Nuclear Fallout Formation in Iron Rich Environments

Above-ground nuclear explosions that interact with the surface of the earth entrain materials from the surrounding environment, influencing the resulting physical and chemical evolution of the fireball. These influences can effect how hazardous radionuclides fractionate and are dispersed in the environment as fallout particles, and can affect their final chemical phase and mobility. The interaction of iron with a nuclear explosion is of specific interest due to the potential for iron to act as a redox buffer and because of the likelihood of significant masses of metals to be present in urban environments. We investigated glassy fallout from a historic surface interacting nuclear explosion conducted on a steel tower and report the discovery of widespread and diverse iron-rich micro-structures preserved within the samples, including crystalline dendrites and micron-scale iron-rich spheres with liquid immiscibility textures. We assert these micro-structures (termed ‘amoeboids’) reflect local redox conditions and cooling rates and can inform interpretation of high temperature events, enabling new insights into fireball condensation physics and chemistry when metals from the local environment (i.e., structural steel) are vaporized or entrained. Amoeboids likely form as a result of decomposition of a single liquid and/or from the emulsification of two compositionally distinct liquids. Amoeboid compositional comparison to computational phase diagram calculations produced using the CALPHAD (CALculation of PHAse Diagrams) method and to silicate liquid immiscibility measurements in other systems suggest they reflect non-equilibrium processes, complicating efforts to make quantitative inferences on fireball conditions. A phase field method (PFM) model shows that the variety of amoeboid morphologies are consistent with a decomposition hypothesis. Principal component analysis (PCA) and multivariate curve resolution (MCR) approaches to spatially resolved compositional measurements of samples were used to estimate the compositions of four distinct precursors and relative contributions to complex melt mixing during formation. These results also suggest that amoeboids may form as a result of spontaneous emulsification between a relatively Fe-poor, well mixed melt (all four precursors) and late entry of one Fe-rich precursor. While radioactive Pu has traditionally been associated with FeCaMg-rich glass in nuclear fallout, MCR models coupled with nano-scale secondary ion mass spectrometry (NanoSIMS) and autoradiography data highlighted inconsistent relationships between these elements in this work. Limited NanoSIMS data on amoeboids and immiscibility textures show that Pu (< 20 ppm) is primarily associated with the relatively Fe-poor phase of amoeboids, which generally supports an emulsification hypothesis. In summary, this work outlines key processes that may be unique to Fe-rich fallout formation, including variations in the relationships of Fe concentrations to other elements of interest, liquid immiscibility, widespread iron oxide crystallization, and limited evidence of intermediate oxygen environments. These processes may offer constraints on fireball conditions as the thermodynamics and kinetics of silicate immiscibility is better understood, and influence the distribution of radionuclides in the environment following a nuclear explosion. These processes should be considered in future quantitative models of fallout formation and radiochemical fractionation.

Advisor: Hosemann, Peter