Thermal-fluid simulation investigation of the reactor cavity cooling system standpipes design for the pebble bed modular reactor
Oosthuizen, Philip Rudolf
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Pebble Bed Modular Reactors are advanced nuclear reactors and are being developed to possess inherent safety and reliability. This is achieved by utilizing a sequence of passive thermal storage and heat transfer mechanisms, to perform long term decay heat removal. The Reactor Cavity Cooling System (RCCS) facilitates this long term decay heat removal. The purpose of this study was to develop a one-dimensional, homogeneous two-phase flow model in order to perform investigative thermal-fluid studies on the concept designs of the standpipes of a typical RCCS system, such as that proposed for PBMR . An extensive literature survey was conducted and revealed that numerous research projects have been done in the field of passive heat removal systems. It was found that the two-phase homogeneous model was used by many authors to investigate the characteristics of natural circulation systems; however, discrepancies were found in the implementation strategy of the two-phase friction multiplier. Research proved that the theoretically derived, homogeneous multiplier were applicable to this study. A simulation model was developed to perform investigative studies on the standpipes of the RCCS. This model is capable of simulating various fundamental phenomena, found in natural convective boilers, including heat transfer in the sub-cooled and nucleate boiling regions. The model was verified with experimental data obtained from Kyung and Lee (1996). The simulation results were in good agreement with the experimental data, even though deviations were observed in the mid heat flux region. These deviations occur due to flow oscillations that were not modelled in this study. The two concepts (U-Tube and Annulus) of typical RCCS stand pipes were compared with each other, subjected to a heat load of 23 MW. The effect of the heater and riser diameter, as well as the effect of different reactor power profiles (constant and axial varying) on the flow characteristics were investigated. It was found that the annulus concept performed better, compared to the U-Tube, due to lower wall temperatures and higher flow rates.
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