More than two years after the earthquake and tsunami that devastated parts of Japan, scientists are still trying to quantify the extent of the damage.
Of particular importance is determining just how much hazardous material escaped into the atmosphere from the stricken Fukushima Dai-ichi nuclear power plant in the period following the disaster on 11 March 2011.
Scientists estimate a 'source term' (the types and amounts of hazardous materials released following an accident) by running computerised atmospheric and oceanic dispersal simulations and collecting samples from seawater. Data from the Fukushima incident is unfortunately plentiful. Immediately after the accident some radionucleids were carried east by a strong jet stream and reached the west coast of North America in just four days; other airborne radionucleids were eventually deposited into the Pacific Ocean. Further releases of hazardous material occurred through accidental and intentional discharges of contaminated water from the plant into the ocean.
Writing in the Journal of Nuclear Science and Technology, a team of researchers from the Japan Atomic Energy Agency now reveal that the previously estimated release rates of 137C and 131I were too low. They present their new source-term estimates for 12-20 March 2011, as refined through four numerical models and seawater data. Their comparison of the statistics obtained using the new source term with those obtained with the initial source term showed that all statistic values were improved by the new calculation. Their study also shows the effectiveness of using radionucleids observed in seawater to estimate the source term of atmospheric release in coastal areas.
Further research and modelling is needed to improve their new estimate, but this study is an important step in understanding the likely effects of the Fukushima incident on the marine environment by providing a clearer picture of how much hazardous material was actually released.
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