Investigation on the formation and break-up of helium stratification inside the COTHYD facility by CFD simulation

The hydrogen generated due to metal-water reaction releases into the containment and may get stratified locally under accident conditions. And the stratification of hydrogen may be eroded by several measures, such as gas jet, spray, ventilation and so on. The COTHYD (COntainment Thermal-hydraulics and HYdrogen Distribution) is a large-scale containment test facility designed and constructed for studying the containment thermal hydraulics behavior and hydrogen risk in PWR during severe accidents by CGN (China General Nuclear Power Group). For the tests on COTHYD, helium is discharged from top, and when the helium stratification phenomenon occurs, gas (air or vapor) is injected vertically upward from a nozzle inside the containment to impact the helium stratification. In this work, numerical investigation on the formation and break-up of helium stratification was carried out using the CFD code FLUENT for experimental scenario definition. In the simulations of the helium stratification's formation, different helium injection rates were selected as the inlet boundary condition. By comparing the stratification phenomena under different conditions, the most suitable helium injection rate was determined. In the simulations of the helium stratification's break-up, different vapor injection rates with different Froude number were selected, and their impact effects were compared. Results showed that the Froude number of the steam jet can be employed to distinguish the plume flow and the jet flow. During the injection of the steam, the corresponding Froude number tends to decrease gradually. (author)