Predictive modeling of atmospheric nuclear fallout microphysics

The capability to predict size, composition, and transport of nuclear fallout enables public officials to determine immediate and prolonged guidance in the event of a nuclear incident. Predictive computer models of fallout can also provide useful insight for nuclear forensic response when detailed radiochemical processes can be reliably included. Current post-detonation nuclear fallout models prescribe particle size distributions empirically or semi-empirically, based on measurements across limited conditions pertaining to tests conducted primarily in Nevada and the Pacific. These empirical fallout relationships may be subject to large uncertainties in particle size and radionuclide activity distribution if used to extrapolate to other regions with different environmental conditions (e.g., urbanized areas). Replacing empirical relationships with physics-based microphysical process modeling can enable significant advances in the fidelity of predictive models simulating distributions of fallout across diverse environments. Particle microphysics describes the formation and evolution of fallout particles, as well as the interaction of radioactive material with entrained particles, which requires accounting for fundamental processes such as nucleation, condensation, and coagulation. The objective of this perspective article is to summarize computational techniques to simulate particle microphysical processes advancing the fidelity of predicting nuclear fallout. We review current empirical models for simulating post-detonation fallout and assess promising research directions moving towards physics-based predictive systems.