BUBBLE BEHAVIOR IN TWO-PHASE FLOW NATURAL CIRCULATION EMPLOYED IN THE PRIMARY SYSTEM ON THE INTEGRATED MODULAR WATER REACTOR (IMR), ANALYZED BY α-FLOW CODE

摘要

The Integrated Modular Water Reactor (IMR) is one of the advanced small-sized reactors. Its rector vessel features the primary cooling system employing two-phase flow natural circulation of light water coolant and built-in-vessel type of steam generators. This primary cooling system, named the Hybrid Heat Transport System (HHTS), enables competitive economy and high-level safety by elimination of piping of large diameter, pumps and pressurizers. The steam generators are two kinds; one (SGV) is located in the vapor region in the reactor vessel to condense vapor and control the primary system pressure, the other (SGL) is submerged in the two-phase mixture region to remove heat from the mixture and control the core inlet temperature down to subcooled state. The SGV and SGL totally extract 1000MW of thermal power from the core. The design target of averaged void fraction in the two-phase upward flow is around 20% to 25%, which is corresponding to bubbly flow. The performance of the HHTS is subject to bubble behavior and void distribution in the circulation. This study deals with bubble behavior investigation for the riser-downcomer sections through analyses by the commercial code, α-FLOW. Free surface vapor separation from the two-phase flow, which is important to determine required capacities of the SGV and the SGL, is also estimated, and vapor separation on the steam-water simulation test with the loop facility of small diameter piping is additionally investigated to clarify its predominant factors of the separation. The analyses show two-phase flow distributions dependent to the given conditions at the riser inlet, which are corresponding to core cycle burn-ups. The analyses estimate around 30% of the generated vapor to separate up to the vapor region in the case of 16GWd/t of cycle burn-up without the internal structures, while the steam-water simulation test data arranged by the induced velocity parameter suggest that predominant factors of the vapor separation are the system pressure and liquid velocity horizontally flowing towards the downcomer section.