## 2 Simulation of Blast Events
### Characteristics of Blast Load and Pressure Wave Propagation  
Blast loads are generally defined by the peak overpressure, the positive phase duration, and the shape of the pressure versus time curve. Although ammonia process units and import terminals with storage tanks for ammonia are already in operation [19], there are no specific codes or industry standards for specifying the blast load for ammonia tank design. A look at LNG process units or even the entire chemical or petroleum industry as well as storage areas shows the same normative regulatory gap. Only guidelines such as the TNO Green Book [16] contain specifications for industrial buildings, for example. The definition of the applicable blast load is typically derived from a project specific risk assessment that considers explosion sources within and outside the battery limit of the new facility. Since ammonia has a relatively high minimum ignition temperature, the risk of an explosion within an ammonia plant is considered very low. However, due to the high toxicity of the ammonia product and the circumstance that new ammonia projects are approaching storage sizes in the range of world-scale LNG facilities, authorities intend to define minimum blast and impact loads to be considered by design [13; 14]. The minimum blast load requirements only specify the peak overpressure and positive phase duration. But the information regarding the shape of the time curve or the cause of the explosion is missing, which is necessary for accurately specifying the overpressure time history. When discussing accidents caused by explosions in tank farms, the Buncefield event is often the first to come to mind. In 2005, a vapor cloud explosion occurred at Buncefield Oil Storage Depot, UK, generating significant blast pressures, which is reviewed, e.g., in [20]. Further reviews of accidental explosions in tank storage facilities can be found in the literature. The authors of [21] studied 242 accidents of storage tanks that occurred in industrial facilities. Their finding is that most of those accidents would have been avoided if good engineering had been practiced. A further summary on explosion events in a tank farm was carried out by [PERSON] [22]. This article gives an overview of accident frequencies of atmospheric storage tanks and past accidents described in the open literature to show the possible accidental sequences, underline the most frequent causes involved in explosions of atmospheric storage tank, and highlight the potential consequences of such accidents [22]. In general, within the wide range of accidental explosion types, vapor cloud explosions are a major concern in the petrochemical and petroleum industry [20; 23; 24; 25; 26]. The same applies to LNG process units [27; 28; 29] and can therefore be adopted to ammonia process units as the main cause for the blast overpressure analysis and design. Furthermore, there is a growing threat of terrorist attacks targeting tank structures, which are considered critical infrastructures. These attacks typically involve the use of explosive materials like TNT to cause significant damage [30; 31; 32]. While the latter event is characterized by an instantaneous increase in pressure and is referred to as detonation with a propagation velocity of the shock wave above the speed of sound, an explosive event due to a vapor cloud explosion can show different characteristics. An extremely energetic vapor cloud explosion can, in the same way, propagate in the form of shock waves in the near and far field [23; 33]. But most vapor cloud explosions are characterized by slow deflagration and propagate in the form of pressure waves that usually travel below the speed of sound, which is slower in comparison to shock waves. In special cases, it is also possible that a pressure wave caused by a vapor cloud explosion transfers while propagating to a shock wave, the so-called deflagration-to-detonation transition (DDT) [23; 33; 34]. However, the possibility of DDTs is lower than the one of deflagrations.