While TEPCO still doesn't know exactly what is causing the decrease in the amount of water being injected into the reactors to (supposedly) cool the reactors (see my previous post), the company has come up with the possible culprits: rust and plankton.
Yes, plankton. Why? Because the water being injected into the Reactor Pressure Vessels of Reactors 1, 2 and 3 at Fukushima I Nuclear Power Plant is not just the treated water after decontamination and desalination. The treated water is mixed with the filtered river water. Clearly, plankton escapes the filter.
From Jiji Tsushin (9/3/2012):
Fukushima I Nuke Plant - Foreign matters in the water for the reactor may be causing the decrease in the water flow
Concerning the frequent problem of decreasing amount of water injected into the reactors for cooling at Fukushima I Nuclear Power Plant, TEPCO announced on September 3 that foreign matters in dark brown color were collected after they took the water from one of the storage tanks that feed the [treated] water to the pumps for water injection and filtered it. The company thinks it is possible that rust and plankton in the water are clogging the pipes, and will conduct the analysis of the foreign matters and clean out the tanks.
I remember reading the tweets from last year by one of the workers who tweet from the plant that the water treatment system (Kurion, AREVA at that time) was being clogged up with jello-like substance because the water being treated was in part filtered river water.
By the way, TEPCO, after consulting the regulatory agency no doubt, has lowered the minimum amount of water necessary for each reactor, so that the continued operation of the plant does not constitute a deviation from the safety regulations set by NISA.
From TEPCO's handout for the press (Japanese) on September 3, 2012:
The minimum amount of water per hour to be injected into the RPV:
Reactor 1: from 4.3 tonnes/hour to 3.8 tonnes/hour
Reactor 2: from 6.1 tonnes/hour to 5.4 tonnes/hour
Reactor 3: from 6.1 tonnes/hour to 5.4 tonnes/hour
There has been hardly any change in temperature at the bottom of the RPV in each reactor anyway. I'm curious to know what would happen if they completely stopped water injection.
There are two lines in the water injection system: CS (core spray) line and FDW (feedwater) line. The one with the greater fluctuation seems to be the FDW line, looking at TEPCO's data on September 3, 2012.