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ABSTRACT: Understanding how the fireball from a nuclear detonation interacts with its environment is essential to predicting the post-detonation environment, including fallout composition and form. Realistic scenarios for nuclear events inevitably involve complex environments, such as urban settings, however the majority of data informing fallout processes come from environments devoid of relevant buildings or other structures. This paper summarizes recent developments in simulations of above-ground nuclear explosions as part of a broader effort to better characterize conditions within a fireball that may influence the chemical evolution of bomb materials and other materials entrained from the local explosion environment. We discuss our recent improvements in modeling of the coupling of radiation transport and mechanical deformation, as well as the transition from intact materials (e.g., rock, concrete, etc.) into airborne particulates. The entrainment process is particularly important to our investigations because entrained materials are a predominant influence on the chemistry and form of resultant fallout. 1. INTRODUCTION This paper discusses recent efforts as part of an internal research project at Lawrence Livermore National Laboratory to improve our understanding of the post nuclear detonation environment, including the chemical evolution of species within the fireball, addressing both bomb debris and entrained material. The motivation is to be able to provide actionable information for both forensic (e.g., establishing responsibility for an event) and consequence management (e.g., predicting the activity of respirable particles). Our goal is to be able to simulate complex scenarios that involve conditions outside of historical testing experience, including, for example, an event at street level in an urban environment. Modeling of such scenarios involves capturing a number of physical and chemical processes that span a range of spatial and temporal scales. Fig. 1 shows the approximate sequence of events and associated processes. At early time, the outgoing shockwave and radiation cause damage and vaporization of immediate geologic materials and structure prior to entrainment into the evolving fireball. It is critically important to capture the geomechanical processes at this stage of fireball evolution. The vaporization, pulverization, and comminution of the geologic materials will determine how much mass introduced into the fireball at early time. This entrained material plays at least two critical roles. First, the entrained mass will cool the fireball, leading to more rapid condensation of materials from the plasma state. Second, the entrained material introduces additional chemical species that contribute to subsequent fallout formation. As the fireball expands and radiates, the initial plasma state cools and individual atoms and molecules can develop. During this phase, it is important to be able to predict what specific molecules develop, because some molecules are more refractory than others. For example, depending upon how much oxygen is available, different oxidation states will be achieved with different melting points. Consequently, modeling the mixing of the fireball with both entrained materials and with the atmosphere is key to predicting the initial formation of fallout relevant radionuclide species. With further cooling, nucleation and condensation of particles occurs, and they are subsequently transported to the surrounding environment. |
