TEPCO estimate sees more radiation than NISA's

The Yomiuri Shimbun

Tokyo Electric Power Co. has estimated the total amount of radioactive substances discharged from its Fukushima No. 1 nuclear power plant measured 760,000 terabecquerels, 1.6 times the estimate released by the Economy, Trade and Industry Ministry's Nuclear and Industrial Safety Agency in February.

One terabecquerel is equal to 1 trillion becquerels.

TEPCO will include the estimate in a final report to be compiled by an in-house accident investigation committee in June. The firm has also begun explaining how it arrived at the figure to local governments in Fukushima Prefecture.

There are two ways to estimate the amount of discharged radioactive substances. One way is to base calculations on the degree of damage to the reactor core. The other is to reverse calculate based on the density of radioactive substances found in the atmosphere and seawater. As a result, there will be differences in estimates depending on how the figures were obtained.

NISA released an estimate of 770,000 terabecquerels in June last year, and another estimate of 480,000 terabecquerels in February. The Cabinet Office's Nuclear Safety Commission released an estimate of 570,000 terabecquerels in August last year.

TEPCO combined the two methods and repeated its calculations under different conditions. It reached a final estimate of 400,000 terabecquerels of iodine-131 and 360,000 terabecquerels of cesium-137.

The amount of radioactive substances discharged in the Chernobyl accident in 1986 was 5.2 million terabecquerels.

"As there wasn't enough available data immediately after the disaster, estimates can differ substantially if conditions change, even just a little," said Prof. Hideo Yamazaki at Kinki University, an expert in environmental analysis. "The discharged amount of radioactive substances increased, but the figure is within the assumed margin of error. There will be no problems in continuing decontamination work and other measures."