Trapping and Measuring Radioiodine (Iodine-129) in Cartridge Filters

Abstract

To understand the distribution of 129I trapped in AgS (silver-impregnated silica gel) adsorbent and to determine a calibration curve for measurement of this 129I, cartridges containing 10 in-line (arranged in series) filter elements, each packed with a 10-mm thickness of adsorbent, were fed from 20Bq (3.3 μg) to 4.6 MBq (768mg) of 129I at 150°C at a gas velocity of 22cm/s. The 129I in each filter element was measured after the adsorbent had been mixed homogeneously until the counting rates at the front and back surfaces of the filter element became equal. The amount of input 129I was allocated to each filter in proportion to the counting rates thus obtained.The first 7 of the 10 filter elements completely confined 4.6MBq of 129I. Therefore, the width of the adsorption zone was 7cm. Although each filter element in the cartridge could theoretically adsorb 2.92 MBq (485mg) of 129I, the first element captured only 1.42 MBq, which was 49% of its saturation value. Its DF had dropped to a level of 2.21 for its adsorption of only 17.4% of the saturated amount. As 129I continued to be deposited, the DF dropped to 1.45. The high gas velocity causes this phenomenon. Plots of counting rates vs. adsorption quantity provide a calibration curve covering a range of 20Bq to 2MBq of 129I with deviation of ± 13%. This curve is expressed by1n y = −0.57 + 0.961n x where y is the counting rate (counts/300s) and x the quantity of 129I in Bq.