Terrestrial Energy has filed its Graphite Topical Report with the U.S. Nuclear Regulatory Commission (NRC), moving its Generation IV IMSR reactor licensing program forward. The document asks the NRC to approve the methodology the company intends to use to qualify the graphite employed in the reactor’s Core-unit — the central component of its molten salt design.
The choice of graphite is far from a minor detail. The IMSR is a graphite-moderated reactor, where this material performs the function of neutron moderator while molten salts act as both the heat carrier and the fuel-bearing medium. Graphite performance directly determines the operational lifespan of the Core-unit, and therefore the economic viability of the entire plant. Terrestrial Energy has been running an experimental qualification program for years: testing is conducted at the High Flux Reactor in Petten, the Netherlands, operated by research institute NRG, across multiple grades of nuclear graphite supplied by multiple manufacturers. That data now underpins the methodology described in the newly submitted report.
The Graphite Topical Report is the second of three reports the company plans to deliver to the NRC during 2026. The sequence reflects a deliberate regulatory strategy: address the most technically significant issues well ahead of the future operating license application, thereby reducing the time and uncertainty of the final review. Approved topical reports can be referenced in subsequent license applications without requiring re-evaluation from scratch, substantially streamlining the authorization process. This is a mechanism the NRC explicitly provides for advanced reactor projects, and Terrestrial Energy is using it methodically.
The backdrop to this move is a strong one. In September 2025, the NRC approved the IMSR’s Principal Design Criteria Topical Report, which establishes design criteria for all major plant systems, including the intrinsic fission power control mechanisms. In May 2026, approval followed for the Postulated Initiating Events Topical Report, which defines the methodology for identifying and assessing events that could challenge safe reactor operation. Those two approved reports form the foundation of the IMSR’s licensing basis for commercial operation. The Graphite Topical Report builds on that structure.
On the business side, the Core-unit — replaced every seven years — sits at the heart of Terrestrial Energy‘s business model: each scheduled replacement generates recurring revenue throughout the plant’s operational life, on top of the initial construction contract. Graphite qualification is therefore also a financial matter, as it sets the parameters within which every replacement cycle occurs. If the NRC approves the methodology, the company will be able to complete material qualification within a framework already validated by the regulator.
With three topical reports filed or approved within twelve months, the IMSR’s regulatory program is advancing at a pace few advanced reactor projects have sustained. If the momentum holds, a formal operating license application could be submitted with much of its documentary foundation already pre-validated, significantly narrowing the window between filing and final authorization.
