Terrestrial Energy has filed the Graphite Topical Report with the U.S. Nuclear Regulatory Commission, marking a concrete step in the licensing process for its fourth-generation molten salt reactor, the 390 MW IMSR. The document asks the NRC to approve the methodology the company intends to use for qualifying the graphite employed in the reactor’s Core-unit — a central component of the overall design.
In a reactor like the IMSR, graphite serves as a moderator: it slows neutrons and sustains the chain reaction. The molten salt, meanwhile, acts as both the heat transfer medium and the fuel carrier. This architecture differs fundamentally from conventional commercial light-water reactors. Although graphite has been used as a moderator for decades, its performance must be demonstrated for each individual design. To that end, Terrestrial Energy has developed a dedicated qualification program, with tests on multiple graphite grades conducted at the NRG Petten research reactor in the Netherlands.
The newly filed report is the second of at least three Topical Reports the company plans to deliver to the NRC during 2026. The first — covering the methodology for Postulated Initiating Events — already received NRC approval earlier this year. Prior to that, in September 2025, the Principal Design Criteria report was approved. Approved documents establish foundational elements of the IMSR’s regulatory basis and can be referenced in future license applications without being re-evaluated, thereby narrowing the scope of subsequent technical reviews.
CEO Simon Irish explained that graphite performance determines the operational lifespan of the IMSR’s Core-unit. The Core-unit is designed to be replaced every seven years — a planned cycle that sits at the heart of the company’s business model: the majority of expected revenues, approximately 79% of total plant lifetime income, comes from the recurring supply of Core-units and fuel salt after initial construction. This structure reduces upfront financial risk and generates a steady revenue stream for decades.
On the commercial front, Terrestrial Energy has already secured significant partnerships. In May 2026, the company announced a collaboration with Riot Platforms to develop IMSR plants co-located with hyperscale data centers dedicated to artificial intelligence, with a potential capacity of up to 4 GW spread across multiple 390 MW units. The IMSR can generate either 390 MW of electricity or 822 MW of thermal power for industrial applications such as metal smelting and fuel refining. In parallel, the TETRA and TEFLA pilot projects — both pursued in partnership with the U.S. Department of Energy — continue to advance. The first commercial deployment is expected in the early 2030s.
