Near-infrared imaging to quantify the diffusion coefficient of sodium pentaborate aqueous solution in a microchannel

Borated water is injected into boiling-water reactors as a neutron absorber, and it is, therefore, essential to understand the effect of the solution concentration on its diffusion. We have developed a technique for measuring concentration distributions using a microchannel and near-infrared imaging. This measurement technique was applied to borated water in the present study, precisely an aqueous solution of sodium pentaborate. Laminar flows of water and this aqueous solution with 0.12 mol/L, 0.24 mol/L, and 0.32 mol/L at temperatures from 30 °C to 70 °C were mixed in a microchannel at Reynolds numbers of less than 2, and images were acquired at a wavelength of 1900 nm. From the obtained images, the concentration distribution was visualized, and the diffusion coefficient was estimated as being between 3.6 × 10⁻¹⁰ m²/s and 9.8 × 10⁻¹⁰ m²/s. The diffusion coefficient was correlated positively with both temperature and concentration.