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Numerical and experimental studies of IFE target layering in a cryogenic fluidized bed


The redistribution of deuterium (DD) or a deuterium- tritium mixture (DT) to form a layer on the inside of spherical inertial fusion energy (IFE) capsules is a challenging problem because of the symmetry requirements of the fuel layer thickness, the smoothness requirement of the outside target surface, the number of targets required, and the time restriction on the production process. Several physical processes have been identified to interact with each other to influence the outcome of the layering process in a fluidized bed. These include the gas -flow-speed-dependent movement of unbalanced spheres through a fluidized bed and the resulting local heat transfer coefficient on the target surface from the cooling gas. The mass redistribution speed of the fuel inside the shell towards a uniform layer and the final layer thickness uniformity depend on the variation in time -averaged local heat transfer coefficient along the outer target surface. While a high gas flow rate through the bed would lead to more uniform time-averaged heat transfer coefficients, the high-Z layer covering the outer target surface has been observed to deteriorate through collisions at high impact velocities which occur during fluidization at high bed expansions. The focus of this work was to develop numerical tools to help model and understand the physics involved in the fluidized bed layering and to assess the influence of key parameters on the layering outcome. Two separate models have been developed independently for particle behavior in a fluidized bed and for the coupled mass and heat transfer processes governing the layering process; these models include unique boundary conditions, beyond the capability of currently found commercial software. The models were validated through comparison with theoretical results and laboratory-scale experiments. They were then combined to model the entire layering process and used for parametric analyses. From these analyses, a window of operating parameters was identified at which a prototypic layering experiment is likely to be successful.

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