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» Interview to the chief researcher at the Meteorological Research Institute on the contamination of the ocean

Radioactive substances from the Fukushima No. 1 nuclear plant will dilute to negligible levels in the ocean, but the area close to the site remains a problem, the chief researcher at the Meteorological Research Institute said at the Scientific Forum of the International Atomic Energy Agency (IAEA) in Vienna in September.

The following is the interview to Mr. Michio Aoyama by Asahi newspapwer about his findings on the circulation of radioactive substances released into the sea.

* * *

Question: Is contaminated water still leaking into the sea?

Aoyama: I’ve pointed out that the leakage has continued since just after the accident, when the concentration of radioactive substances began declining at a slower rate. You would expect the concentration to decrease in the waters right by the plant if there were no leaks, but the monitoring results show that is not the case.

Q: Why is that?

A: At present, the concentration of radiation in waters very close by is holding at a certain level. It is believed that new radioactive water is leaking in from the land and replacing what is flowing out to the open sea. Estimates suggest there are 30 gigabecquerels of cesium-137 and 30 gigabecquerels of strontium-90 flowing out to the ocean each day. (One gigabecquerel equals 1 billion becquerels). This is a negligible amount when you consider the Pacific Ocean as a whole.

Other than in the area right next to the nuclear power plant, the radiation should become diluted and have no effect on fish.

Q: Around how much cesium-137 has the accident released into the open sea?

A: Twelve to 15 petabecquerels (one petabecquerel equals 1,000 trillion becquerels) of cesium-137 released into the atmosphere fell into the Pacific Ocean via rainfall. It is believed that an additional 3.5 petabecquerels flowed into the ocean when water used to cool the melting fuel rods immediately after the accident was contaminated and released.

Q: What can we learn by comparing the nuclear accident with the spread of radioactive substances caused by nuclear weapons testing?

A: In 1970, a total of 290 petabecquerels of cesium-137 was released in the North Pacific due to nuclear weapons testing. The level was 69 petabecquerels in 2011, after the cesium-137 decayed over its 30-year half-life and some of it flowed into other waters.

In comparison, the amount released due to the Fukushima accident is around 15 petabecquerels.

There were 1 to 2 becquerels of cesium per cubic meter in the seas off Japan before the accident, but the level rose, on average, by 20 percent immediately afterward.

However, the additional cesium has not had a significant effect on the vast waters, and the concentration in the open ocean is returning to pre-accident levels.

Q: What will happen to the radioactive substances?

A: Ninety-nine percent of fission products produced by large-scale nuclear weapons testing rose into the stratosphere and got carried around by the winds. But some have fallen in large amounts in the ocean east of the Japanese archipelago.

Following that cesium, we have learned that they proceed eastward into the Pacific Ocean and sink into deep waters before reaching the North American continent. They then turn south and head into the Indian Ocean, the South Pacific or return to the Japanese archipelago.

Surveys conducted after the Fukushima accident have shown the same phenomenon. I believe the radioactive substances will follow a similar pattern of behavior in the future as well.
<Media Report>
POINT OF VIEW/ Michio Aoyama: Fukushima radiation circulating but diluting in Pacific (Asahi Newspaper)

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