|Fig. 1: Fukushima Nuclear Power Plant aerial view post-explosion. (Source: Wikimedia Commons)|
The Fukushima Nuclear Power Plant (FNPP) accident of March 2011 is particularly notable in the history of nuclear accidents due to its close relationship with the ocean. A tsunami incited most of the damage following the earthquake and the ocean bore a significant portion of the nuclear fallout. We will review here the incidents at Fukushima from the ocean's perspective, highlighting the short-term risks, the long-term resilience of the ocean, and long-lasting impacts of the accident on our understanding of the ocean and its resources.
The Great Tohoku earthquake (M=9.0) of March 11, 2011 triggered an extremely large tsunami off of the coast of Fukushima Prefecture in Japan.  The tsunami easily washed over the seawalls of the FNPP, disabling the plant and its backup diesel generators. Without power, cooling pumps could not be run to remove the residual heat produced by fuel rods in the six reactors and spent fuel pools.  Although the immediate cause has not been conclusively reported, reactor units 1, 3, and 4 exploded in the following days.  The accident was classified as level 7 on the International Nuclear and Radiological Event Scale and worldwide attention turned to investigating health and environmental impacts. 
It was fortunate for Japan that the prevailing westerly winds of spring transported much of the airborne fallout toward the ocean. Subsequent rain, dry atmospheric deposition and direct discharge from the plant into the ocean resulted in approximately 80% of the total fallout entering the ocean.  Although terrestrial fallout posed the greatest immediate danger to Japan, contaminated seafood is also an indisputable risk. Ocean resources are indisputable to the island country and it comes as no surprise that there is great consumer concern over radionuclide levels in fish. Sea animals concentrate certain trace substances from their environment in a phenomenon called bioaccumulation, so even relatively low levels of radionuclides in sea water can be magnified to harmful levels within sea food. In particular, fish have been found to concentrate cesium by a factor of about 100 from seawater.  Thus, seawater radionuclide concentrations and distribution patterns are immensely relevant to fisheries.
Cs-134 and Cs-137 been particularly notorious radionuclides in post-Fukushima marine studies because of their health risk and the high solubility of Cs-134 in water.  The radioactivity ratios of these two isotopes has been used to distinguish between different types of nuclear fallout (i.e. nuclear weapons versus reactors), and researchers have found a characteristic Cs-134/Cs-137 ratio of almost exactly 1 in samples from Fukushima.  This indicator has bolstered confidence that the cesium in seawater samples taken across the Pacific originated from Fukushima, since the half life of Cs-134 is just two years.
Cesium timeseries in seawater across several studies have shown that radioactivity levels have decayed to safe levels within six months, even in the immediate vicinity of the FNPP. [5,6] Further across the Pacific, ocean mixing lowered radioactivity levels far lower than the near-field values.  What is fascinating, however, is that as late as December, 2012, a fish has been sampled at FNPP with 254,000 Bq/kg.  This is far above the Japanese regulatory limit of 100 Bq/kg, suggesting that there must be a contamination source that is still releasing radioactive materials.  Whether or not the FNPP is still actively leaking is unclear; even if the plant is no longer leaking, further study of radioactive fluxes in the environment and food chain are necessary.
In the midst of the vast negative impacts of radioactive fallout on the people and marine economy of Japan, there is one way to take advantage of dispersed radionuclides in a constructive way: using them as scientific tracers. As mentioned earlier, the short half life and water solubility of Cesium-134 make it an attractive tag for tracking the migration of sea life and ocean circulation. 
© Kevin Mori. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 M. Honda et al., "Dispersion of Artificial Caesium-134 and -137 in the Western North Pacific One Month After the Fukushima Accident," Geochem. J. 46, e1 (2012).
 M. Prigg, "Fish Caught Near Crippled Japanese N-Plant With 2,500 Times the Legal Limit of Radioactivity for Human Consumption," Daily Mail, 21 Jan 13.