The temperature of the Fukushima Daiichi unit 4 spent fuel pool has dropped significantly since the circulating cooling system began operation on 31 July. A support structure for the same pool has also been completed.
The alternative cooling system for the spent fuel pools has been designed to pump warm water out of the spent fuel pools, cool it using a heat exchanger, and then return the cooled water back to the pool. TEPCO began utilising this method for cooling of the spent fuel pool at unit 4 at 0:44am on 31 July. The temperature fell from 83C on 20 July to 63C by 31 July, according to the operator's report. TEPCO had previous set up similar systems for the spent fuel pools at units 2 and 3 in May and June, respectively.
TEPCO also reported that it had completed construction of a support for the unit 4 spent fuel pool. TEPCO decided to build the support after a structural evaluation of the reactor building found that the outer wall was heavily damaged. It chose to install the support even though the structural analysis determined that the spent fuel pool could withstand a large seismic force such as the Great East Japan earthquake without the additional supporting structure. TEPCO reported that installation of steel support pillars below the spent fuel pool was completed on 20 June. Concrete pouring was completed on 21 July and grouting was finished on 30 July, finishing the structure, according to a TEPCO report on 1 August.
Finally, TEPCO reported the results from dust samplings that were carried out near the openings of the units 1,2 and 3 reactor buildings between 22 and 24 July. TEPCO used an unmanned remote control helicopter (T-Hawk) to carry out the sampling. The analysis measured the concentration of I-131, Cs-134 and Cs-137 and also measured dose rates. Exposure doses ranged from 0.18-0.47mSV at unit 1; 0.21-0.38mSv at unit 2 and 0.24-0.46mSv at unit 3.
Meanwhile, other developments at the Fukushima Daiichi site have included completing the installation of an additional cesium removal system SARRY(Simplified Active Water Retrieve and Recovery System) on 31 July. TEPCO said it would begin trial operation from 1-5 August, with full scale operation starting later that month.
TEPCO also submitted a report to the Japanese regulator (NISA) on 28 July, which included seismic analysis results for units 1&3 and vibration data from when the 11 March earthquake hit the plants. This report confirmed that the calculated stress did not exceed the technical standard and it is thus presumed that the major equipment linked to the safety operation had been able to maintain the safety functions when the earthquake hit the plants.
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