Spatiotemporal Evaluation of Bubble Characteristics in Sodium Using Real-Time X-Ray Imaging

Sodium boiling in a fuel subassembly of a sodium-cooled fast breeder reactor is a very critical phenomenon as void propagation may lead to loss of heat removal and increase in reactivity. Hence, sodium boiling behavior from the incipient stage and evolution of bubble dynamics need to be thoroughly understood toward validating various thermal-hydraulic models pertaining to safety analysis of fast reactors. An experimental program has been taken up at Indira Gandhi Centre for Atomic Research to investigate incipient sodium boiling toward spatiotemporal evaluation of bubble characteristics in sodium. Real-time X-ray radioscopy has been employed for in-sodium visualization of bubbles generated during sodium boiling. The raw images obtained during the experiment were processed using an in-house-developed image processing algorithm that includes background subtraction, noise filtering, and segmentation techniques for enhancement of foreground features. A max filter-based tracker is also devised for clear depiction of bubble boundary and motion with time during the boiling process. Attributes like bubble diameter, nucleation site density, and bubble velocity are determined effectively from the captured images using the algorithm. The paper focuses on the experiment, imaging technique using X-ray radioscopy, and image processing toward depicting the bubble characteristics during sodium boiling in a vertical channel.