Research on radioactivity in the marine environment
In order to monitor the safety of national fishing grounds in the vicinity of nuclear power plants and the offshore areas of nuclear fuel cycle facilities, we carry out radioactivity researches targeting marine organisms, seafloor sediments and seawater, and basic data are compiled as part of the radioactivity research and comprehensive assessment project in the marine environment by the government.
Research on radioactivity in the marine environment
Temporal variations of cesium-137 and strontium-90 concentrations in surface seawater in the vicinity of 15 nuclear power plants are shown in figure below. The strontium-90 was found in the marine environment in FY1983 when the survey began, and subsequent researches have confirmed a gradual decreasing trend. The cesium-137 concentrations in FY1986 showed high values, probably due to the impact of the Chernobyl Nuclear Power Plant accident. Similar to the strontium-90 concentrations, the cesium-137 showed a gradual decreasing trend after FY1986. However, in FY2011, high values for cesium-137 and strontium-90 concentrations were observed in some stations as a result of the TEPCO Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. In the researches conducted since FY2012, high concentrations exceeding the values measured in the five years before the accident (FY2006-2010) were observed in some areas, mainly off the Fukushima Prefecture. In FY2020, the cesium-137 and strontium-90 concentrations in marine organisms and seawater were comparable with the values before the TEPCO FDNPP accident. |
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| Seafloor sediment sampling work using a multiple corer |
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| Results of radionuclides concentrations in surface seawater in 15 sea areas around nuclear power plants, etc. |
| * Conventional beta-ray (β-ray) measurements cannot distinguish between cesium-137 and cesium-134. Since the values for radioactive cesium may contain cesium-134 derived from the TEPCO FDNPP accident, the analytical results for FY2011 are shown as the values of radiocesium (cesium 134+137), combining both cesium-134 and cesium-137. In FY2012, the samples were analyzed using gamma-ray (γ-ray) spectrometry that can distinguish cesium-134 and cesium-137 separately, and the figure shows the value for cesium-137 only. The analysis up to FY2010 was also based on beta-ray measurement, cesium-134 was not actually included, all values are shown as cesium-137 only. |
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Analysis for comprehensive assessment
The radionuclides concentrations vary depending on the sea area and the biological species tested. We are, therefore, conducting complementary support and analysis research to ensure more comprehensive assessment, such as clarifying the causes and mechanisms responsible for the observed differences.
As an example of analytical results, the relationship between feeding habit data, based on the composition of fish stomach content, and the percentage of cesium-137 in the edible parts * is shown. Even in the same fish, this percentage tends to be higher up the food chain.
* The value shown here is the ratio of the radionuclide concentration in the organism to the radionuclide concentration in seawater (the value for the radionuclide concentration in the organism divided by the radionuclide concentration in seawater). Radionuclide concentrations in organisms are also influenced by differences in radionuclide concentrations in seawater. |
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| Differences in radioactivities due to differences in feeding habits |
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Wide-area monitoring off the coast of Fukushima Prefecture and the surrounding waters
Following the accident at the TEPCO FDNPP on March 11, 2011, urgent monitoring of the surrounding sea area (outside the 30 km radius) began on March 23. In addition, in response to the “Environmental Monitoring Enhancement Plan” established by the Nuclear Emergency Response Headquarters on April 22, 2011, wide-area monitoring was not only off the Fukushima Prefecture but also off the Miyagi to Ibaraki Prefectures from May 2011. Furthermore, additional monitoring was in the vicinity of TEPCO FDNPP (within a radius of 10 km) from November 2013. These monitoring results are posted on the website of the Nuclear Regulation Authority as preliminary values. |
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●Seawater
The cesium-137 concentrations in seawater around the TEPCO FDNPP (outside the 30 km radius) began to increase in late March 2011, immediately after the accident; a maximum for cesium-137 concentrations was identified on April 15; they decreased from mid-April 2011. Since mid-September 2011, the fluctuations of cesium-137 concentrations have become smaller but they still showed a decreasing trend.
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Variations in cesium-137 concentrations in seawater. Data up to February 2021
* Green shaded area (■) indicates the concentration range of cesium-137 (0.0011 to 0.0024 Bq/L) in surface seawater during the five years (FY2006-FY2010) before the TEPCO FDNPP accident in the Miyagi, Fukushima Daiichi, Fukushima Daini, and Ibaraki areas. |
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In early November 2019, slightly higher concentrations that deviated from the decreasing trend were measured in surface water samples at some stations. Since January 2020, it has shown a decreasing trend again, and it is thought that it was a temporary increase due to flooding after rainfall on nearby land.
The cesium-137 concentrations have continued to decrease gradually and they are now falling within the concentration range of the five years before the accident. However, the concentrations in the vicinity of TEPCO FDNPP (within a radius of 10 km) are approximately one order of magnitude higher than the values outside the 30 km radius. |
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●Marine sediment
The cesium-137 concentrations in the surface layer (0-3 cm) of seafloor sediments decreased less rapidly than in seawater. Although there were some variations between the stations after the accident, a maximum for cesium-137 concentrations was identified in the fall of 2011, and then it showed a gradual decreasing trend. On the other hand, the horizontal concentration distribution is not solely determined by the distance from the nuclear power plant but may also be related to the migration route of seawater immediately after the accident and the compositions of seafloor sediments, etc. The decrease in cesium-137 concentrations in the seafloor sediments was mainly attributable to resuspension and horizontal movement of surface sediments. Another scenario for decrease in concentrations could be downward migration of surface sediment due to bioturbation and elution and desorption from the seafloor sediments.
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| Variations in cesium-137 concentration in seafloor sediments |
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