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

2024/06/05

When considering what to write regarding the theme of "Ocean Sustainability" and my research subject, hydrates, I think that 20 years ago, it would have been appropriate to write about the resource development of methane hydrates. However, given recent societal movements toward carbon neutrality, I believe it is not wise 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 the topic of tritiated water separation as a subject 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 no technology exists to separate (or concentrate) tritiated water and purify ALPS-treated water. 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 joint research with Image ONE Co., Ltd. and So-Innovation Co., Ltd.

There are two reasons for the current conclusion that no technology exists to separate and concentrate tritiated water from ALPS-treated water: 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 radioisotope only at 1 billion Bq/kg or higher; from the perspective of radioactivity concentration, it is a low concentration that does not fall under the category of a radioisotope (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, the development of separation and concentration technologies for tritiated water has progressed for military purposes (e.g., hydrogen bomb production) or as part of nuclear power-related technologies, but there have been no cases targeting such low-concentration, large-volume tritiated water as mentioned above. However, since isotope fractionation always occurs if distillation, freezing, electrolysis, etc., are performed, separation and concentration are possible to a greater or lesser extent. 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 (around 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 exchange 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; in the case of low-concentration, large-volume treatment, the cost would likely be astronomical. Due to this situation, it is currently stated that no technology exists to purify the ALPS-treated water at the Fukushima Daiichi Nuclear Power Plant.

Tritiated water separation and concentration 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/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 composed of water. Although it varies slightly depending on the guest substance (the non-water substance), heavy water hydrates at a temperature approximately 2°C to 3°C higher. Due to 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 a crystal growth characteristic where the interface between the guest and water becomes the preferred 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 generated in the author's laboratory. You can see that it is formed like shaved ice. Since the efficiency of separation increases as the contact area between the solid and liquid becomes larger, 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 this coprecipitation simply, it is a phenomenon where "like attracts like." The idea is to remove tritiated water more efficiently by utilizing the fact that the physical properties of heavy water and tritiated water are very similar when comparing light water, heavy water, and tritiated water.

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, a heavy water hydrate consisting of heavy water and HFC-134a is generated. By discharging the heavy water that remains without being hydrated from the bottom of the vessel, a porous body of heavy water hydrate is formed.

Figure 2 shows an observation image of the hydrate generated by this experimental operation. The porosity of this hydrate porous body is about 50%. Subsequently, while maintaining the temperature and pressure conditions under which heavy water hydrates 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 obtained in the early stages of experimental research, where a tritiated water concentration of about 500,000 Bq/kg was reduced to below 1,500 Bq/kg through the one-hour treatment described above, 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, for the time being, it will be stored on the site of the Fukushima Daiichi Nuclear Power Plant. 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 scaling up in the future. Considering that ALPS-treated water is still increasing by about 100 tons per day and that over 1 million tons are already stored, the processing volume required for a practical technology would be several hundred tons per day. While this amount seems astronomical from a laboratory scale, 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.

As part of the scale-up from the 100 cm³ scale device, the author's laboratory has 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 scaling 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 joint research by 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 position of providing academic evaluation and advice. Leveraging 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