The United States Department of Energy has identified the pebble-bed reactor as a high priority for US research with the end goal of licensing a pebble-bed reactor (PBR) for operation in the United States. Before this can be accomplished significant research must be done regarding the safety of the pebble-bed reactor. Of high importance is using existing experimental data and behavioral models of graphite to expand its knowledge base, and to understand the effect that changing graphite properties can have on the safety behavior of the PBR core.
In this dissertation, summaries are presented of pertinent, existing data on: graphite material properties, theoretical graphite behavior models and characterizing experiments, simulation methods for PBRs including the discrete element and finite element methods, and empirical correlations for evaluating the safe operating metrics of the PBR.
Using these tools and data, novel simulation methods were developed to combine graphite behavior with these methods to estimate significant changes to PBR safe operation. Control simulations using static graphite properties were built alongside the developed simulations using dynamic graphite properties, and the resulting differences evaluated.
Results of this work agreed with previous experimental and simulation works from the German AVR, HTR-10, and HTR-PM reactor programs, and with data from international and national benchmarks. The dynamic graphite properties were not conclusively shown to significantly affect the core temperatures and neutron physics behaviors but did show that at high temperatures and over long irradiation periods, changing graphite properties could significantly change the crushing forces on graphite fuel elements.
From the developed work, a better understanding of graphite and PBR core behavior was produced. The phenomenon of dust generation and pebble cracking in the core appear to be better simulated using these dynamic graphite properties. This research also provided useful design insights for PBRs. The results of the research suggested that future design work should include consideration of: once through fuel cycles, graphite thermal expansion properties as a major driver for material selection, and the sharpness of the designed bottom conus of the core.
The data and mathematics used, along with the developed graphite property models and simulation tools, are made available here to expand the knowledge base of graphite behavior to the wider scientific community. This research provides valuable information which contributes to the goal of licensing safe PBR designs in the United States.