Thermo-fluid-dynamic evaluation of a microsystem to analyse radioactive solutions

G. Janssens-Maenhout
Joint Research Centre Ispra
Ispra, Italy

It has become common place to use micro-electromechanical systems (MEMS) to evaluate the chemical properties of solutions. However, such microchips have not yet been applied to the analysis of radioactive solutions, for the purpose of nuclear safeguards, in the nuclear reprocessing industry. Implementing MEMS in this area results in a reduced volume of the sample to be analysed. This has many advantages over conventional methods, icluding a reduction in radiation. The radioactive solution is sucked through a microchannel (300 x 300 ìm) in the chip, releasing heat due to radioactive decay. The flow and heat characteristics of microchannels have been observed to deviate from conventional and well established theory. This work initially evaluates these differences with an in-depth literature study examining the reasons for these differences. The microscopic effect of the electrical double layer (EDL) is focused on. These investigations on the validity of traditional macroscopic models lead to the adaptation and application of classical theories to suit the microscopic situation. It is concluded that the EDL influences the flow. FEMLAB 3 (COMSOL AB 2004) is then utilised to model both fluid dynamic and thermodynamic aspects of the microchip. Subsequent thermal stresses are also investigated. Conditions on the released heat are derived that guarantee no deformation of the chip and no temperature shift for absorptiometry measurements.

G. Janssens-Maenhout was one of the keynote speakers at the COMSOL User's Conference, fall 2005 in Milano