Water ingress is one of the peculiar accidents of high temperature gas-cooled reactors (HTR). Since the pressure of the secondary circuit is higher than that of the primary circuit, when the break of heating tubes of the SG occurs, liquid water and steam will enter the primary circuit from the secondary side, and then enter the reactor core together with the helium coolant. This will result in introduction of a certain positive reactivity, rising of the primary pressure and corrosion of the in-core fuel elements and graphite components. Therefore, water ingress is a complex physical processes involving water spout, flash evaporation, multi-phase flow, heat transfer and other phenomena, and in-depth research of this accident will benefit further understanding of the characteristics of HTRs.

Based on the preliminary design of the Chinese high temperature gas-cooled reactor pebble-bed modular (HTR-PM), assuming that the break of steam generator (SG) heating tube can be considered as a round pressure nozzle, the characteristics of the flow line at breaks with different diameters, as well as the property of the droplet spectrum are studied. According to the analysis of droplet properties of flow and evaporation in the primary circuit, the preliminary result shows that under the design basis accidents of water ingress, the water will enters the reactor core in the form of steam. This analysis result can provide a basis for the further research of water ingress of HTR in the future.

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