Fusion neutron leakage into air produces atmospheric C-14 at 0.25-0.50 atoms per 14.1 MeV neutron

The arXiv preprint models terminal neutron streaming through open ports and ducts using MCNP6.2 with 14.1 MeV DT and softer spectra. Conversion probabilities range 0.25-0.50 for air nitrogen interactions, rising for lower-energy leakage. Scaling to 2500 GWe fleet capacity shows that a mean leakage fraction of order 10^{-6} is required to keep fusion-derived radiocarbon below 10 percent of natural cosmogenic input.

Natural ^{14}C production is dominated by cosmic-ray spallation at roughly 1.6 atoms cm^{-2} s^{-1}. The fusion pathway adds a controllable, ground-level source whose isotopic signature and vertical distribution differ from stratospheric input. Siting and regulatory frameworks developed for fission and tritium will therefore require extension to neutron shielding specifications that explicitly bound atmospheric activation.

Existing fusion environmental assessments have focused on tritium and activation products inside the vessel. This work identifies an external, non-point-source term that scales directly with integrated neutron output and port area. Designs incorporating beamlines or diagnostic ducts face the tightest constraints; fully enclosed stellarator or inertial concepts may relax them.

Next steps include coupling these source terms to global carbon-cycle models and performing site-specific Monte Carlo runs for proposed demonstration plants. Experimental validation with neutron beams on instrumented air volumes would reduce the present order-of-magnitude uncertainty.