Study on thermal-hydraulic response characteristics of internal compartments in the containment building under LOCA conditions

Abstract

Investigating the thermal-hydraulic response characteristics of internal compartments within the containment structure during LOCA accidents holds significant scientific, engineering, and safety value for elucidating accident evolution mechanisms, evaluating compartment structural loads, and optimizing containment design. During LOCA events, high-energy fluids released through breaches create highly non-uniform velocity, Distribution of Water Vapor, temperature, and pressure distributions within the complex multi-compartment space of the containment structure. This generates intricate dynamic structural loads, making the investigation of these non-uniform response characteristics crucial for assessing and ensuring containment integrity. Taking the Hualong One containment as the research subject, a three-dimensional computational model incorporating the main compartments and connecting passages was established. Transient numerical simulations were conducted for four accident scenarios: passive cooling system with cold-end breach closed, passive cooling system with hot-end breach closed, passive cooling system with cold-end breach open, and passive cooling system with hot-end breach open. The analysis examines the evolution of velocity fields, temperature distributions, temperatures in the breach area, pressures in the breach area, and pressure differentials between the breached compartment and adjacent compartments during the initial high-velocity jet and backflow formation. Regarding velocity, high-velocity zones primarily concentrate near the breach location, forming high-flow regions within the pressure vessel compartment and the top of the containment structure. After activating the passive cooling system, the velocity of moist air within the large upper containment space decreases, while the velocity of moist air within the pressure vessel compartment increases.Regarding water vapor, the PCCS effectively reduces the vapor content within the containment structure through wall condensation, thereby suppressing the rate of temperature rise and pressure increase. For hot-end breaches, where steam locally accumulates, the PCCS directly weakens the pressure differential driving force through immediate condensation near the breach, yielding a slightly higher suppression effect than for cold-end breaches.Regarding temperature, the side of the main pump compartment adjacent to the breach exhibits higher temperatures. Due to insufficient coolant, the pressure vessel experiences a rapid temperature rise, resulting in the highest temperatures in the central region. Temperatures decrease progressively from top to bottom within the large space. The temperature trends in the breach area are similar across the four accident scenarios: a rapid initial increase followed by relative stability. The final temperature in the hot-end breach area is higher than that in the cold-end breach. After activating the passive cooling system, temperature conditions in both cold-end and hot-end breach scenarios resemble pre-activation patterns but exhibit significantly reduced temperatures. Regarding pressure, breach zones experience rapid pressure surges within seconds, followed by minor fluctuations before the rate of increase slows. Hot-end breaches exhibit higher pressure values and faster rise rates than cold-end breaches. Activating the passive cooling system mitigates pressure growth in breach zones. Regarding inter-compartment pressure differential, the maximum pressure differential in the cold-end breach scenario peaked at 0.142 MPa within 0.25 s, while the hot-end breach scenario reached a maximum peak of 0.153 MPa at 0.31 s. After activating the passive cooling system, the pressure differential transient curve trend resembled that with the passive cooling system deactivated. However, the peak pressure differentials for both breach scenarios were effectively suppressed, resulting in a flatter curve profile. The maximum compartment pressure differential for the cold-side breach accident scenario peaked at 0.123 MPa at 0.24 s, while the maximum compartment pressure difference in the hot-end breach accident scenario peaked at 0.132 MPa at 0.26 s. Through multi-scenario computational simulations, the thermal-hydraulic response patterns of multi-compartment containment structures during LOCA accidents were obtained, leading to the following strategies: consider local structural dynamic loads for each scenario; optimize equipment layout and thermal insulation protection in high-temperature zones based on temperature field distribution; and evaluate natural circulation equipment layout in conjunction with flow field changes.