Scientists from the Korea Atomic Energy Research Institute and Korea Institute of Ocean Science and Technology undertook a large-scale research to trace back in detail the path of radioactive substances discharged into the atmosphere as a result of the accident at Japan’s nuclear power plant Fuklushima-1 in 2011. The two key isotopes: iodine-131 and cesium-137 are meant here. The results of this work permit a new evaluation of the atmosphere role in the contamination of the World Ocean.
For a long time it was believed that the main contamination of the Pacific Ocean after the accident at Fukushima is attributable to the direct discharge of the radioactive water from the plant. However, the Korean researchers decided to verify what role the atmosphere had played. To this end, they used the LADAS model which permits tracing the movement of contaminants in the air by dividing two sources: direct discharge into the ocean and fallout from the atmosphere. For this purpose, the scientists restored the pattern of release spread from March to April 2011 based on the data on the releases and weather conditions.
The disaster happened on March 11, 2011: after a powerful earthquake and the following tsunami the power supply and cooling systems of the Fuklushima-1 nuclear plant were disabled which caused the overheating of reactors, hydrogen explosions and the release of radioactive substances into the atmosphere and the ocean. The main flow picked up by west winds crossed the Pacific Ocean within a matter of days and as early as March 18 already reached the west coast of the USA. By March 24, the discharge traces were recorded in Europe and about two and a half weeks later the radionuclides spread all over the Northern Hemisphere. The scientists discovered a shorter route, too: a part of the cloud that first went towards the ocean, changed direction in the Kamchatka area and on March 24 reached the Korean Peninsula via Manchuria. Later, on April 6, one more contamination wave arrived in the region: this time directly from Japan because the winds changed.
The main result of the research work is attributable to where eventually the releases happened to be. Calculations showed that just a small part of substances – about 5% of iodine-131 and 15% of cesium-137- fell out on the ground. However, the main mass was deposited on the surface of the ocean: about 54% of iodine and 76% of cesium. This radically changes the idea about accident consequences. It turns out that a significant part of the radioactive contamination of the ocean was formed not because of direct leaks, but because of the atmospheric transport and subsequent fallout.
This conclusion is corroborated by the results of field observations. The radioactive cesium detected in the central part of the Pacific Ocean and at the US coastline as early as spring 2011 could not have been delivered there by oceanic currents: it would have taken years. It means that it got there from the atmosphere having deposited on the water surface.
The comparison of these calculations with the data of the international monitoring network, including CTBTO and observation stations in South Korea, indicated that the model quite accurately reproduces both the time of arrival of the radioactive masses and the concentration levels.
Therefore, the Korean researchers proved that the ocean is becoming the main place of radioactive releases accumulation at the earlier stages after the disaster, while the atmosphere is the main channel for a fast global radioactive transport. It is important to have that in mind when assessing risks and developing a system of monitoring and responding to such events in the future.
