Mitsui & Co., Ltd. ("Mitsui", Head Office: Tokyo, President and CEO: Kenichi Hori) has subscribed to a third-party allotment of new shares by MiRESSO Co., Ltd. ("MiRESSO"), a Japan-based company engaged in fusion energy–related technologies. This marks Mitsui's third investment in the fusion energy field, following its investment in Kyoto Fusioneering Ltd. in May 2023 and Commonwealth Fusion Systems LLC in September 2025.

Fusion energy is a power generation technology that produces large amounts of energy by fusing two light atomic nuclei, mimicking the same principle that powers the sun. As a sustainable energy source that emits no CO₂ during power generation, fusion energy is expected to be commercially deployed starting from the 2030s.
Countries around the world, including Japan, are increasingly positioning fusion energy development as a national strategy, accelerating R&D support and the development of regulatory frameworks. While many startups are focusing on reactor development, the importance of building a commercially viable supply chain is growing as fusion moves closer to commercial applications.

MiRESSO is a Japanese startup aiming to establish a competitive refining process for beryllium, a rare mineral that is essential for fusion reactor operations.

Beryllium is an indispensable material for fusion power generation, as it enables the production of tritium— the fuel required for fusion reactions to occur—inside the reactor. Fusion reactions occur when deuterium and tritium collide at extremely high speeds. While deuterium can be sourced from seawater, tritium is scarcely found in nature, making it necessary to create an environment within the reactor in which tritium can be artificially and continuously bred. Beryllium plays a key role in promoting this tritium production.
While several hundred tons of beryllium are required for the initial loading of a single fusion reactor, current global production remains at approximately 300 tons per year, highlighting the need to significantly expand supply to enable the widespread deployment of fusion power.

MiRESSO was established as a certified venture company of the National Institutes for Quantum Science and Technology (QST) and is one of the very few companies worldwide working to expand the supply of beryllium. The company has developed an innovative low-temperature refining technology that combines chemical processing with microwave heating, enabling beryllium ore—traditionally requiring high-temperature treatment of approximately 2,000°C due to its low solubility—to be refined at around 300°C under atmospheric pressure. Compared with conventional methods, this technology offers a lower-cost and more energy-efficient approach, and MiRESSO is advancing its commercialization.

Mitsui aims to explore a wide range of potential collaborative opportunities with MiRESSO, including the procurement of beryllium ore and the distribution and sales of beryllium products.

Through its investments in Kyoto Fusioneering, Commonwealth Fusion Systems, and MiRESSO, Mitsui is committed to building cutting-edge expertise and know-how across the entire fusion value chain and contributing to the early realization of fusion energy.

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