Advanced light water reactor systems are designed to use passive emergency core cooling systems with horizontal pipes that provide highly subcooled water from water storage tanks or passive heat exchangers to the reactor vessel core under accident conditions. Because passive systems are driven by density gradients, the horizontal pipes often do not flow full and thus have a free surface that is exposed to saturated steam and stratified flow is present. The subcooled water flows into the reactor vessel via the downcomer—which is an annular region separated from the core by a cylindrical, steel, inner liner. Under accident conditions, where the horizontal pipes are only partially full, both saturated water and saturated steam from the downcomer are likely to be present in the pipe. The saturated water is quasi-static and the fraction of the free surface that is saturated water is dictated by the physics of the flow. The saturated steam is usually flowing in the countercurrent direction to the subcooled water. Consequently the flow may be a three-layered system with saturated steam over a static saturated liquid layer over a flowing subcooled layer. The conditions leading to a saturated liquid layer that separates the saturated steam from flowing subcooled water are explored. The variables that influence the formation of the saturated liquid layer and enable the saturated layer to be maintained for a spectrum of conditions, including steam flow in the countercurrent direction to the subcooled water, are derived, and compared to experimental data. Conclusions regarding this type of flow are given. Finally, typical steam flow velocities that may induce wave-bridging, leading to condensation-induced-water hammer, are identified.

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