A scaling approach for fluid mixing in rapid boron-dilution transients

The issue of rapid-boron dilution transients has been investigated for a decade with few definitive conclusions. The potential reactivity insertion that would result from the transport of a boron-dilute slug to the core region depends on the spatial and temporal concentration of neutron poison in the fluid that penetrates the core. Difficulties in assessing the consequences of a boron-dilution transient lie in the diversity of postulated on-set scenarios, and in the large dependence of the resulting reactivity insertion on plant conditions and geometric features. The University of Maryland (UM) 2 x 4 Thermohydraulic Loop Facility is collecting code validation data for rapid boron-dilution transients. The data has to be representative of the prototype. scaling thus becomes an important consideration. The mixing of the boron-dilute slug with the highly borated coolant present in the primary system is advective-diffusive. Therefore, the scaling approach involves extracting relevant parameters from the advection-diffusion equation. The dimensionless numbers that govern mixing are shown to be the Strouhal and Schmidt numbers. They have to be considered simultaneously to properly scale the flow in the model facility. A closer investigation of the relation between these two numbers reveals that, for fully developed turbulent flows, matching only one of these numbers is sufficient. This conclusion is substantiated by experimental results from Vattenfall. (author)