Release, Dispersion and Fate of Radioactive Strontium From Fukushima in the Northwest Pacific Ocean

By Jay T. Cullen

The purpose of this diary is to summarize recent models and measurements of the release of strontium-90 (90-Sr, half life 28.8 yr) to the ocean resulting from the triple meltdowns at the Fukushima-Daiichi nuclear power plant in March 2011. This post is part of an ongoing series aimed at understanding the impact of the disaster on the North Pacific Ocean and residents of the west coast of North America. 90-Sr is a beta-emitting element that is a radiological health concern given its relatively long half life and similar chemistry to the nutrient calcium (Ca). Previous peer-reviewed work indicate that releases of 90-Sr were about 30-10,000 fold less than 137-Cs and similar to the release of 90-Sr from the Chernobyl disaster in 1986 and about 600-fold lower than the releases from atmospheric weapons tests that peaked in the mid-1960’s. Given maximal release rates after the disaster, modeled activities of 90-Sr in the marine foodweb and in fish that accounts for bioconcentration and accumulation predict maximal dose rates from Fukushima to human consumers three orders of magnitude less than doses owing to the presence of 137-Cs in marine products and thus well below maximum dose limits thought to be detrimental to public health.

A 3D numerical model designed to track the dispersion and fate of 90-Sr in the waters and biota of the northwest Pacific Ocean was published by Maderich and colleagues in the peer-reviewed journal Science of the Total Environment. The authors used a dynamic model including the marine food chain to assess the fate of 90-Sr in the northwest Pacific from 1945-2010 and the radiological health risk from Fukushima through marine 90-Sr exposure pathways from 2011-2040. The model is designed to predict the dispersion of 90-Sr derived radioactivity in the water, sediments and the transfer of the isotope through the marine foodweb resulting in doses to humans through the consumption of marine products. The model accounts for transfer of 90-Sr from the terrestrial environment to the ocean over time and tracks the transfer of the isotope from phytoplankton, zooplankton, molluscs, crustaceans to fish as shown schematically in the following figure.

Schematic of radionuclide transfer in the Maderich et al. (2014) model.

The model domain in the northwest Pacific is shown in the following figure which identifies numbered model compartments.

The compartment system. Shaded boxes represent the deep water boxes (> 1000 m). The compartments representing estuaries of large rivers (174 — the Chang Jiang River, 173 — the Huanghe River and 175 — the Han River) are shown by arrows with numbers of compartments. The location of NPPs are indicated by filled circles. Letter “F” represents the Fukushima Dai-ichi NPP. The intermediary regional compartment no. 176 around the FDNPP is also indicated.

The model well predicts the temporal evolution of 90-Sr in the northwest Pacific post World War II. The figure below compares measurements of 90-Sr with model output for numbered compartments of the model domain which show good agreement.

Calculated concentrations of 90Sr in surface waters for box no. 37 (A) and no. 30 (B) in the East China Sea, and for box no. 96 (C) in the Korea/Tsushima Strait and for box no. 149 (D) in the East/Japan Sea. Predicted concentrations in the East China Sea were estimated with and without considering riverine inputs.
The calculated activities of 90-Sr in water, bottom sediment and marine biota are in good agreement with measurements made in the coastal area around Fukushima before the accident. Fewer direct measurements of 90-Sr exist compared to Cs isotopes because the analysis of 90-Sr requires significantly more sample processing and handling to separate it from other beta-emitters. For this reason the model assumes that 90-Sr releases from Fukushima to the ocean are related to 137-Cs releases in ratios determined by direct measurement after the disaster. While releases are ongoing the model determined that the activities in sediments and marine foodstuffs depend primarily on the initial releases in March and April 2011 when rates of release were greatest. Using a conservative estimated release of 640 TBq (TBq = 10^12 Bq) the model predicts the following individual dose rates to Japanese consumers of sea organisms where the average consumer eats 23.4 kg of fish, 2 kg of crustaceans, 1.3 kg molluscs and 3.7 kg of macroalgae per year. It was also assumed that 50% of the fish consumed included the organisms bones.
Individual dose rates for Japanese consumers of 90-Sr contaminated seafood in box 176 and 90 of the model domain before and after Fukushima.

The maximum dose rate in the coastal region at Fukushima was 0.66 microSv/yr which is an order of magnitude greater than the maximal dose rate from 90-Sr released through weapons testing in 1959. This maximal dose rate from 90-Sr is three orders of magnitude less than the dose rate from 137-Cs in the most contaminated marine environment off of Fukushima Daiichi. Given mixing and dilution of the contaminated plume of seawater the annual doses owing to 90-Sr from Fukushima are much less significant in model domains distant from the disaster site. The authors considered a worst case scenario where the public only consumed seafood from the Fukushima coast over the course of a year which resulted in a dose rate of 15 microSv/yr which is well below dose rates thought to represent a significant radiological health risk for the public.

While the modeled activities of 90-Sr in fish agrees well with limited measurements made in fish the model tends to slightly underestimate the activity of 90-Sr in fish. More measurements of this radionuclide in seawater, sediments and biota will improve our understanding of how 90-Sr moves through the environment. Ongoing releases of 90-Sr from the Fukushima site also dictate that monitoring of the levels in the marine environment are necessary and prudent to determine the radiological health risk to seafood consumers.

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