The 1946 declassified Los Alamos report addressing whether a nuclear detonation could ignite a self-sustaining fusion chain reaction in the atmosphere’s nitrogen or in the oceans’ hydrogen/deuterium, before the Trinity test. Concludes propagation requires the energy gained from nuclear reactions to exceed radiative-cooling losses in the (largely radiation-transparent) atmosphere, and finds this condition is not met at any credible bomb temperature — ignition is “impossible,” with a large safety margin. Widely cited as the founding case of formal pre-experiment catastrophe risk assessment in physics, structurally paralleled by the RHIC and LHC safety reviews (each: identify the specific runaway/chain-reaction mechanism, show quantitatively it can’t cross threshold). relevance_note: canonical methodological precedent for the “official safety report” genre the LSAG/CERN-2003-001/RHIC reports belong to — useful for assessing whether that genre has a track record of getting these calls right (LA-602’s conclusion held; Trinity did not ignite the atmosphere).