflex] to polyethylene pipes (15 centimeter in diameter). The shavings are considered to have been generated when the pipes were being cut. White substances that looks like the shavings have been found inside the [buffer] tank and on the filter of the cooling equipment.
If it is true, I don't know what to say, other than that TEPCO is fast running out of money and quality subcontractors. This is decidedly not the "nuclear plant" spec.