Application of membranes to monitoring for tritiated water vapour

Nafion, a copolymer of tetrafluoroethylene and various perfluoro-sulphonic acids, is very permeable to water compared with other polymers. In the form of tubing, it allows the transfer of tritiated water vapour (HTO) from samples of gaseous effluents to a counter-current gas stream that passes to a radiation detector. The permeation rates for tritiated hydrogen (HT) and radioxenons (Xe) are approximately 10-3 and 10-4 that for HTO. The results of measurements with an assembly of Nafion tubes have demonstrated that, by careful selection of sample and detector flow rates, discrimination factors against HT and Xe of these orders may be attained. At the same time the concentration of HTO in the gas passing to the detector is within a few per cent of the concentration in the air sample. Direct capture of airborne HTO by permeation through a Nafion tube into liquid scintillator has also been demonstrated. With a mixture of 20% water in liquid scintillator and a mixture flow of only 0.1 cm3/min, concentrations of HTO in air down to 250 Bq/m3 (7 nCi/m3) may be measured. (author)