In the conceptual modular high temperature gas-cooled reactor (MHTGR), the reactor cavity cooling system (RCCS) is designed for decay heat removal. Following the occurrence of a design basis accident such as a loss of forced circulation, the RCCS operation is designed to transition from the active to passive convective heat transfer modes. The RCCS is a cross-cutting concept for any reactor type and in practical design can be air or water-cooled. Here the water-cooled RCCS design consists of multiple standpipes surrounding the RPV and as such provides natural circulation decay heat removal to the ultimate heat sink, the ambient environment. The decay heat load on the full-scale RCCS, of a reference MHTGR, during a transient event varies over time with an estimated peak power of 1.5 MW. This work experimentally investigates the two-phase performance using a 1/4-scaled, three channel natural circulation loop at different radiative-convective thermal loads representing decay heat, and the corresponding 'linear to oscillatory' thermalhydraulics as anticipated under single to two-phase flashing flow.
