Tokyo Electric Power Co. recently released a more detailed schedule to install a water treatment system at its severely damaged Fukushima Daiichi nuclear plant. The company also released further details on an earlier explosion at unit 4 and revised fuel damage estimates.

Remote-controlled equipment operators at Fukushima Daiichi, Source: TEPCOBloomberg Thursday quoted TEPCO officials as saying a floating barge to store contaminated water at the site will arrive in mid-May. Along with land-based storage containers shipped to the site, the “megafloat” will store irradiated water flooding reactor tunnels and basements that is currently preventing repair work on the units.

A water treatment unit provided by Areva and Kurion is expected to begin removing contaminants from that water in June, the news agency reported. In concert with storage and desalination systems by Toshiba and GE-Hitachi, the treatment system will then feed the water back into the three damaged reactors at the site. Doing so would represent a major step in controlling the reactors, which have been without internal cooling systems since Japan’s March 11 earthquake. Water pumped into the reactors externally to cool them is believed to be leaking from at least one of the units’ containment.

Earlier, TEPCO said temporary storage tanks with 31.4 million liters of capacity will be available by early June, and afterward the company plans to add 20 million additional liters of storage each month.

Crews also continue to pour tons of water into spent-fuel storage tanks at four units. Stored fuel overheated early in the crisis at unit 4, and on Thursday the Daily Yomiuri newspaper reported that TEPCO has a new theory on why that unit’s fuel was not damaged further. According to the newspaper, TEPCO officials said that a hydrogen explosion in the unit March 15 may have damaged a gate holding back water from another part of the unit. The reactor was down for maintenance, with its vessel flooded to facilitate removal of fuel to the adjacent spent fuel tank. Water from other parts of the reactor may have leaked into the spent fuel tank after the explosion, quenching it and reducing damage to the fuel rods. The theory would also explain why water levels in the tank have not risen significantly, even as a rig for pumping concrete has filled it with water continuously for weeks. Water now might be leaking back into other parts of the reactor through the damaged wall.

Estimating the exact damage at each unit has been extremely difficult, as high radiation prevents workers from approaching much of the plant. Earlier this week, TEPCO revised estimates of the percentage of fuel damaged in each of the three units operating at the time of the earthquake. Based on radiation readings taken from the reactors’ containment vessels, TEPCO lowered its estimate of fuel damage in unit 1 to 55 percent. The company increased slightly its estimate of damaged fuel in unit 2 to 35 percent and in unit 3 to 30 percent.

(Photo: TEPCO workers operate remote-controlled heavy equipment at Fukushima Daiichi. Source: TEPCO)