Radioactive particles are released from the source, carried by the wind vector plus a random turbulent-diffusion walk, and settle onto a ground grid seen here from directly above — a simplified Lagrangian stand-in for the Gaussian plume model used in real fallout dispersion. Each landed particle adds its activity to the surface contamination density C(x,y) of the cell it lands in; falling particles are drawn shrinking as they descend so you can still read their height off the 2D view.
Surface dose rate: Ḋ(x,y,t) ≈ k · C(x,y) · A(t)/A₀
Radioactive decay: A(t) = A₀ · (1/2)^(t / T½)
Cumulative dose: D = ∫ Ḋ dt (integrated at the fixed monitor point ●)
- Wind speed / direction — sets the advection vector; a stronger wind stretches the plume further downwind before particles settle.
- Source release rate — how many contamination-carrying particles leave the source per second.
- Isotope — Iodine-131's 8-day half-life visibly decays within the slider's range; Caesium-137's 30-year half-life barely moves over the same window, illustrating why long-lived isotopes dominate long-term land contamination.
- Time since release — applies the decay law above uniformly to everything already deposited.
- Remediate hotspot — finds the worst-contaminated cell and simulates topsoil removal: it and its immediate neighbours are zeroed, the standard emergency-response technique for cutting local dose rate quickly.
Real-world relevance: this is the same logic environmental-monitoring teams use after an accident — deposition mapping to find hotspots, decay-corrected dose projections, and targeted remediation of the worst cells rather than the whole territory. Drag to pan the map, scroll or pinch to zoom.