Experiments addressing Passive Containment Cooling Systems for small modular reactors in the PANDA facility

Abstract

Passive Containment Cooling Systems (PCCS) are relevant features of many Small Modular Reactor (SMR) concepts, where natural circulation and condensation provide long-term containment heat removal without external power or operator intervention. The performance of in-containment condensers is strongly affected by the presence of non-condensable gases and by large-scale mixing and stratification in the containment atmosphere, which are difficult to predict with system codes and require high-quality data for the validation of multi-dimensional simulation tools. This work presents an experimental investigation of a simplified in-containment PCC system performed in the large-scale PANDA facility within the OECD/NEA PANDA-1 programme. The P1A4 test series is devoted to a generic in-containment PCC loop representative of SMR applications. The present paper focuses on experiment P1A4_7, which establishes two quasi-steady states at containment pressures of 2.07bar and 2.46bar, respectively. The configuration includes a single vertical condenser tube connected to elevated pools at constant temperature and a containment volume initially filled with air. Steam is injected until the condensation rate on the PCC balances the injection rate, resulting in quasi-steady conditions of pressure and PCC loop mass flow rate. The facility delivers global measurements (pressure, temperatures, steam and water mass flows, condensate collection, pool stratification) and local data (axial water temperature and heat flux in the PCC, gas temperature and composition profiles, and gas velocities from Particle Image Velocimetry). Two well-defined operating points are obtained, with PCC powers of 12.2kW and 16.5kW and non-uniform axial heat-flux distributions along the condenser. Vertical gas temperature and steam concentration profiles reveal a stratified structure with air-rich layers below the steam injection level and nearly uniform steam mass fractions of about 0.6 and 0.7 above the injection elevation. Time-averaged PIV measurements near the PCC show very low gas velocities, indicating that condensation is primarily controlled by local diffusion of steam and non-condensable gases rather than by strong convective flows. An energy-balance is used to infer the condensation mass flow rate, which agrees with the injected steam mass flow within combined measurement uncertainties, showing a consistent global mass balance. The experiment thus provides a clean, well-characterized benchmark with two quasi-steady operating points, combining global integral quantities with spatially resolved fields that are well suited for the validation and improvement of system and CFD codes for SMR containment analyses.