Nanoplastics rarely enter the environment as manufactured nanoparticles — most form when millimetre-scale microplastic is weathered by sunlight and mechanical stress until it fractures, again and again, into ever-smaller fragments. This simulator renders that stochastic fragmentation cascade in 3D: a chamber of microplastic particles accumulates a UV photo-oxidation dose (rate set by the chosen polymer's susceptibility) plus an independent mechanical-abrasion term, and each particle's per-step fracture probability rises with dose and inversely with its own radius. Every fracture event conserves volume while splitting the parent into 2–4 daughters, so live readouts track simulated exposure time, particle count, the nanoplastic (<1 µm) fraction, mean diameter and total surface area — the last of which grows every event even though total mass never does, which is exactly why the resulting nanoplastics leach additives and adsorb pollutants far more readily than the microplastic they came from.