In the hypothetical liquid metal reactor SGTR (Steam Generator Tube Rupture) accident, the high-pressure water in steam generator tube will be injected into the high-temperature liquid metal. During the water-liquid metal interaction, water vaporized and vapor bubbles migrated into liquid metal, which could be resulting a sharp pressure raise and threatening the safety of the reactor. This multi-phase system can be abstracted as a system in which a high-pressure subcooled fluid (overheating state at atmospheric pressure) is injected into another higher temperature fluid (normal pressure) to form a two-component (water and liquid metal) system in which multiphase (fluid and vapor) were existed.

There are two effect could cause bubbles expansion which lead to overall system pressure increase, they are flash evaporation of pressurized water since the depressurization and high-temperature glycerin heating the water/vapor. In this paper, the theoretical analysis of the single bubble expansion model is used to evaluate the effect of these two phenomena on the system pressure rise. First of all, this paper uses the force balance point of view to derive the single bubble expansion equation inside the inviscid fluid, and obtain the relationship between bubble expansion speed and bubble internal pressure. Secondly, through conservative assumptions, the bubble expansion speed caused by the heating effect of the high temperature fluid on the bubble is evaluated. Finally, this paper compares the strength of these two effects in the expansion of a single bubble, and theoretically explains the driving force of the pressure increase in the simulation experiment and the potential flow pattern.

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