Ever since the Fukushima Daiichi nuclear power plant accident that caused the discharge of radionuclides from the power plant into the ocean, operators at the Tokyo Electric Power Company (TEPCO) have been implementing measures to reduce groundwater inflow into the damaged reactor buildings. TEPCO has also been pumping water into the reactors since the accident to cool them.
The cooled water is then treated using the Advanced Liquid Processing System (ALPS), which removes all radioactive materials from the water except for tritium – which is very difficult to remove and has a half-life of 12.32 ±0.02 years. This treated water ended up being accumulated and stored at the site, with limited space to store it.
To combat this storage issue, the Japanese government implemented a new policy in 2021 focused on discharging the ALPS-treated water into the ocean using a 1 km long tunnel. The release of the treated water (containing tritium) began on 24th August 2023, and the plan is to continue releasing it until 2050. The government set a threshold for tritium suspension levels of 700 Bq/L in the discharge outlet vicinity and 30 Bq/L in the ocean. If the concentrations exceed these thresholds, then the discharging must stop immediately.
Researchers at the University of Tokyo have now collaborated with Fukushima University to investigate the effects of discharging tritium into the local ocean environment, and whether the discharging of this treated water is actually having an adverse impact. The study used an ocean general circulation model known as COCO4.9 to look at the influence of climate conditions – such as long-term global warming – on the discharge scenarios of tritium from the power plant. The researchers examined multiple discharge scenarios (based on the amount of tritium released) up until 2099.
Previously, no modelling had been performed looking at long-term impacts relating to the changing environmental conditions of the planet. In a press release from the University of Tokyo, lead author Alexandre Cauqouin states that: “In our global ocean simulations, we could investigate how ocean circulation changes due to the global warming and representation of fine-scale ocean eddies influence the temporal and spatial distribution of tritium originating from these treated-water releases”.
It is important to find out how fast and far the tritium discharge spreads because both climate change and eddies in water currents can speed up the movement of tritium through the ocean.
The study revealed that in all but one of the modelled scenarios (and at the release location, which has a much higher concentration because the treated water hasn’t dissipated yet), the tritium concentration in the ocean remained almost the same, and at a very low concentration. This was true for both long- and short-term scenarios – showing that the discharge from the Fukushima Daiichi nuclear power plant has an almost negligible impact on the ocean.
Other than the worst-case scenario, the model discovered that the increase in tritium from the treated water is 0.1% or less of the tritium background concentration of 0.03–0.2 Bq/L within 25 km of the discharge site in the Pacific Ocean. This is well below detection limits – such a small amount that the presence of the added tritium from the treated water cannot be measured directly in the seawater. The results are also far below the safety standards of 10,000 Bq/L set by the World Health Organization and consistent with physical seawater monitoring being performed today.
Even in the worst-case scenario, the levels of tritium still fell well below the detection limits, but the model did find that in such a high-CO2 emission scenario, there would be an increased concentration of tritium in the south of Japan due to the Kuroshio current – which could theoretically reach the western coast of the US, but in insufficient concentrations to have any adverse effects throughout the Pacific Ocean.
Overall, the study showed that the long-term safety threshold won’t be exceeded under the current treated water release plans. The study could also help with building future models to better understand how tritium moves through both water vapour and ocean water – as tritium could be used in the future as a chemical tracer to track atmospheric and oceanic circulation, precipitation patterns, river catchments, moisture sources and groundwater flow.
The research is published in Marine Pollution Bulletin.
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