A new modeling study examines an environmental consideration for future fusion energy systems: the production of atmospheric carbon-14 from neutron leakage. Using MCNP6.2 neutron-transport simulations, researchers calculated the probability that neutrons escaping from fusion reactors undergo the nuclear reaction ¹⁴N(n,p)¹⁴C—the same process that creates natural radiocarbon in Earth's atmosphere.
The study focused on deuterium-tritium (D-T) fusion systems, which produce 14.1 MeV neutrons. The modeling shows conversion probabilities of 0.25 to 0.50 across representative geometries, meaning that a significant fraction of leaked neutrons interact with atmospheric nitrogen to create carbon-14. For a single 1 GWe fusion power plant, the analysis indicates that percent-level neutron leakage into air would generate carbon-14 production rates comparable to natural global sources.
At the scale of a potential 2500 GWe global fusion fleet, the implications become more stringent. To keep fusion-derived radiocarbon below 10 percent of natural atmospheric production, the mean neutron leakage fraction must be limited to approximately one part per million. This extremely tight tolerance presents a design challenge for reactor architectures with open ports, beamlines, ducts, and other potential streaming paths for neutrons.
The research provides a "screening-level" source-term estimate—an initial assessment to guide fusion safety analysis and regulatory frameworks. Unlike traditional fission reactors, which contain their nuclear reactions within pressure vessels, fusion systems using neutral beam injection or other external systems may have inherent leakage pathways that require careful engineering.
These findings emphasize that fusion's environmental credentials depend not only on zero carbon emissions during operation but also on rigorous neutron containment design. As fusion technology advances toward commercialization, developers must incorporate these constraints into engineering specifications and safety analyses to demonstrate compliance with radiological protection standards. The work underscores fusion's need for robust shielding and architectural design choices that minimize uncontrolled neutron escape.
