Five Years After Fukushima, What Is It Really Doing To The Oceans?

The Fukushima daicii nuclear power planet after being hit by a tsunami. Despite ongoing leakage, radiation levels nearby are falling. Digital Globe. CC BY-SA 3.0 via wikimedia commons

Spend too much time in the wrong parts of the Internet and you'll find claims the Fukushima nuclear disaster is turning the entire Pacific Ocean into a dead zone. A review of the evidence collected presents a far more rosy picture, as long as you stay away from Fukushima harbor.

After the Tohoku tsunami struck Japan in March 2011, large amounts of radioactive material was vented from the Fukushima Daiichi nuclear reactors. Of this, 80 percent is thought to have fallen in the Pacific Ocean, while some of the material that fell on land was subsequently washed out to sea in heavy floods. Moreover, direct discharges from the plant to the ocean have added about a third of the original release.

The main source of radiation released by Fukushima is cesium-137, other radioactive isotopes were either too rare to pose a major threat from the start, or, like iodine-131, have such a short half-life they have ceased to be an issue. According to a paper published in the Annual Review of Marine Science, the amount released was similar to that released by the UK's Sellafield, so it would be strange indeed if it was having apocalyptic effects on a planetary scale. Moreover, the report notes this is about one-fiftieth of that released by all the atmospheric nuclear tests conducted in the mid-20th century combined and about one-fifth the amount released by the Chernobyl accident of 1986.

Most estimates place the cesium-137 released to equal 15 to 25 x 1015 Becquerel, (a Becquerel, Bq, is one nuclear decay per second) although the paper notes, “All of these estimates are limited by the shortage of vertical concentration profiles [measurements at depth] and spatial coverage,”

Seen the image supposedly of Fukushima's radiation, but actually of the height of the tsunami? Here is what is really happening. Buesseler et al/Annual Review of Marine Science

Such figures sound large, and indeed would be if the radiation was contained in a small area. However, the Pacific Ocean is enormous, and even with most of the effects concentrated in the area around Japan, the radiation has been greatly diluted. Measurements taken offshore from April to July 2011 observed a 50 percent decrease in cesium-137 at the ocean surface every seven days. Since cesium-137 has a 30-year half-life this reduction represents the radioactive atoms sinking or being dispersed, rather than decay.

In early April 2011, 68 million Becquerels per cubic meter was recorded not far offshore of the power plant, but this plunged by a factor of a thousand within a month. Despite continued leaks, radiation nearby has continued to fall as dispersion outpaces arrivals.

Cesium from Fukushima was first detected off the coast of Canada in June 2013. Measurements in the eastern Pacific are still rising, but the highest found so far – 10 Bq/m3 – remains far below levels considered unsafe.

Where things are serious is in the marine life off the coast of Japan. Sampling of fish and invertebrates has been done both by the Japanese authorities and some independent studies.

Prior to the disaster, Japan considered fish with radioactivity levels of more than 500 Becquerels per kilogram to be unsafe, but this was lowered to 100Bq/kg as a result of public concerns after the disaster. “In 2011 approximately half the fish sampled in coastal waters off Fukushima Prefecture had radiocesium levels above 100 Bq/kg,” the paper reports. By 2015 only 1 percent exceeded this level.

Nevertheless, fish caught within Fukushima harbor remain highly radioactive, which the authors attribute to a combination of cesium trapped in sediments on the harbor floor and continued leakage from the reactors.

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