23/04 Sea leaks would be rated Level 5 or 6

The Yomiuri Shimbun

If estimates of leaks of radioactive material into the sea from Tokyo Electric Power Co.'s stricken Fukushima No. 1 nuclear power plant were judged according to the International Nuclear Event Scale, the severity of marine contamination would be rated Level 5 or 6.

The Three Mile Island nuclear accident in the United States was rated Level 5 on the scale introduced by the International Atomic Energy Agency that runs from zero to seven and is based on radiation leaks into the atmosphere.

The radiation leaks into the sea have caused serious contamination of the marine environment, TEPCO's estimates show. Water that leaked into the Pacific Ocean from the No. 2 reactor's water intake contained an estimated 4,700 terabecquerels of radioactive substances, TEPCO announced Thursday.

On April 4-5, the company took the emergency action of discharging 10,000 tons of relatively low-level radioactive water from a fuel waste disposal facility, causing an international stir.

The estimated total amount leaked, 4,700 terabecquerels, is 30,000 times the amount of radioactive material contained in the water discharged on April 4-5.

It is also about 20,000 times the amount of radioactive substances the plant is legally permitted to release into the environment in a year. One terabecquerel equals 1 trillion becquerels.

On April 12, the government's Nuclear and Industrial Safety Agency raised the severity level of the ongoing crisis to Level 7, citing the estimated 370,000 terabecquerels of radiation released into the air from the reactors.

TEPCO has said leakage into the sea started April 1, but records show radiation levels in seawater peaked on March 31. It is therefore highly likely that radioactive water leaked into the ocean before April 1.

Jun Misono, a senior researcher of the Marine Ecology Research Institute, said: "At the moment, [radiation] levels higher than government-set limits have been detected in small fish that eat plankton. Tests should also be conducted on various other marine species."

In the past, accidents outside Japan have resulted in sea contamination. At the Sellafield nuclear facility in Britain, massive amounts of radioactive substances were discharged into the sea for 30 years until the 1980s. The total amount remains unknown, but according to a Norwegian research institute, the leak peaked in 1975 with radiation of about 9,000 terabecquerels.

(Apr. 23, 2011)