Tsinghua University team simulates Japan's nuclear wastewater discharge into the sea: it takes 240 days to reach the coastal areas of China

On August 22, 2023, Japanese Prime Minister Fumio Kishida announced that he would begin discharging contaminated water from the Fukushima Daiichi nuclear power plant into the ocean from the 24th. Tokyo Electric Power Company has announced detailed steps for discharging into the ocean. According to the plan, a large amount of seawater will be added to the treated water before discharge. If the concentration is confirmed to have decreased to the expected level, the first batch of 7800 tons of nuclear contaminated water will be discharged within 17 days. The estimated emissions for 2023 are about 31200 tons, with a total tritium amount of 5 trillion becquerels, which is about 20% of the upper limit of TEPCO's annual planned emissions.

This morning, a news article titled "Research simulating the diffusion process of Japanese nuclear wastewater into the sea: 240 days to reach the coast of China, 1200 days to cover the North Pacific" sparked heated discussions among netizens.

It is understood that the study came from a team at Tsinghua University.

In 2021, Tsinghua University conducted an experiment on the diffusion mechanism of nuclear wastewater in the Pacific Ocean for wastewater discharge.

Academician Zhang Jianmin and Associate Professor Hu Zhenzhong from the Institute of Ocean Engineering at Tsinghua University Shenzhen International Graduate School have established diffusion models of radioactive substances at the ocean scale from two different perspectives, macro and micro, and have achieved long-term simulation of the Fukushima nuclear wastewater discharge plan.

Macro diffusion simulation results of tritium

The macro simulation results show that nuclear wastewater will reach the coastal waters of China 240 days after discharge, and will reach the coast of North America and cover almost the entire North Pacific after 1200 days. Subsequently, pollutants rapidly spread along the coast of the Americas towards the South Pacific under the influence of the equatorial ocean currents, while on the other hand, they transferred to the Indian Ocean through the northern waters of Australia.

It is worth noting that although the emission location of pollutants is near Fukushima, over time, the high concentration areas of pollutants will extend eastward along the 35 ° N line, spreading from the waters near East Asia to the waters near North America. On the 2400th day, the waters along the southeast coast of China mainly showed a low concentration of light pink, while the waters on the west side of North America were basically covered by a high concentration of red.

Changes in pollutant concentrations in three coastal cities and their vicinity

Researchers further selected three coastal cities, Miyazaki in Japan, Shanghai in China, and San Diego in the United States, for comparison. From the curve of pollutant concentration changes, it can be found that on the 4000th day, the concentration of pollutants near San Diego is about 0.01 units, which is about three times that of Miyazaki and about 40 times that of Shanghai. The main reason for this phenomenon is the strong ocean currents near Japan. Fukushima is located at the confluence of the Japan Warm Current and the Kuril Cold Current, so most pollutants do not migrate along the land edge in a north-south direction, but spread eastward with the North Pacific Warm Current.

This result also means that in the early stages of nuclear wastewater discharge, its impact on the Asian coast should be primarily considered. However, in the later stage, as the concentration of pollutants in the North American coastal waters will continue to be higher than most of the East Asian coastal waters, it is necessary to focus on the affected situation in the North American coastal waters.

Microscopic diffusion simulation results of tritium

In addition to macroscopic diffusion, researchers also simulated tritium diffusion from a microscopic perspective.

Unlike macro diffusion analysis that focuses on the overall distribution of pollutants, micro diffusion analysis focuses more on the behavior of individual pollutants, and therefore can support the analysis of pollutant diffusion pathways.

For example, for a certain three pollutant particles that reach the coastal waters in the simulation results, their motion trajectories are obtained with a sampling interval of 400 days. Based on these motion trajectories, it can be inferred that pollutants in the coastal waters of the Americas mainly reach through crossing the Pacific Ocean.

The movement trajectory of partially polluted particles

It is worth noting that according to Japan's emission plan, the concentration of one unit of tritium pollutant corresponds to approximately 0.29 Becquerels per cubic meter, which is not significant compared to the background concentration of tritium in the ocean. However, this study still has significant implications for predicting the long-term diffusion of pollutants, reasonable response to nuclear wastewater discharge plans, and subsequent monitoring of radioactive substance concentrations. On the basis of this study, further experiments are needed to explore the sensitivity of the ecological environment to radioactive substances, determine the degree of impact of increased radioactive substance concentration on the marine ecological environment and human living environment, and ultimately determine the impact of the discharge of nuclear wastewater on the entire ocean and human life.

The related achievements were published in the journal National Science Review under the title of "Macro and Micro Simulation of Water Discharge from Fukushima Nuclear Accident Treatment".