Coupled Calculation of Reactor Core Thermal-Hydraulic Characteristics in the Early Blowdown Phase of LBLOCA

Loss of Coolant Accident (LOCA) is one of the design-basis accidents for light water reactors. Its blowdown phase is a complicated transient process, especially the early depressurization blowdown, during which the thermal-hydraulic characteristics of the coolant change drastically. To accurately evaluate the phenomena and safety margin of a pressurized water reactor (PWR) during the early blowdown phase of a large break loss of coolant accident (LBLOCA), the numerical simulation is conducted. Firstly, based on the independently developed system analysis program, a one-way coupling framework with the subchannel analysis program is established through secondary development. Secondly, the primary loop system of Qinshan Phase I nuclear power plant is selected, modeling and steady-state calculation were performed to obtain the initial parameter distribution for transient analysis. Finally, by solving the postulated cold-leg and hot-leg LBLOCA conditions, the variation curves of key parameters such as the inlet-outlet pressure difference, mass flux, and minimum departure from nucleate boiling ratio (MDNBR) of the hottest channel, as well as the core flow field distribution, were analyzed. The results show that the coupling framework can effectively obtain the detailed transient response of the reactor core, and the safety limits of nuclear power plants can be met. This study is an important work and provides a reliable numerical tool for the safety analysis of the early phase of PWR LBLOCA, which has practical application meaning.