In a Nagasaki bomb simulation, dust and radioactive material took different paths

The model found that ground dust rose through the cloud’s center, while bomb-derived radioactive material gathered around its spreading head.

Editorial illustration — not from the study.

Researchers modeled the blast wave, fireball and later mushroom-cloud rise in three stages. They tracked ground-derived dust and bomb-derived radioactive material separately as they moved through the simulated airflow.

The simulation reproduced the hot fireball’s upward rise, the cloud head’s horizontal spread and a rotating ring structure. The two materials followed different routes: dust was drawn mainly into the central updraft, while radioactive material was preferentially carried into the ring around the cloud head. The results are intended to provide starting conditions for later simulations of radioactive “black rain.”

How the materials moved

The calculations showed that the reflected shock wave deformed the fireball and created a strong upward flow along the cloud’s central axis. As the mushroom cloud rose, its hot core continued upward, its head spread sideways and a vortex ring developed around the head.

Ground-derived dust—used in the model to represent non-radioactive smoke and dust from fires and disturbed surfaces—was entrained mainly through the central updraft. Bomb-derived radioactive material, assumed to represent plutonium released from the bomb, was preferentially incorporated into the vortex-ring region around the cloud head.

The study also found that the simulated cloud rise depended on numerical resolution and the background atmospheric profile. Coarse grids blurred the interaction between the reflected shock and fireball and underestimated the later cloud-top height. A dry adiabatic atmosphere agreed better with historical nuclear-test data than the International Standard Atmosphere used in another comparison.

Why the paths matter

The results indicate that radioactive material and ordinary ground dust did not necessarily move together within the Nagasaki mushroom cloud. Distinguishing their three-dimensional distributions can provide more specific starting conditions for meteorological simulations examining how radioactive material and dust may have contributed to “black rain.”

Model evidence and limits

This is a numerical modeling study, not a direct measurement of the 1945 cloud or its contamination. The researchers represented the two materials as passive tracers carried by simulated airflow; the radioactive component was assumed to be plutonium, and the dust component represented non-radioactive smoke and disturbed surface material. The findings depend on the model’s grid resolution, atmospheric profile and other setup choices. The study provides initial conditions for later weather simulations rather than a complete simulation of black rain or a direct reconstruction of measured fallout.

// Source

Progress in Earth and Planetary Science · 2026 · DOI: 10.1186/s40645-026-00841-8

View paper (DOI)Open access versionPublished 2026-08-24

Authors: Kenta Nakajima, Akiko Matsuo

Institutions: Keio University

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