A factual clarification and chemical-technical reassessment of the 1921 Oppau explosion disaster - the unforeseen explosivity of porous ammonium sulfate nitrate fertilizer

The Oppau factory was a chemical production facility constructed by the leading German chemical company BASF in the time period shortly before the First World War. The facility was located along the river Rhine, a few kilometers north of the town of Ludwigshafen in Germany. The purpose of the chemical manufacturing at Oppau was the production of nitrogen fertilizers on the basis of the then recently developed Haber-Bosch synthesis of ammonia. It was the first facility of its kind anywhere in the world. On the morning of 21 September 1921, hundreds of tons of ammonium sulfate nitrate (ASN) fertilizer, kept in a storage silo at the Oppau factory site, decomposed explosively when piles of hardened fertilizer material were broken up with the help of explosives. The event, referred to as the Oppau explosion disaster, killed more than five hundred people, wounded nearly two thousand more, and destroyed a large part of the factory site and surrounding residential area. The incident remains one of the worst industrial disasters and most iconic hazardous material events of all times. The Oppau explosion disaster is different from most other explosion disasters since it was not the result of an uncontrolled fire. The procedures leading to the disaster had actually been applied at the same facility thousands of times prior to the disaster. In spite of the exceptional assembly of notable experts summoned for the ensuing inquiries, the alleged causes of this industrial calamity – while partially identified – were never completely understood. The objective of the present report is to clarify actualities and to ascertain the chemical-technical causes of the tragedy at Oppau, with the highest degree of certainty possible. To that end, this investigation has involved two main approaches. The first is a historical-technical approach, comprising a detailed reassessment of the original investigation material in light of the current state of knowledge within the science of energetic materials. The second is a chemical-technical approach, involving computerized thermochemical modeling of ASN decomposition, in an attempt to shed light on the energetic potential of ASN mixtures. It is clarified how the understanding of energetic materials at the time of the accident was inadequate for an appropriate understanding of the phenomena involved. It is implied that the special physical characteristics imparted on the Oppau fertilizer material, by the application of a new spray-drying procedure some time prior to the accident, constituted an unforeseen disaster in waiting. The subsequent investigations thus struggled in their efforts to align experimental results and the conclusions extracted from these, with the theoretical framework of explosive materials then in existence. The presented account renders a prime example of the dangers confronted at the intersection of large-scale chemical manufacture and a, to some degree, incomplete scientific foundation. Moreover, it underscores the unpredictable behavior of ammonium nitrate materials.