Determination of low concentrations of iodine-131 in fast-reactor coolant systems

A method has been devised for the determination of iodine-131 in the presence of mixed fission products in reactor coolants, by oxidation to elemental iodine followed by sublimation on to copper gauzes. The method reduces to a minimum contamination of the copper gauzes by ruthenium-103 and ruthenium-106 and eliminates contamination due to technetium-99 and caesium-137. It is possible to determine 10(2)cpm ml from iodine-131 in a total count of 2 x 10(6)cpm ml of fission products, with an accuracy of +/- 3%. The only activity collected on the copper gauzes was iodine-131 (62% collection efficiency) and ruthenium-103 (0.005% of total activity) which gave comparable count rates. The activity of these two isotopes was separated by single-channel gamma analysis.