A TEPCO investigation into a slow leak affecting the Fukushima Daiichi unit 4 spent fuel pool alternative cooling system has found a likely cause. Corrosion was found on a weld between a flexible and fixed hose connection. TEPCO's preliminary analysis concludes that the corrosion was most likely caused by the high temperature of the water at the start (80°C) and high chloride (salt) content of the water (2000 ppm). A destructive test using penetrant testing and electromicrosopy revealed the hole.
The test is important because the same hoses are being used for a new alternative cooling process for unit 3; TEPCO switched from water injection from the feedwater system, which cools the reactor from the bottom, to using the core spray line, which cools the core from above. Because the injected water has a lower temperature (40°C) and a lower chloride content (20 ppm), TEPCO concluded that the core spray system would not be as likely to leak. As a result, it started up core spray on 1 September. Monitors at the site have so far not picked up any increased radiation as a result of the change in unit 3 core cooling.
TEPCO has revised the decontamination factor of the second caesium decontamination line in the floodwater treatment system. The previous figure it published, 10,000, was found to be lower than the current operational decontamination factor, estimated at greater than 370,000 times for Cs-134 and 480,000 times for Cs-137. The decontamination factor is the density of radionuclides in sample before treatment/density of radionuclides in sample after treatment.
