TOKYO — Japanese Prime Minister Shinzo Abe has insisted that seafood caught near the crippled Fukushima nuclear plant is safe to eat. Scientists have voiced concerns, however, that radioactive isotopes could accumulate in fish and pose a danger to human health.
Well before dawn on a cool October morning in Soma port, 30 kilometers north of the crippled Fukushima nuclear plant, fishermen prepare their nets and get ready to head out to sea.
Fishing resumed here last month, following the lifting of a ban imposed after it emerged in July that radioactive water had leaked into the ocean.
Buesseler said a bigger concern is the accumulation of isotopes in marine life. Earlier this year, cesium isotopes from Fukushima were found in tuna caught off California.
“The tuna that were caught off San Diego with the Fukushima cesium isotopes, they were 10 to 20 times lower than they had been off Japan. Now the new releases, the leak from the tanks - they’re changing in character. Strontium 90 has become of more concern because it’s a bone-seeking isotope. That will stay in fish much longer,” he said.
Scientists who have been studying the situation were not surprised by the revelation, since radiation levels in the sea around Japan have been holding steady and not falling as they would if the situation were under control. In a 2012 study, Jota Kanda, an oceanographer at Toyko University of Marine Science and Technology, calculated that the plant is leaking 0.3 terabecquerels (trillion becquerels) of cesium-137 per month and a similar amount of cesium-134.
While that number sounds mind-boggling, it’s actually thousands of times less than the level of radioactive contamination that the plant was spewing in the immediate aftermath of the disaster, estimated to be from 5,000 to 15,000 terabecquerels, according to Buesseler. For a comparison, the atomic bomb dropped on Hiroshima released 89 terabecquerels of cesium-137 when it exploded. (See related: "Animals Inherit a Mixed Legacy at Chernobyl.")
Let’s begin by considering the maximum concentration of Cs-137 in seawater reported in the quote above, about 150,000 Bq/m3, near the Japanese coast. Incidentally, Ken Buesseler has told The New York Times that he has received samples of seawater taken in July  from near the plant that contained 10,000 Bq/m3. The corresponding level last year, only months before the disaster, was just 1.5 becquerels. Since there are 1,000 liters of water in a cubic meter and a liter of water has a mass of one kilogram (kg),150,000 Bq/m3 corresponds to 150 Bq/liter (or Bq/kg) of seawater. We further assume that the cesium bio-accumulates in fish flesh such that the concentration of cesium in the fish meat is 100 times the average concentration of cesium in the water there the fish is swimming. This is at the upper end of the range of measured ratios. Consequently, fish meat from fish taken in waters contaminated at 150 Bq/liter will be expected to have cesium-137 concentrations below 15,000 Bq/kg.
Let’s assume we are very risk averse and we want to maintain the risk of getting cancer from eating contaminated fish to less than 1x10-5, that is, the probability of getting a cancer from eating the contaminated fish will be less than one in 100,000. Keep in mind, for an average individual in the United States, the probability of getting cancer is about 40 percent and the probability of dying of cancer is about 20 percent. The U.S. Environmental Protection Agency (USEPA) estimates that for dietary intake, the risk of getting cancer (risk of cancer morbidity) from ingesting food containing cesium-137 is 1x10-9 cancers/Bq of ingested cesium-137 (USEPA, “Health Risks form Low-Level Environmental Exposure to Radionuclides,” Federal Guidance Report No. 13, - Part 1, Interim Version, EPA 402-R-97-014, January 1998, p. 35). That is a risk of one in a billion per Bq of ingested cesium-137.
To keep the risk below 1x10-5 the consumer must limit his/her dietary intake to less than 10,000 Bq of cesium-137. Therefore this risk limit would be reached after eating about 0.7 kg of fish meat. While this is a conservative estimate of what is required to achieve a low risk, one could make a good case for quarantining fishing off the Japanese coast near Fukushima, which of course is what the Japanese government has done.
Near the west coast of the United States the maximum projected concentration is about 30 Bq/m3 some three years after the initial release. This is 5,000 times less than the 150,000 Bq/m3 concentration we have assumed near the Japanese coast. Therefore, to keep the risk below 1x10-5 the consumer must limit his/her dietary intake to less than about 3,000 kg (3 tonnes) of fish. In other words, do not worry about eating fish taken from US coastal waters. Since the concentration projected for waters near the Hawaiian Archipelago are even less than that projected for the West Coast, the same admonition applies to Hawaii.h
Are fish such as tuna that might have been exposed to radiation from Fukushima safe to eat?
Seawater everywhere contains many naturally occurring radionuclides, the most common being polonium-210. As a result, fish caught in the Pacific and elsewhere already have measurable quantities of these substances. Most fish do not migrate far from home, which is why fisheries off Fukushima remain closed.
But some species, such as the Pacific bluefin tuna, can swim long distances and could pick up cesium in their feeding grounds off Japan. However, cesium is a salt taken up by the flesh that will begin to flush out of an exposed fish soon after they enter waters less affected by Fukushima. By the time tuna are caught in the eastern Pacific, cesium levels in their flesh are 10-20 times lower than when they were off Fukushima.
Moreover, the dose from Fukushima cesium is considered insignificant relative to the dose from naturally occurring polonium-210, which was 1000 times higher in fish samples studied, and both of these are much lower relative to other, more common sources, such as dental x-rays.
More about the dose and associated risk (pdf) of radiation from Fukushima to marine life and humans.
a primordial nuclide that decays via double beta decay with an observed half-life of 2.0×1019 years; it can also undergo single beta decay which is not yet observed, but the theoretically predicted value of T½ is 2.4×1020 years. The second most stable radioisotope is 93Zr which has a half-life of 1.53 million years.