Hydrate Research in the Era of Carbon Neutrality

With the hosting of COP25 and COP26, calls for decarbonization and carbon neutrality have grown stronger worldwide, and it can be said that the time has come to conduct hydrate research in a somewhat different context than before. I myself became strongly aware of this starting in the fall of 2020, and at the beginning of fiscal year 2021, I decided to review all of my research group's projects. However, since we had long been conducting research for the development of environmental and energy technologies utilizing the physical properties of hydrates, reviewing our research group's projects in the context of the carbon-neutral era did not require a complete overhaul. In fact, much of what we had been doing was already important research for the carbon-neutral era. Research on thermal energy storage technology using hydrates is one such example. The full-scale development of thermal energy storage technology began quite some time ago, with hydrate thermal storage research starting in the 1980s. In the past, the idea was to generate hydrates at night when the supply from baseload power sources was excessive, and then use the cooling energy obtained by decomposing them during the day, when demand was high, to provide air conditioning. While this is becoming a thing of the past, thermal energy storage remains important in the carbon-neutral era as a technology to compensate for the instability of renewable energy. From a broader perspective, the importance of energy storage in the carbon-neutral era seems to have increased even more than before. For example, there is a relatively new academic journal called the *Journal of Energy Storage*, and its Impact Factor has shown a dramatic 90% increase from 3.5 to 6.6 over the past three years. Our research group also published three papers in this journal between 2021 and 2022.

It's not just that the importance of our work has remained unchanged despite the changing times; we have also started new projects geared toward the carbon-neutral era. A prime example is our tritium water separation and concentration technology. The tritium concentration in the ALPS-treated water stored at the Fukushima Daiichi Nuclear Power Plant is about 1 million becquerels per liter. While 1 million becquerels may sound like a very high concentration, when expressed in terms of mass or molar concentration, it is a low concentration of less than one ppm. Until now, it was believed that no technology existed to separate and concentrate such low-concentration tritiated water. However, in recent years, it has been shown that separation and concentration by a factor of several hundred to 1,000 is achievable through hydrate formation. This utilizes the physical property that tritiated water forms hydrates at a temperature about 3°C higher than light water—in other words, it is more prone to forming hydrates. This tritium water separation and concentration is a technology that can also be applied to nuclear fusion. The Ohmura group is advancing the development of this technology as a joint research project with ImageONE Co., Ltd. and So-Innovation Co., Ltd., and we hope to advance it to the stage of social implementation.