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Physical Similitude in Hierarchical Engineered Systems


Engineers interested in predicting the behavior of complex engineered systems have a strong need to prioritize phenomena by importance due to limitations in simulation and experimentation. The hierarchical nature of reactor systems allows for this organization of complex interactions. By breaking down the system into a series of levels, or stratas, the hierarchical architecture of the system can be established and provide a rational basis for the top-down and bottom-up scaling analyses. From the global response of the reactor using the integral forms of the fundamental transport equations to the atomic interactions such as vacancy creations in reactor materials, the level of acceptability for model fidelity varies enormously and depends strongly on how much of the application domain is encompassed by the validation domain.

In this dissertation, physical similarity criteria are derived at three distinct levels of an advanced reactor: system, subsystem, and component levels. For the purpose of this work, the Advanced High Temperature Reactor (AHTR) is used as a reference reactor. The AHTR is a liquid-salt cooled, high temperature reactor that uses coated-particle high temperature reactor fuel. At the system level, the focus is on developing similarity criteria first at the highest level of the reactor system where the focus is on the dynamic interaction of the components in the system or the global response. Fractional scaling and causative process related scaling methods are introduced where numerical values for each non-dimensional group are determined. At the subsystem level, the concept of a buoyantly-driven shutdown rod system for reactivity control is introduced. The scaling rationale for the shutdown rod system is determined where limitations in subsystem scaling are discussed. Results from a reduced-scale experiment are presented. The shutdown rod performance with respect to system response is also analyzed. At the component level, the efficacy of a fluidic diode concept in high temperature reactors is examined. Similarity criteria and results from a fluidic diode reduced-scale experiment are presented. An empirical approach is used to determine design optimization. The methods developed in this dissertation should be equally applicable to other reactor types and complex engineered systems outside of the nuclear industry.

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