Radioiodine Deposition Losses from Sample Air Containing Particulate and Gaseous Iodine in Nuclear Reactor Sample Lines

Mechanisms of radioiodine deposition from sample air containing both gaseous and particulate radioiodine in reactor sample lines are studied, and experimental methods are developed. A short half-lived radioiodine tracer, ¹²⁸I (t_1/2 = 25 min), is used in the chemical forms of molecular iodine and methyl iodide. An effort is made to investigate the type of particles for particulate iodine. Of the various types of particles tested, only tobacco smoke particles have a sufficiently high iodination rate to be used in these studies. The 609.6-cm (20-ft)-long sample lines of Types 316 and 304 stainless steel tube (2.29 cm i.d.) were tested for the sample flow rates of 28.3 ℓ/min (1 ft³/min) and 56.6 ℓ/min (2 ft³/min). In-tube measurements using a calibrated thin-walled Geiger tube are conducted to determine the penetration factor and space-dependent deposition velocity profile of radioiodine. Methyl iodide is not reactive for either the tube surfaces or aerosol particles. The overall deposition velocity of the mixture of the smoke particles and molecular iodine is higher than that of molecular iodine alone for similar sampling conditions. It is concluded that the high deposition rate of radioiodine in the sample air mixed with smoke particles and molecular iodine is caused by the different sample line surfaces that are contaminated with smoke particles.