Mitsui Invests in MiRESSO to Strengthen Fusion Beryllium Supply Chain

Mitsui & Co., Ltd. has taken an equity stake in MiRESSO Co., Ltd., a beryllium refining startup headquartered in Misawa City, Aomori Prefecture, through a third-party allotment of new shares announced March 5, 2026. The Tokyo-based trading house, led by President and CEO Kenichi Hori, said it aims to explore collaborative opportunities with MiRESSO spanning procurement of beryllium ore through to distribution and sales of finished beryllium products.

The deal is Mitsui's third fusion-field investment, following its stake in Kyoto Fusioneering Ltd. in May 2023 and Commonwealth Fusion Systems LLC in September 2025. Mitsui framed the portfolio as part of its commitment to "building cutting-edge expertise and know-how across the entire fusion value chain and contributing to the early realization of fusion energy."

What makes MiRESSO worth backing for a major trading company comes down to a striking temperature differential. The Aomori startup has developed a beryllium refining process that operates at roughly 300°C under atmospheric pressure, against approximately 2,000°C required by conventional approaches. MiRESSO is certified by Japan's National Institutes for Quantum Science and Technology and describes its mission as contributing to fusion's social implementation through stable mineral resource supply.

The strategic urgency behind that mission is visible in the numbers. Current global beryllium production sits at approximately 300 tons per year, while a single fusion reactor requires several hundred tons for its initial loading alone. MiRESSO's planned pilot plant, targeting a site of approximately 300 square meters in Shinden, Kawaragi, Hachinohe City, Aomori Prefecture, will produce just 1 ton per year when it comes online in fiscal year 2027. That figure underscores how much scaling still lies ahead for the industry, but the pilot is designed to demonstrate the low-temperature refining technology before larger expansion.

MiRESSO CEO Masaru Nakamichi welcomed Mitsui's participation: "We are very pleased to receive investment from such reliable investors. Based on the funds raised, we will accelerate the pilot plant demonstration and start the technology platform business. In the future, we plan to expand partnerships with companies interested in the beryllium business and the technology platform business."

The company has assembled substantial backing ahead of the pilot build. MiRESSO raised ¥250 million in its first external round in March 2024, with Genesia Ventures leading and JGC MIRAI Innovation Fund L.P. and MITSUI SUMITOMO INSURANCE Venture Capital Co., Ltd. participating. A subsequent Series A first close brought in 1.83 billion yen (approximately USD 12 million), led by Spiral Capital, Inc., with Pacific Metals Co., Ltd. and Genesia Ventures rejoining. The Ministry of Education, Culture, Sports, Science and Technology also selected MiRESSO for a ¥2 billion SBIR Phase 3 grant in October 2023. Total funds raised since founding, including subsidies, now stand at approximately 4.28 billion yen (approximately USD 28.5 million) by MiRESSO's own accounting.

Alongside the pilot demonstration, MiRESSO intends to launch what it calls a technology platform business and is actively hiring for CXO-level positions. Mitsui's involvement adds distribution muscle to that ambition: the investment signals a potential pathway for ore procurement and product offtake that a pure materials startup would struggle to build independently. The financial terms of Mitsui's subscription, including the investment amount and ownership percentage, were not disclosed in the announcement.

With Commonwealth Fusion Systems representing reactor technology, Kyoto Fusioneering covering fusion engineering, and MiRESSO addressing materials supply, Mitsui's fusion portfolio is now spread deliberately across the value chain. Whether the 300°C process can scale from a 300-square-meter pilot to volumes capable of feeding commercial reactors remains the central question, and fiscal year 2027 will be the first real test of that answer.