TEPCO workers have completed a steel pipe sheet pile wall at the southern end of the water intake canal of Fukushima Daiichi.
The wall was damaged in the 11 March tsunami, and the new barrier helps isolate the plant from the sea by preventing radioactive water leaking out of the canal. There have been several documented cases of contaminated water leaks from the plant into the intake canal. The wall supplements other protection and mitigation measures taken in the intake canal to reduce the spread of contaminated water, including installation of concrete panels to seal off reactor openings, floating booms and powered decontamination filtration of the intake water.
In other news, TEPCO continues preparatory works to install primary containment vessel gas control systems to mitigate the risk of hydrogen explosions in units 1, 2 and 3. Although TEPCO's position is that the inert nitrogen gas that has been injected for months in the reactors should mitigate the risk of explosion, it is still planning to install the system for safety's sake.
In unit 1, gas concentrations were measured in the containment cooling system piping. According to TEPCO, hydrogen explosions only occur when hydrogen gas concentrations are above 4%, oxygen gas concentrations are above 5% and there is an ignition source. The survey found a hydrogen gas concentration of 63% and oxygen gas concentration of 0%. From this, TEPCO concludes that there is no risk of explosion. It is now determining how to purge the hydrogen gas. In units 2&3, a survey by remote-controlled robots of radiation and internal conditions has found that installing the gas system would be feasible.
TEPCO reported on the status of contaminated water treatment. So far 83,930 tons of water has been treated.
In other news, TEPCO workers tested a plan to install a cover for the unit 2 and 3 spent fuel pools to protect them from debris falling from above. Now they have started work to install them. TEPCO plans to install 400 air-filled sealed plastic floats in each pond. Eight 50cm square floats will support a 3mm-thick steel sheet. A drawing shows that the floats will be raised on a crane platform to a point slightly higher than the edge of the spent fuel pool, and roll down a feeding ramp to the pool. Some parts of the water will be left uncovered for monitoring purposes.
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