Feasibility study of a microsystem to analyse radioactive solutions

摘要

The application of micro-electro-mechanical systems (MEMS) to evaluate the chemical properties of radioactive solutions has been investigated with the example of a liquid sample taken from reprocessing plant vessels. For radiochemical solutions the application of a microvial instead of a millivial bears more advantages than for other chemical solutions because of the strongly simplified sample preparation and significantly reduced dose uptake. The scaling down of the liquid sample might also cause negative implications on the radiochemical analyses with regard to accuracy and representativeness. All the consequences on replacing a liquid sample of several millilitres by one of less than 1 μl are investigated. This paper reports in particular on a first feasibility study of the replacement of a millivial by a microvial for the analysis of spent fuel solutions in a reprocessing plant for the purpose of nuclear safeguards. Implementation of MEMS in this area results in a reduction in dose that is almost proportional with the reduction in size. This brings about a simplification in sample preparation and a significant reduction in dose uptake for the analyst with many advantages over conventional methods. The MEMS designed for analyzing a spent fuel solution consists of three microchannels: one channel for the sample, one for a reference solution, and one is the blank. The concentration of the solution is determined by the photospectra of the light transmitted along the channel axis and absorbed at nuclide-specific wavelengths. Absorptiometry experiments with a micro-volume demonstrated the validity of the Beer-Lambert law and derived the limits in precision as a function of the concentration. A photospectrometric database for the reference solution of aqueous solutions of nitric acid with the neodymium surrogate was setup. Electrophoretic forces fill the subject microchannel with the solution, which will release heat due to radioactive decay. The flow and heat characteristics of microchannels have been observed to deviate from conventional and well established theory. These differences have been evaluated and the reasons examined. The microscopic effect of the electrical double layer (EDL) is focused on. These investigations on the validity of the traditional macroscopic models allowed application of classical theories within a well defined validity range and the adaptation of these theories to suit microscopic models. It was concluded that for acid solutions the EDL can be neglected. With thermodynamic simulations the stresses were evaluated. Conditions on the released heat were derived that guarantee no deformation of the chip and no temperature shift for the absorptiometry measurements.