Experimental Study of Bubble Dynamics and Concentration Variation in the Presence of Boron in the Liquid

Accumulation of neutron absorber (boron or boron compounds) within the porous crud layer on the fuel rods in the cores of pressured water reactors (PWR) results in the so-called axial offset anomaly (AOA). There is practically little information on the gradients of neutron absorber concentration near a nucleation site on the cladding surface during the growth of a bubble. The objective of the present work is to study bubble dynamics and associated concentration field of aqueous boron. As a first step in solving the complete problem, dynamics of single bubble was studied under pool boiling conditions. Distilled water and an aqueous solution containing 3,000ppm by weight of boric acid were used as test liquids. Single bubble was generated at a micro-fabricated cavity on a polished silicon wafer. It was found that the growth and departure processes of single bubbles are similar for both the test liquids. A miniature sensor for measurement of concentration was developed and calibrated. Concentration variation near the liquid-vapor interface was detected successfully. The measured concentration variations at different radial locations from the center of cavity have the same trend as given by the numerical simulations but the magnitude is much smaller.