Applying Extreme Value Theory to a Century of Ammonium Nitrate Disasters: Persistent Safety Risks in Chemical Supply Chains.

We have analyzed 39 major ammonium nitrate (AN) accidents (1916-2022) that reveal systemic catastrophic risks have not diminished over the past century. This challenges the prevailing narrative of improved process safety of AN on account of iterative guidelines and technology advances. Applying extreme value theory (EVT), we model temporal occurrence as a homogeneous Poisson process (maximum likelihood estimate λ ≈ 0.23 events/year) and casualty severity as heavy-tailed (generalized Pareto distribution, shape parameter ξ ≈ 1.13). These findings indicate that there are persistent systemic patterns. This is shown by the implied "shadow mean" that exceeds the sample mean by 1.5-19-fold, and societal risk profiles occupy the "intolerable" region per frequency-number curve criteria. To address this problem, we establish an EVT-based systemic monitoring framework with quantitative benchmarks, identifying 50-year (∼315 fatalities) and 100-year (∼715 fatalities) return levels as a data-driven baseline for assessing safety performance. This ensemble framework provides a necessary tool for stakeholders to monitor realized systemic risks over mechanism-driven simulations or narrative expectations.