|
|
| Fig. 1: Illustration of Fukushima reactors. (Source: Wikimedia Commons) |
On March 11, 2011, a magnitude 9.0 earthquake occurred 45 miles east of the Oshika Peninsula, lasting six minutes and causing a fifteen meter tsunami. It was the most powerful earthquake ever recorded in Japan. Eleven reactors at four nuclear power plants in the region were operating at the time and all shut down automatically when the earthquake hit. Eight of the eleven units were able to run the residual heat removal (RHR) system cooling pumps using power from the grid or backup generators. The other three, at Fukushima Daiichi Nuclear Power Plant, lost power when the entire site was flooded by the tsunami, disabling twelve of thirteen backup generators onsite and also the heat exchangers for dumping reactor waste heat and decay heat to the sea. The three units lost the ability to maintain proper reactor cooling and water circulation functions (see Fig 1. Hydrogen arising in unit 3 and reaching unit 4 by backflow in shared ducts when vented caused an explosion in unit 4 on March 15.
On December 16, 2011, Japanese authorities announced that the three reactors reached cold shutdown, marking the end of the accident phase of events at Fukushima Daiichi. The achievement of cold shutdown was predicated on achievement of target values for certain plant parameters, such as stable cooling of the reactors, as well as significant suppression of radiological releases and steady decline of radiation dose rates as defined by Tokyo Electric Power Company Holdings (TEPCO).
Many radionuclides were released into the atmosphere on a large scale and deposited on land and ocean since the meltdown of the three reactors, including I-131, which has a half-life of 8 days, and Cs-137, which has a 30-year half-life. [1] Research in 2019 shows that the concentrations of radionuclides with short half-lives have decreased below detectable levels, while those of radionuclides with long half-lives have existed in the environment and subjected the public to constant exposure. [2]
There were also direct releases of radionuclides into the sea via firstly, direct release of radioactive water approved by the government to free-up storage for more contaminated water, and secondly, leakage of contaminated water from trenches carrying cabling and pipework. This had a larger effect on the coastal environment compared to atmospheric deposition of radioactive materials to the ocean. [3] The open nature of the Fukushima coast resulted in a rapid flushing of radionuclides in coastal seawater, although research shows that radioactivity in fish appears not to follow the rapid decrease in seawater radioactivity. [4] Radionuclides were also released into the environment as a consequence of transportation from land via river runoff and also as a consequence of redissolution from beach sands due to submarine groundwater discharge. [5]
Since the nuclear accident, water is needed to continually cool the melted fuel and fuel debris at units 1-3 of Fukushima Daiichi Nuclear Power Plant. Sources of contaminated water also include groundwater that seeped into the site, and rainwater that fell into the damaged reactor and turbine buildings. These contaminated waters were then treated through a filtration process known as Advanced Liquid Processing System (ALPS). The process uses a series of chemical reactions to remove 62 radionuclides from contaminated water, but is not able to remove tritium. [6] Tritium is a radioactive form of hydrogen that is part of the water molecule itself and has a radioactive half-life of 12.32 years.
The treated water was then stored in 1000 tanks located at the Fukushima Daiichi site, designed to hold 1.3 million cubic meters of water. The total amount of tritium in the tanks was reported to be around 1 PBq (1.0 × 1015 decay events per second), which is significantly lower than the 8000 PBq of tritium still remaining from global atmospheric nuclear testing in the 1960s. [6] However, in mid-2018, TEPCO disclosed the presence of more dangerous isotopes in the tanks, such as Sr-90, and the concentrations of these isotopes are highly variable from tank to tank. [6] While the volume of contaminated water generated annually has decreased, concerns of reaching storage capacity limits and annual expenses incurred (approximately USD 912.66 million per year) prompted the Japanese government to announce plans for the discharge of treated water into the Pacific Ocean in April 2021. [7]
Assessing the risk of discharging 1.3 million tons of treated wastewater requires knowing radioisotope amounts and their ability to concentrate in seafloor sediments and biological tissues. [6] The radiation concentration for each nuclide allowed in the release of wastewater by the Japanese government is listed out in Table 1. Wastewater will be treated again if radiation levels are higher than the discharge limits. [8] During discharge, the Fukushima wastewater is diluted 100 times so that tritium levels will be 2.5% of the Japanese Government regulatory limit and the sum of the 30 other relevant radionuclides will be less than 1% of the limits. [8]
|
||||||||||||||||||||||||||||||
| Table 1: Concentrations of radioactive isotopes required by Japanese law. [6] |
Seafloor sediments are important for the sequestration of particle-reactive radionuclides and serve as a contaminant reservoir for organisms that feed and live on the seafloor. [9] It was found that radioactive contamination in bottom sediments from sites off the east coast of Japan were dominated by Cs-137 and Cs-134. [9] Quantities of Sr-90 in the bottom sediments were almost three orders of magnitude lower than the quantities measured in the water column due to its low affinity to particles. [9] Cs-137 was found to generally decrease in sediments with increasing distance from the nuclear power plants and increasing water depth because of lower water column particle abundances. However, there is high spatial and temporal variability in sedimentary Cs-137 activities.
Radionuclides can be incorporated into marine organisms via uptake from seawater or food ingestion. Most research has been dedicated to understanding the concentration of Cs-137 due to its long half-life. The concentration factor (which refers to the ability of an organism to accumulate an element) for cesium in fishes is approximately 100, which is lower compared to some other elements like mercury. [9] Cesium also has limited biomagnification in marine food chains (a factor of approximately 2). However, research found that the concentration ratio of Cs-137 in fish to water is higher for fish of freshwater habitats than in those of coastal waters, especially freshwater fish distributed within the designated evacuation zone. [10] Concentration factor of Cs-137 in fish is dependent on ecosystems, and differences can be caused by suspended solid concentration, total organic carbon, and salinity, among other environmental factors. [11]
Since the first release began on 24 August 2023, publicly available information from TEPCO, the IAEA and the Japanese Ministry for Agriculture, Fisheries and Food, reveals that tritium levels are negligible within 3 km of the site. [8] With the fourth release of wastewater in March 2024, only 10 of the 1000 storage tanks will be emptied, and fully discharging the treated wastewater will take decades. Even though planned releases at Fukushima will likely be the most closely monitored wastewater discharge from a nuclear site, more research is needed to understand the contamination levels of radioisotopes other than tritium, and the resulting public health impacts. [8]
© Joyce Lin. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. P. Povinec et al., "Dispersion of Fukushima Radionuclides in the Global Atmosphere and the Ocean," Appl. Radiat. Isot. 81, 383 (2013).
[2] K. Saito, et al., "Summary of Temporal Changes in Air Dose Rates and Radionuclide Deposition Densities in the 80 km Zone Over Five Years After the Fukushima Nuclear Power Plant Accident," J. Environ. Radioac. 210 105878 (2019).
[3] D. Tsumune et al., "One-Year, Regional-Scale Simulation of 137Cs Radioactivity in the Ocean Following the Fukushima Dai-ichi Nuclear Power Plant Accident," Biogeosci. Discuss. 10, 6259 (2013).
[4] N. Yoshida and J. Kanda, "Tracking the Fukushima Radionuclides" Science 336, 1115 (2012).
[5] J. A. Kenyon et al., "Distribution and Evolution of Fukushima Dai-ichi Derived 137Cs, 90Sr, and 129I in Surface Seawater off the Coast of Japan," Environ. Sci. Technol. 54, 15066 (2020).
[6] K. O. Buesseler, "Opening the Floodgates at Fukushima," Science 369, 621 (2020).
[7] M. Das, "IAEA Reviews Plan to Release Treated Water from Fukushima," Lancet Oncol. 23, 574 (2022)
[8] J. Smith, N. Marks, and T. Irwin, "The Risks of Radioactive Wastewater Release," Science 382, 31 (2023).
[9] K. Buesseler et al., "Fukushima Daiichi-Derived Radionuclides in the Ocean: Transport, Fate, and Impacts," Annu. Rev. Mar. Sci. 9, 173 (2017).
[10] T. Wada et al., "Strong Contrast of Cesium Radioactivity Between Marine and Freshwater Fish in Fukushima," J. Environ. Radioact. 204, 132 (2019).
[11] Y. Ishii, S.-I. S. Matsuzaki and S. Hayashi, "Different Factors Determine 137Cs Concentration Factors of Freshwater Fish and Aquatic Organisms in Lake and River Ecosystems," J. Environ. Radioact. 213, 106102 (2020).
.PNG){kind=link}