[Feature: Ocean Sustainability / Special Feature: 150th Anniversary of the Yochisha Elementary School] Ryo Ohmura: Tritiated Water Separation Technology Using Clathrate Hydrates

Regarding the relationship between the theme of "Ocean Sustainability" and my research subject, hydrates, I think that 20 years ago I would have written about the resource development of methane hydrates. However, considering recent social movements toward carbon neutrality, I believe it is not good to cling to methane hydrates. The hydrates referred to here are substances academically known as clathrate hydrates; they are crystals formed when molecules of a substance other than water (guest molecules) enter a cage-like structure formed by water molecules. Since the main part of the crystal structure is composed of hydrogen bonds between water molecules, and more than 85% of the composition is water on a molar basis, they can be considered a type of ice. This time, I would like to raise tritiated water separation as a topic where hydrates and the ocean can be related.

Now that the ocean release of ALPS-treated water accumulated on the site of the Fukushima Daiichi Nuclear Power Plant has been decided and is being implemented, many people may wonder if that water really has no choice but to be released. ALPS-treated water is wastewater from which most radioactive substances have been removed by Advanced Liquid Processing Systems. However, only tritiated water cannot be removed even with ALPS treatment. It can be said that the problem with ALPS-treated water is the residual tritiated water. Tritiated water is water in which tritium (hydrogen-3), an isotope of hydrogen, has replaced the hydrogen in the water. If tritium is represented as T, the chemical formula is T₂O or THO (or HTO).

The conclusion of TEPCO and the government is that technology to separate (or concentrate) tritiated water and purify ALPS-treated water does not exist. However, in the author's laboratory, we have succeeded in a separation experiment—albeit on a small scale with an internal volume of about 100 cm³—to treat tritiated water with a concentration of about 1 million Bq/kg, equivalent to the ALPS-treated water at Fukushima Daiichi, and reduce it to a concentration below 1,500 Bq/kg, which is the standard for ocean release. This research and development is being conducted as a joint research project with Image ONE Co., Ltd. and So-Innovation Co., Ltd.

There are two reasons for the current conclusion that technology to separate and concentrate tritiated water from ALPS-treated water does not exist: its radioactivity concentration and its total volume. At the Fukushima Daiichi Nuclear Power Plant, more than 1 million tons of ALPS-treated water (1,000 tanks of 1,000 tons each) with a concentration of 1 million Bq/kg are stored. While 1 million Bq/kg intuitively feels like a very high concentration, tritiated water is treated as a radioactive isotope at 1 billion Bq/kg or more; from the perspective of radioactivity concentration, it is a low concentration that does not fall under the category of a radioactive isotope (on the other hand, since the WHO guideline for drinking water is 10,000 Bq/kg or less, it cannot be said that it is safe to drink).

In the past, development of separation and concentration technology for tritiated water has progressed for military purposes (e.g., hydrogen bomb production) or as part of nuclear power-related technology, but there have been no cases targeting such low concentrations and large volumes of tritiated water as mentioned above. However, since isotope fractionation always occurs if distillation, freezing, electrolysis, etc., are performed, separation and concentration to a greater or lesser extent is possible. What is required for the current treatment of ALPS-treated water is a technology for large-scale treatment that has the separation performance to further reduce the concentration of low-concentration water of about 1 million Bq/kg to 1,500 Bq/kg or less, and can handle a total volume of over 1 million tons. The CECE method, which combines electrolysis and chemical replacement using a catalyst, is considered to have the best separation performance among existing technologies, but this method is suitable for high-concentration, small-volume treatment, and in the case of low-concentration, large-volume treatment, the cost would likely become astronomical. Due to this situation, it is currently stated that technology to purify the ALPS-treated water at the Fukushima Daiichi Nuclear Power Plant does not exist.

Separation and concentration of tritiated water by the hydrate method utilizes the phenomenon where water containing hydrogen isotopes—namely, tritiated water and heavy water (D₂O)—solidifies at higher temperatures. While the melting and freezing point of light water (H₂O) is 0°C, it is 3.8°C for heavy water and 4.5°C for T₂O. A similar phenomenon occurs not only in ice formation but also in hydrates, which are solids consisting of water. Although it varies slightly depending on the guest substance (the substance other than water), heavy water forms hydrates at temperatures approximately 2°C to 3°C higher. Because of these physical properties, if water containing tritiated water is solidified, the tritiated water is concentrated and incorporated into the solid side. While it could be said that using ice formation, which does not require a guest substance, is simpler, hydrates have the physical property of forming at higher temperatures than ice, which is advantageous in terms of cooling costs.

Hydrates have crystal growth characteristics where the interface between the guest and water becomes the preferential site for crystal growth; therefore, they tend to form not as a lumpy solid but as a porous body with many gaps. Figure 1 shows an observation image of hydrates formed in the author's laboratory. You can see that it forms like shaved ice. Since the efficiency of separation increases as the contact area between the solid and liquid increases, the characteristic of hydrates forming in a porous state is considered one reason for the separation performance that cannot be obtained with ice formation.

The hydrate method developed by the authors utilizes another important phenomenon: coprecipitation using heavy water. To put it simply, this coprecipitation is a phenomenon where similar things gather together. Comparing the physical properties of light water, heavy water, and tritiated water, the idea is to remove tritiated water more efficiently by utilizing the fact that heavy water and tritiated water are very similar.

The experiment begins by forming a porous body of hydrate using heavy water. HFC-134a, which allows for hydrate formation at relatively low pressures, is used as the guest substance. By bringing heavy water and HFC-134a into contact within a pressure vessel for a certain period, heavy water hydrate consisting of heavy water and HFC-134a is formed. By discharging the heavy water that remains without becoming hydrate from the bottom of the vessel, a porous body of heavy water hydrate is formed.

Figure 2 shows an observation image of the hydrate formed by this experimental operation. The porosity of this hydrate porous body is about 50%. Subsequently, while maintaining temperature and pressure conditions under which heavy water hydrate can grow, tritiated water with a concentration of about 1 million Bq/kg (simulated ALPS-treated water) is injected into the gaps of this hydrate porous body and circulated for about one hour using a pump prepared outside the device. Figure 3 schematically illustrates this operation. As the hydrate grows little by little during circulation, tritiated water is preferentially incorporated into the hydrate, causing the tritiated water on the liquid side to be concentrated on the solid side.

The results of reducing the tritiated water concentration of about 500,000 Bq/kg obtained in the early stages of experimental research to below 1,500 Bq/kg through the above one-hour treatment have already been published as an academic paper (Reference 1). For those interested in more academic content, please refer to the literature (the author would like to highlight that it was published in a top-rated academic journal in the field of chemical engineering).

In this hydrate method, the tritiated water concentration on the water side is lowered by concentrating the tritiated water on the hydrate side. If treatment continues, the tritiated water concentration on the hydrate side will increase. As for what to do with that concentrated tritiated water, it will be stored on the site of the Fukushima Daiichi Nuclear Power Plant for the time being. By concentrating it on the hydrate side, the number of storage tanks, which currently exceeds one thousand, can be significantly reduced. As for what to do with the water whose concentration has decreased, it will still be released into the ocean, but the amount released will be significantly reduced to about 1/1000th of the current value, which relies solely on dilution.

This technological development is currently at the stage of successful laboratory-level operation, and it is necessary to proceed with scale-up in the future. Considering that ALPS-treated water is still increasing by about 100 tons per day and that more than 1 million tons are already stored, the processing volume required for practical technology is about several hundred tons per day. While this amount seems like an astronomical value from the scale of a laboratory, it is a fairly small scale when considering the size of water treatment facilities such as water purification plants and seawater desalination plants operated in society.

In the author's laboratory, as part of the scale-up from the 100 cm³ scale device, we have already designed, manufactured, and started operating a hydrate generation device with an internal volume of 34 liters, which is two orders of magnitude larger (see Figure 4). Further scale-up is necessary to achieve a processing capacity of several hundred tons per day. About 10 years ago in Japan, a bench-scale project for hydrate production, storage, and transportation was conducted as a joint research project between Mitsui Engineering & Shipbuilding, Chugoku Electric Power, and NEDO, succeeding in producing 5 tons of natural gas hydrate per day. I have experience participating in that project from the standpoint of providing academic evaluation and advice. Utilizing that experience, we are proceeding with research and development in collaboration with industry, aiming for practical application within a few years.

(Reference 1) Satoshi Nakamura, Toshihiro Awata, Hitoshi Kiyokawa, Haruki Ito, Ryo Ohmura, “Tritiated water removal method based on hydrate formation using heavy water as coprecipitant”, Chemical Engineering Journal, Vol. 465, 2023, Paper ID: 142979; DOI: 10.1016/j.cej.2023.142979