An existing geometric and fluid-fluid scaled facility is applied to investigate the transport of borated safety injection (SI) fluid in the Westinghouse AP600 reactor vessel during a main steam-line rupture (MSLR) event. The AP600 reactor has coaxial injection into the vessel downcomer rather than the cold-leg cross-flow injection typical of operating power reactors. This gas-flow test facility has unique detail in the representation of the SI nozzle-to-core inlet path most important to SI transport. Analysis of the transport phenomena expected in the reactor and the scaled facility, given MSLR conditions, indicates that both buoyancy and turbulent diffusion can have comparable influences on SI transport. It is shown that different reactor-to-experiment velocity ratios are required to scale each phenomenon. Tests are performed to evaluate transient SI fluid concentration at the core inlet using the appropriate velocity ratios to scale buoyancy and diffusion. Two asymmetric loop-flow boundary conditions representative of the MSLR event as well as a symmetric flow condition are applied. While no one test result is fully similar to the expected reactor transport, this ensemble of tests provides data that are valuable for AP600 numerical model benchmarking.