Coolant mixing experiments in the upper plenum of the ROCOM test facility

Highlights: • Coolant mixing in a geometry similar to upper plenum of a four-loop PWR measured. • Transfer function from all fuel element positions to reactor outlet nozzles. • Strong temporal fluctuations of the transport scalar profiles in hot legs found. • Measuring results explain temperature gradients found in the hot legs. • Data suitable for CFD code validation. - Abstract: The coolant outlet temperature of a Pressurized Water Reactor (PWR) is a result of the turbulent mixing of coolant streams from different fuel elements taking place in the upper plenum of the reactor. Experiments were performed at the ROCOM test facility representing an adiabatic fluid dynamic model of a KONVOI type PWR in the scale of 1:5. Salt tracer solution was injected into each fuel element position of a full symmetry sector of the core and the arrival of the thacer was detected by wire-mesh sensors in the four outlet nozzles of the reactor. It was demonstrated that the mixing is incomplete. The flow coming from a certain fuel element position arrives in a small area inside the cross-section of the hot legs of the main circulation pipes. The location of the maximum share of the flow from a given fuel element position is very unstable. It fluctuates in a chaotic manner and in case of a perturbation close to a symmetry axis; it jumps from one outlet nozzle to another. A characteristic periodicity is not observed; also this phenomenon is rather chaotic. The tendency was found, that coolant from peripheral fuel element positions arrives in the outlets close to the bottom, while the point of the maximum average moves up when the fuel element position is moved toward the center of the core. On the basis of the experimental data, temperature profiles in the hot legs of the primary circuit were estimated which show characteristic temperature differences relevant for the interpretation of the hot leg temperature measurement