Gaseous Radioiodine Deposition Losses in Nuclear Reactor Sample Lines

Experimental methods are developed, and the mechanisms of airborne radioiodine deposition in reactor sample lines are studied. A short-half-lived radioiodine tracer, ¹²⁸I (t_1/2 = 25 min), is used in the chemical forms of molecular iodine and methyl iodide. In-tube measurements using a calibrated Geiger tube are conducted to determine the space-dependent iodine deposition rate and the penetration factor. The reproducibility of average deposition velocity and thus penetration factors for a given sample line under similar experimental conditions show good improvement over those of previous researchers. For the three stainless steel tubes tested under comparable conditions, the deposition velocities are tube specific, with the difference in deposition velocities being a factor of >10. The most important factors that determine the I₂ deposition rate are organic contamination, sample air relative humidity, and sample line inside surface structures. Heat tracing and passivation procedures are found to be effective in reducing I₂ deposition rate. The CdI₂ filter in the iodine sampler system showed a retention efficiency of ∼81% under the test conditions rather than the 98% reported by the manufacturer. In conclusion, in-plant testing is necessary to determine the deposition losses of airborne radioiodine in the existing plant sample lines. The sample lines should be cleaned at regular intervals and heat traced to minimize the deposition losses. For very long sample lines, passivation procedures may be required.