Retention efficacy and release of radioiodine in fume hoods

Procedures to determine the release of hazardous gaseous substances including radioactive iodine are covered by different norms such as the European standard EN 14175 and the German national standard DIN 25466. The detection of sulphur hexafluoride (SF₆) is required to comply with the prescribed methodology. The detection limit of this test is 4.5·10^-7 mol/m³ in exhaust air. This detection limit would represent a very high activity in the region of 0.27 TBq/m³ leading to an unacceptable risk. We therefore developed a test using a filter system, consisting of a combination of filters capable of separating various chemical forms of airborne radioiodine. Air samples were collected directly in front of the fume hood and in the laboratory beside two different fume hoods of a similar construction with a final activated carbon filter for retention of radioiodine. Particular attention was therefore paid to air samples taken after passage over the filters. Significant differences in the degree of retention of iodine were found between the two fume hoods investigated. In one test a malfunction of the fume hood was demonstrated. In this case 0.148 × 10^-3% of the total released activity per m³ air was found 1 cm in front of the hood sash. A remarkably high fraction of the activity released in the fume hood (1.3 × 10^-3%/m³ air) was measured after the activated carbon filter. In the ambient air, values of up to 8.6 × 10^-6% pro m³ laboratory air sampled were measured, despite a 6-8-fold air exchange. The selected procedure is a factor of 10¹¹ (Schomäcker et al., 2001) more sensitive than the standard recommended methods (EN 14175). The standard test prescribed by the DIN/EN failed to reveal any inadequacy in the protective function of the radionuclide hood with respect to radioiodine retention.