The Japan Atomic Energy Agency (JAEA) has carried out expeditions in the Japanese and Russian exclusive economic zones (EEZ) of the Japan Sea for 10 years to clarify seawater circulation and transport processes of materials in the sea. As a result, for the first time JAEA has made distribution maps of anthropogenic radionuclides in the Japan Sea and outlined the transport processes of radionuclides in the sea.

The Japan Sea expeditions at JAEA started with its participation in the Japanese-Korean-Russian joint expeditions of 1994 and 1995. With the Mutsu Establishment as a research base, JAEA has since continued the expeditions in the Japanese and Russian EEZs under the International Science and Technology Center (ISTC) partner project, entrusted research by the Science and Technology Agency/the Ministry of Education, Culture, Sports, Science and Technology of Japan (STA/MEXT) and cooperative research with Hokkaido University and Kyusyu University. Through these expeditions, JAEA succeeded in covering almost all areas of the Japan Sea that could be observed at present.

The findings of this study were obtained through 18 expeditions in the Japanese and Russian EEZs of the Japan Sea. These findings enable understanding of seawater circulation and transport processes of materials in the sea. Furthermore, the dataset on the distribution of anthropogenic radionuclides established by this study provides background data in assessing the radiological consequences in cases of radioactive waste disposals and/or nuclear emergency response for accidental releases of radionuclides in or near the Japan Sea. The results of these expeditions are summarized below.

1) Concentrations of anthropogenic radionuclides in the Japan Sea vary over regions and depths. Radionuclides were detected in middle-deep layers of the sea, and penetration leaches deeper in the northwestern region than the southeastern region. Distributions of radionuclides in middle-deep layers of the sea show a southeastward lateral transport of radionuclides from the Japan Basin to the Yamato Basin, detouring around the Yamato Rise due to seawater movement (Figure 1). Vertical convection of seawater in winter plays an important role in transport of radionuclides to the interior of the northwestern Japan Sea.
2) Investigation of settling particles show accumulation of particulate radionuclides in the seabed of the Japan Sea from lateral transport of particles originating in the East China Sea and the Japan Islands, as well as from yellow dust from the Asian Continent via the atmosphere (Figure 2).
3) Anthropogenic radionuclides detected in seawater and seabed sediments in the Japan Sea are inferred to originate from global fallout from atmospheric nuclear weapons testing. The radiological consequences of these radionuclides on human physiology are negligible.

In February 2006, JAEA published a report (in English) of the results of these Japan Sea expeditions.

• Supplementary documents
• Glossary