TEPCO will use tubs for detailed survey of radiation leak

Tokyo Electric Power Co. is making a detailed survey of the rate of discharge of radioactive substances from its damaged Fukushima No. 1 nuclear power plant, with the help of some old-fashioned tubs filled with water.

TEPCO plans to install the tubs at 11 sites, one on the premises of the nuclear plant and 10 more in the vicinity, to help review the concentrations of radioactive fallout.

TEPCO said the rate of discharge has so far been estimated at 200 million becquerels per hour, but the measurements that provided a basis for this estimate may have included radioactive materials that became airborne after falling to the ground.

Suppressing the additional discharge of radioactive substances is one of the goals of Step 2 in the road map developed by TEPCO and the central government toward ending the nuclear crisis, which they plan to accomplish between October and January.

The goal is defined as reducing the exposure to radioactive fallout along the outer boundary of the grounds of the nuclear plant to below 1 millisievert per year.

In July and August, TEPCO measured the concentrations of radioactive materials near the plant's west gate, which is about 1 kilometer from the nuclear reactors. On the basis of those concentrations, TEPCO estimated the hourly discharge at 200 million becquerels, one ten-millionth of the corresponding discharge rate at the onset of the crisis and one-fifth of the measurements as of late June. This corresponds to an annual exposure of 0.4 millisievert, TEPCO said.

TEPCO, however, decided to conduct a more detailed survey to review this estimate, saying that it may be an overestimation.

The water tubs are to be installed on platforms 90 centimeters above the ground or on building rooftops. TEPCO has already installed one on the premises of the plant, and is in the process of installing 10 more at distances of 5-10 kilometers from the plant across the municipalities of Tomioka, Namie, Futaba and Okuma. Eight were installed by Aug. 22.

Besides the water tub stations, concentrations of radioactive substances are also being measured at 12 new stations, including in the air above the No. 1 through No. 3 reactor buildings, inside containment vessels and in the vapor rising from spent fuel storage pools. A precise estimate of the discharge rate will be available by the time the road map is reviewed in September.

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