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| Fig. 1: Diagram of a boiling water reactor. [12] (Source: Wikimedia Commons) |
The Fukushima Daiichi power plant consisted in 6 boiling water reactors. (See Fig. 1.)
On March 11, 2011 units 1, 2 and 3 were operating at full power, unit 4 was not fueled, and units 5 and 6 were in cold shutdown. [1] At 2:46 pm, an earthquake of magnitude 9.0 occurred off the east coast of Japan. The seismic sensors in all operating reactors triggered the insertion of the control rods, stopping the fission reaction.
However, residual heat from fission product decay was still being produced inside the reactors and needed to be removed. Since all the units had lost power supply due to the earthquake, emergency diesel generators took over, providing necessary power to cool down the units. Unfortunately, about an hour later, 2 tsunami waves hit the nuclear plant, damaging the seawater pumps and drowning the diesel generators. This left the reactors without any cooling system to remove the residual heat. Steam was then generated in the reactor pressure vessels, and interaction between this steam and the fuel's zirconium cladding produced hydrogen.
The hydrogen release did not present any immediate risk at the time because the containment building was filled with nitrogen. [2] However, the rise in temperature progressively led to a meltdown of the fuel, thus causing the depressurizing valves to open, pushing the radioactive gas to the wetwell suppression pool. The water acted as a filter, trapping most of the radioactive particles, but it soon started boiling, reducing its efficiency. Water injection then began. But the various systems injecting water failed over the next days, so the fire pumps took over, injecting seawater due to the absence of fresh water on the site. [2]
To avoid containment rupture, the operators decided to release the gas into the atmosphere. This should have led all the gas to escape through the venting lines. However, hydrogen was escaping through uncontrolled leakage pathways into the reactor building. Now reacting with the oxygen in the air, this hydrogen caused an explosion of the top of the structure, releasing radioactive elements not yet trapped in the suppression pools.
Explosions in units 1 to 4 happened during the four days following the tsunami. [2] By the end of March, fresh water had replaced seawater, and the reactors' cooling systems were again in operation in July.
Over the months following the accident, both seawater and fresh water were used to cool the damaged reactors. Large volumes of water were pumped into the reactors to absorb heat and prevent further nuclear meltdowns. By week 3, removing the contaminated water from the reactors has become one of the main challenges.
During the first week of April, TEPCO (Tokyo Electric Power Company) released 10,400 m3 of slightly contaminated water into the sea to free up storage for more highly contaminated water from unit 2, which was the main source of contamination. Despite coming from the least radioactively contaminated pool, the discharged water was still 100 times above the legal limit. [3]
In addition to the water used for cooling after the accident, it has been found that about 400 m3 of goundwater went daily into the different parts of the plant. This needed to be treated and stored. Every day, a total of at least 700 m3 of water (taking the cooling water and the influx of natural water together) was entering the buildings. [4] Of the water collected for treatment, half was re-used for cooling and half went to storage.
To face this storage challenge, more than 1000 tanks with a capacity of 1200 m3 were progressively set up in the Fukushima facility, reaching a total capacity of 1.3 million m3. Taking all these numbers into account, one could calculate the amount of time before Fukushima storage tanks saturate which would be around 1.3 × 106/(350 × 365) = 10.2 years.
The cooling water used in the different reactors was continually being exposed to the reactor core as well as other damaged reactor components. This resulted in the generation of a large volume of radioactively contaminated waste-water. A need for water treatment became quickly apparent for (1) desalination and (2) the removal of radionuclides.
The seawater that had been injected into the reactors to cool them in the days following the accident contained salts and minerals that were not suitable for cooling nuclear reactors. When seawater is used without desalination, these salts and minerals leave deposits on the reactor components, affecting their efficiency and potentially causing damage. Additionally, the presence of certain ions in seawater can enhance the corrosive properties of the water, posing a risk to the reactor infrastructure.
Radioactive nuclides also needed to be removed before the water could be used again or stored. Among the radioactive isotopes, Cs-137, Cs-134, I-131, I-129 and Sr-90 were present in significant quantities. [4,5]
In 2011, a new wastewater treatment facility was developed using US, French and Japanese technologies. This enabled reduction of the concentration in Cs by a factor of 10,000 via an adsorption-based treatment system.
In September 2013, TEPCO began implementing the ALPS (Advanced Liquid Processing) system. It was capable of processing 750 m3 per day (3 units with a capacity of 250 m3/day) to remove 62 remaining isotopes. It did this by means of a hybrid treatment process consisting of multiple precipitation, ion exchange and adsorption steps. [5,6] However ALPS couldn't remove the Tritium and Carbon-14 present in the water. This then required dilution of the water to meet the drinkable standards.
In March 2022, ALPS-treated water samples were taken to be analyzed by three IAEA laboratories and four third-party laboratories. In addition to TEPCO, these included Spiez Laboratory in Switzerland, Institut de Radioprotection et de Sûretét Nucléaire in France, Los Alamos National Laboratory in the USA, and Korea Institute of Nuclear Safety in the Republic of Korea. The results showed that every radionuclide except Tritium met the regulatory limit for discharge.
The activity concentration of tritium in ALPS-treated water is around 152,300 Bq/L, well above the regulatory limit of 60,000 Bq/L. According to those results, the total activity concentration of source term radionuclides in ALPS-treated water is around 152,321 Bq/L, making tritium responsible for 99.99% of the radioactive activity. [7]
Since 2011, more than 1.3 million m3 of water has been stored in the tanks at the plant. In 2021 it was announced that the facility would run out of land by 2022. This is consistent with the estimate mentioned above.
In April 2021, the Japanese government approved the discharge of the water in the Pacific Ocean over a course of 30 years starting in 2023. The water, whose only remaining radioactive element is Tritium, would first be diluted with sea water and stored to verify its Tritium level. It would then be released into the ocean through a 1 km undersea tunnel into an area where no fishing would be allowed. TEPCO stated that the water discharged from the power plant would contain no more than 1,500 Bq/L, a level that is lower than the maximum recommended by the World Health Organization for drinkable water (10,000 Bq/L). [8]
In comparison with other countries, the amount of Tritium being released at Fukushima is far less than that from many nuclear facilities around the world. For instance the annual amount of discharge Tritium at La Hague reprocessing plant in France reaches 10,000 TBq, whereas Fukushima is only 22 TBq. [8]
This plan has been approved by the International Atomic Energy Agency (IAEA). It will maintain a continuous on-site presence to monitor the release and provide online data from the discharge facility.
Despite scientific approval and government assurances, the release of treated water still raises concerns among local and international populations. In a survey conducted by the newspaper Asahi Shimbun, 51% of the Japanese public said they supported the discharge plan while 41% said they did not. [9]
Fishing communities, in particular, voiced apprehension about the possible stigma attached to seafood from the region. Even though samples collected from fish in the Fukushima area did not show any trace of tritium after the release, the fishing industry might still be affected through international policies and public fear. [10] Indeed, after the plan to release the water was made public, China announced that an existing ban on seafood imports from Fukushima would be immediately extended to cover the whole of Japan. [11] China imports almost half of Japan's seafood exports, making this policy economically damaging.
Other governments, including South Korea, Russia, Germany, New Zealand and several South American countries, subsequently expressed concerns or opposition to the plan, citing worries about potential contamination of marine resources.
© Theo Zivre. 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] S. Tanaka, "Accident at the Fukushima Daiichi Nuclear Power Stations of TEPCO - Outline and Lessons Learned," Proc. Jpn. Acad. Acad. B: Phys. Biol. Sci. 88, 471 (2012).
[2] "Fukushima, One Year Later: Initial Analyses of the Accident and Its Consequences," Institut de Radioprotection et de Sûreté Nucléaire, IRSN/DG/2012-003, March 2012.
[3] S. Westall and F. Dahl, "Japan to Dump 11,500 Metric Tons of Radioactive Water," Reuters, 4 Apr 11.
[4] "Fukushima Daiichi Nuclear Accident - Management of Contaminated Water From the Damaged Reactors - Situation as of End June 2013," Institut de Radioprotection et de Sûreté Nucléaire, August 2013.
[5] P. Sylvester, T. Milner, and J.Jensen, "Radioactive Liquid Waste Treatment at Fukushima Daiichi", J. Chem. Technol. Biotechnol. 88, 1592 (2013).
[6] "Fukushima Daiichi Nuclear Accident - Management of Radioactive Water From the Damaged Reactors - Situation in March 2016," Institut de Radioprotection et de Sûreté Nucléaire, March 2016.
[7] "IAEA Review of Safety Related Aspects of Handling ALPS-Treated Water at TEPCO's Fukushima Daiichi Nuclear Power Station," International Atomic Energy Agency, 2023.
[8] J. Smith, N. Marks, and T. Irwin, "The Risks of Radioactive Wastewater Release," Science 382, 31 (2023).
[9] T. Ishimoto, "Survey: 51% Support Releasing Treated Nuclear Water Into Ocean", Asahi Shimbun, 20 Mar 23.
[10] "No Tritium Found in Fish 1 Month After Fukushima Water Release", Kyodo News, 25 Sep 23.
[11] T. Wong, "Fukushima: China Retaliates as Japan Releases Treated Nuclear Water," BBC News, 24 Aug 23.
[12] J. Xiao et al., "GASFLOW-MPI: A New 3-D Parallel All-Speed CFD Code for Turbulent Dispersion and Combustion Simulations. Part II: First Analysis of the Hydrogen Explosion in Fukushia Daiichi Unit 1," Int. J. Hydrog. Energy 42, 8369 (2017).
