Abstract:
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.
