The time-averaged particle image velocimetry technique has been applied to measure flow mixing in the Michigan Multi-jet Gas-mixture Dome (MiGaDome) facility, a 1/12th, scaled-down model of the high-temperature gas-cooled reactor upper plenum. Measurements were first conducted with one jet injection into the upper plenum for various Reynolds numbers (Re = 1022, 2038, 4097, and 6021). The experimental region of interest includes a plane within the dome located above one of the jet inlets of interest. First- and second-order statistics are presented and discussed to analyze the local mixing process and turbulent characteristics under the effects of jet spreading and jet impingement. Results have shown that the normalized statistics of the jet reach asymptotic behavior as the inlet Reynolds number is increased. By investigating the two-dimensional budgets for the momentum equation on the measurement plane, it was concluded that the contribution of turbulent diffusion is minor near the enclosure surface where strong convection is present due to impingement. An additional measurement on a triple-jet injection case has shown that jet spreading is suppressed by a recirculation zone, which causes a redistribution of turbulent fluctuations. The detailed local fluctuation patterns/coherent structures have been examined through a proper orthogonal decomposition analysis.