Super-Droplet Method to Simulate Lagrangian Microphysics of Nuclear Fallout in a Homogeneous Cloud

Nuclear detonations produce hazardous local and global particles or fallout. Predicting fallout size, chemical components, and location is necessary to inform officials and determine immediate guidance for the public. However, existing nuclear detonation fallout models prescribe the particle size distributions based on limited observations. In this work, we apply the super-droplet method, which is a numerical modeling technique developed for cloud microphysics, to simulate size distributions of particles in a mushroom cloud formed post-detonation of a nuclear device. We model fallout formation and evolution with homogeneous nucleation and condensation of a single species and a Monte Carlo coagulation algorithm. We verify the numerical methods representing coagulation and condensation processes against analytical test problems. Additionally, we explore several scenarios for the integral system mass and yield in equivalent kilotons (kt) of TNT (trinitrotoluene). The fallout size distribution median diameter d(pg) follows a scaling law based on the integral system mass m(v0) kg and yield Y kt: d(pg) = 0. 647m(nu 0)(0.609)/Y-0.436 nm. We test the effect of cloud turbulence, enhanced nucleation and growth, and vapor volatility with a sensitivity study. The range in median diameter predictions for simulations of historical tests performed over the Pacific encompass the measurements of particles sampled from the cloud caps. Predicted median particle size ranges up to 217, 123, 86, and 35 nm for historical tests with yields of 0.2, 0.7, 2, and 10 Mt, respectively. This work can be expanded in many different directions to build a more predictive model for fallout formation. Plain Language Summary Nuclear detonations can result in devastating effects on the surrounding area. In the aftermath, a cloud forms containing radioactive materials formed post-detonation. These radioactive particles can fall out and deposit locally or globally depending on their size. It is important to accurately predict the amount of fallout and fallout particulate size distributions to know the extent and degree of hazard from associated radioactivity. This work applies a technique developed over the last decade for cloud precipitation modeling to the formation and evolution of fallout in mushroom clouds. The numerical techniques implemented to simulate particle growth were verified. Next, we calculated the particle sizes formed under several different nuclear detonation scenarios, varying the integral system mass and the detonation magnitude (energy output). Additionally, we perturbed the underlying assumptions on the rates of particle growth and formation as well as the volatility of the vapourized materials. The range in particle size predicted across these perturbed scenarios encompasses the observed size from atmospheric nuclear tests and prior theoretical estimates.

Automated summary

Nuclear detonations produce hazardous particles that can fall out and deposit locally or globally. Accurately predicting the size and distribution of these fallout particles is crucial for assessing the extent of radiation hazard. This study applies a cloud precipitation modeling technique to simulate the size distribution of particles in mushroom clouds formed after nuclear detonations. The simulation results are validated against historical test measurements, and different detonation scenarios are explored to predict particle sizes.