Nuclear power plant simulation with a moveable-boundary steam generator model

A fast-running model has been developed for typical pressurized water reactor transients. The model is based on a moveable-boundary steam-generator formulation whereby the phase-change boundaries are tracked as they move along the channel. The advantage held by this formulation, as compared to the usual fixed mesh formulation is due to the fact that the phase-change boundaries, such as the boiling and superheat boundaries, represent sharp discontinuities in the physical properties of flow and heat transfer. Therefore, tracking these boundaries allows for a more homogeneous representation of those properties, even when using a rather coarse-mesh structure such as one node each for the subcooled, boiling and superheat regions of the heat exchanger. In this formulation, the heat exchanger secondary side is divided into three moving nodes, namely a subcooled node, a boiling node, and a superheat node. The primary side mesh structure follows that of the secondary side. The two extra nodes before and after the heat exchanger zone represent the inlet and outlet plena which are fixed zones. The combination of some or all of these boundaries depends on the existing flow regimes and their upward or downward movement depends on the nature of the transient involved. Some test results are presented and briefly commented. (Nogami, K.)