Terrestrial Energy has submitted a Graphite Topical Report to the U.S. Nuclear Regulatory Commission (NRC), advancing the regulatory program for its Integral Molten Salt Reactor (IMSR) as the company works toward commercial deployment.
The report seeks NRC approval of the company’s methodology for qualifying graphite used in the IMSR Core-unit, a central component of Terrestrial Energy’s Generation IV molten salt reactor design.
The submission is the second of at least three IMSR Topical Reports the company expects to submit to the NRC in 2026. These reports are intended to address important technical and regulatory questions before Terrestrial Energy submits a future plant license application.
Graphite is central to the IMSR design
The IMSR is a graphite-moderated molten salt reactor, meaning graphite is used as the reactor moderator while molten salt serves as the reactor’s fuel-bearing and heat-transfer medium.
Although graphite has been used as a moderator in commercial nuclear reactors for decades, its behavior has to be demonstrated for each specific reactor design. Terrestrial Energy therefore has a dedicated qualification program to establish that its selected graphite can perform under the conditions expected inside the IMSR Core-unit.
The company says it has spent several years collecting graphite performance data through testing at the NRG High Flux Reactor in Petten, Netherlands. The program has examined multiple nuclear graphite grades from multiple suppliers.
Those results will support the qualification methodology described in the newly submitted report. According to Terrestrial Energy CEO Simon Irish, graphite performance ultimately determines the service life of the IMSR Core-unit. The company says the new submission begins the NRC review of the methodology it intends to use to qualify the material for commercial operation.
Building the IMSR licensing basis
The Graphite Topical Report is part of a bigger pre-application engagement program between Terrestrial Energy and the NRC. The company previously received NRC approval for its Principal Design Criteria Topical Report in 2025 and its Postulated Initiating Events methodology Topical Report in 2026.
Topical reports allow regulators to review specific technical and regulatory issues to be reviewed ahead of a full plant license application. Terrestrial Energy says approved reports can establish foundational elements of the IMSR licensing basis that may subsequently be referenced in future applications without requiring the same technical issues to be reevaluated.
The company expects to submit at least three additional topical reports during 2026, addressing areas it considers consequential to the eventual licensing of an IMSR plant.
The regulatory work is being carried out alongside engineering, testing, supply-chain development and commercial planning for Terrestrial Energy’s planned first commercial IMSR deployment in the early 2030s.
A replaceable reactor core
The graphite report also relates to an unusual aspect of Terrestrial Energy’s proposed commercial model. The IMSR Core-unit is designed to be replaceable.
The company plans sequential Core-unit replacements on a roughly seven-year cycle during an IMSR plant’s operating life. Terrestrial Energy says this approach is central to its capital-light business model and could create recurring component-supply revenue after a plant is built. An IMSR plant can deliver 822 MWt of net thermal energy or 390 MWe of net electricity, or a customized combination of both.
While the graphite qualification process remains an important regulatory step, the newly submitted report does not itself approve the IMSR for construction or operation. Rather, it asks the NRC to review the methodology Terrestrial Energy intends to use to demonstrate that its graphite can meet the requirements of the reactor’s intended operating conditions.