Boron mixing in the lower plenum of a BWR

Current ATWS procedures are rather complicated because of uncertainties regarding the flow behavior of the liquid poison injected into the lower plenum for control. The concern, supported by some old small scale experiments (at General Electric Co.), is that due to strong buoyancy forces the natural circulation flow (less than 30% of rated) would be insufficient to entrain and convect the cold boric acid solution upwards and into the core area. On the other hand, the applicability of these experimental results is also in doubt. This is because Froude number scaling becomes problematic at small scales. The purpose of this work is to bypass such concerns by carrying out a set of experimental simulations at full scale. Such full-scale simulations are practical because of the high degree of compartmentalization of a BWR lower plenum by the control rod guides and the nearly longitudinal flow of coolant in the interstitial spaces. The experimental facility represents a lower plenum slice corresponding to 9 guide tubes in the neighborhood of the injection standpipe (also included at full scale). The structural material is acrylic, allowing full visualization. The base flow is created by recirculating from a 2000 gal capacity (water) tank at velocities up to the full natural circulation strength expected in the reactor. The injection flow consists of a NaCl solution. In addition to visualization the facility is instrumented for mapping out the local/instantaneous concentration (conductivity probes) and velocity (fiber optic laser doppler anemometer)fields. Data from the first round of experiments are presented and initial conclusions regarding stratification potential are drawn