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Retention efficacy and release of radioiodine in fume hoods
Journal of Environmental Radioactivity xxx (2016) 1e6
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Retention efficacy and release of radioiodine in fume hoods €cker a, *, T. Fischer a, B. Zimmermanns a, J. Bregulla a, F. Sudbrock a, O. Prante b, K. Schoma A. Drzezga a a b
Department of Nuclear Medicine, University Hospital of Cologne, Germany Department of Nuclear Medicine, University Hospital Erlangen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2015 Received in revised form 18 December 2015 Accepted 12 January 2016 Available online xxx
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$107 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 0.148 103% 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 (1.3 103%/m3 air) was measured after the activated carbon filter. In the ambient air, values of up to 8.6 106% pro m3 laboratory air sampled were measured, despite a 6e8-fold air exchange. €cker et al., 2001) more sensitive than the standard The selected procedure is a factor of 1011 (Schoma 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. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Radioiodine Fume hood Radioiodine retention DIN standards
1. Introduction Fume hood extraction units are installed in nuclear medical and radiopharmaceutical facilities in order to maintain a working environment with a low level of microbacterial contamination for the handling of unsealed, radioactive materials. These units should be suitable for the production of ready-to-use radioactive pharmaceuticals, in accordance with GMP (GMP ¼ Good Manufacturing Practice) conditions. Those generally available are a combination of a laminar-flow cabinet and a radionuclide hood. Special construction and operating modifications guarantee protection of personnel from incorporation, contamination and external radiation exposure. Here the retention capability of the combined fume hood unit
* Corresponding author. Department of Nuclear Medicine, University Hospital of Cologne, Kerpener Str. 62, 50937 Cologne, Germany. E-mail address: [email protected] (K. Schom€ acker).
assures that no dangerous levels of airborne radionuclide are incorporated by the operator standing in front of the fume hood. This is particularly important in the case of radioactive iodine. The volatility of radioiodine depends on its chemical form. In order to prevent release of 131I-iodine, with its half-life of ca. 8 days, into the environment, radionuclide hoods are fitted with additional, activated carbon filters. This should guarantee a reliable decontamination of air leaving the fume hood unit. To guarantee that the fume hood unit offers full protection, safety equipment must be regularly maintained and monitored to ensure that it functions effectively. The regulations for the design and testing of radionuclide hoods are laid-down in the form of DIN/ EN standards. These DIN/EN standards can be national, European or international. They apply to the performance of standard technical tasks and frequently provide a basis for regulatory measures. The behaviour of released radioactive substances and of gaseous 131 I-iodine in particular is currently assessed in the respective DIN/
http://dx.doi.org/10.1016/j.jenvrad.2016.01.006 0265-931X/© 2016 Elsevier Ltd. All rights reserved.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
€cker et al. / Journal of Environmental Radioactivity xxx (2016) 1e6 K. Schoma
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EN standards as equivalent to non-radioactive chemically hazardous substances. Thus the German DIN standard for radionuclide hoods (DIN 25466) stipulates that the air technology type examination should follow recommendations set down in DIN EN 14175 (DIN EN 14175-1eDIN EN 14175-4). The regulations for type testing laid down in the standard EN 14175-3 (DIN EN 14175-3) in particular call for test measurements with the tracer gas sulphur hexafluoride. This requires a detection limit of at least 108 volume fractions (0.01 ppm). Assuming the gas density of SF6 to be ca. 6 kg/ m3, this is the equivalent of a molar concentration of ca. 4$107 mol/ m3 air. With regard to volatile radioiodide compounds, in the case of 131I this amount would correspond to a radioactivity concentration in the order of magnitude of Terabequerel per cubic meter. A theoretical exposure distributed over 1000 individuals with average thyroid retention of 55% to such a radioactivity of 131I would lead to an effective dose of ca. 6 Sv. For this reason, the current test specifications, based on DINstandards, are unsatisfactory. Below is a proposed method of high sensitivity for measurement of radioiodine concentrations in the air. This offers a suitable procedure for testing the protective function of radionuclide extraction units with regard to protection of personnel (retention of radioiodine in the area around the fume hood) and environmental protection (decontamination of exhaust air from the radionuclide hood).
Fig. 2. Measurement of the containment capability of the fume hoods. For air sample collection the filter system was positioned to the left of the fume hood at a distance of 1 cm from the front sash.
2. Materials and methods 2.1. The fume hoods The fume hoods investigated in this study are shielded hoods for manipulations at medium activity, with frontal sliding shield and laminar flow. The workstation should be suitable for the breakdown of air-hanging radioisotopes coming from the manipulation of liquid or volatile gaseous substances. The interior is made of stainless steel (AISI 304). An integrated ventilator draws the process air from the working area through an activated carbon filter combination within a separate filter cabinet that is also made of stainless steel. The bag change technology, which is based on research from the field of nuclear technology, guarantees a safe filter change (according to the manufacturer). 2.2. Immission in the laboratory 2.2.1. Description of the measuring equipment For the determination of 131I-iodine concentrations in air we developed a test using a filter system (Figs. 1e3) for selective adsorption of chemically different radioiodine species as described by Schom€ acker et al. (2001, 2011). This filter system was mounted in two radionuclide laboratories in separate locations within the area around the radionuclide hood at various measuring positions.
Fig. 1. Sketch of the air sampling system. 0: GF50 ¼ aerosole filtre, 1: BE 110 ¼ cadmium iodide, 2: BE 110 ¼ cadmium iodide, 3: Silverzeolithe filtre, 4: TEDAcoated charcoal.
Fig. 3. Test of the decontamination capacity of the activated carbon filter combination in the filter unit to the right of the fume hood via a removal device (tubing joint) was fitted.
The air was drawn in by a portable constant flow air-sampler Model AVS-28 A from the company SAIC/RADeCO (USA) through a filter system consisting of an aluminium mounting bracket with 5 tightly packed individual filters. The filters were serially arranged in the following order (Fig. 1): one circular glass fibre filter GF50 (Schleicher & Schuell), two BE-110 cadmium iodide filters (SAIC), one GY-130 silver zeolite filter (SAIC) and one CP-100 filter (TEDAimpregnated activated carbon, SAIC, USA). The diameter of the filters was 5.77 cm, with a thickness of 2.54 cm (except for the round glass-fibre filter). The portable constant flow air-sampler and the filters were supplied by the company “Technisches Büro Schütz”, Rimbach, Germany. The listed adsorption materials have been used for many years to contain iodine released during accidents in nuclear power stations. The arrangement of filters used here, or similar combinations of such filters, often serve in nuclear reactor operations or in hot cells to establish the chemical composition of the iodine-radioactivity contained in the exhaust air (Emel et al., 1977; Giraud, 1985; Lee et al., 1991; Wilhelm, 1977, 1982).
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
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The circular glass-fibre filter is composed of borosilicate glass fibres. According to the manufacturer's information, aerosol particles of <1 mm can be separated out, particularly high separation rates (99,993%) being achieved for aerosol particle diameters of 0.5e0.3 mm. The two cadmium-iodide filters contain CdI2-impregnated Chromosorb-P (grain size ca. 30 mesh, mass: ca. 45 g, ca. 15 mass% CdI2). These filters are highly effective for retaining molecular I2. Retention occurs through a combination of isotope exchange and the formation iodine containing cadmium compounds (Keller et al., 1970; Huang et al., 1987). The silver zeolite filters contain ca. 75 g of the molecular sieve 13x-Ag (grain size: 16e40 mesh, pore size: 0.6e0.7 nm), impregnated with 37% (percent by weight) silver in the form of silver nitrate. Organically bound iodine reacts with the highly reactive silver surface to form silver iodide (Maeck et al., 1969; Evans and Jervis, 1992; Lothar and Jurgen, 1990). The last of the five serially arranged filters contains TEDA (triethylene-diamine, 5% by weight), impregnated with activated carbon (ca. 50 g, grain size 40e50 mesh). This type of filter can retain molecular as well as organically bound iodine. The iodine retention is brought about by primary adsorption and the formation of quaternary ammonium salts on the activated carbon surface (Kovach, 1992; Wren et al., 1999). Depending on its chemical form, airborne radioiodine can thereby be retained, quantified and differentiated into molecular I2, organically bound iodine or iodine as an aerosol. The air volume flow through the filter combination was ca. 4.5 m3/h. The activity of the individual filters was measured with a NaI(Tl) scintillation detector (Scintibloc 8SF/2A-X, Crismatec, Photomultiplier ScintiPack, model 296 EG&G ORTEC, Software UMS) from Berthold Technologies GmbH. The total activity was calculated as the sum of the activities in all the filters. The scintillation counter was calibrated with a “standard filter” containing a defined activity of 131I. This calibration was carried out in precisely the same geometry as that used for exhalation measurements. The “standard filter” was prepared using an aliquot of a 131 I sample for which the activity had been measured beforehand in a calibrated ionisation chamber (Physikalisch-Technische Bundesanstalt PTB, Braunschweig, Germany). Our scintillation detector was calibrated to convert count rates into activities by means of the above-mentioned calibration and was recalibrated 5 times. The mean value for the efficiency was taken as 11.6 Bq/cps. Each count rate (in cps) measured in the energy window between 320 and 400 keV was thereby converted into a radioiodine activity (in Bq). As typically low count rates were expected, the background radiation was determined prior to measurements on the filters and for air-flow samples. 2.2.2. Calibration of the filter combination Before commencement of air measurements, the effectiveness of the filter combination in separating the various iodine species was tested as follows: Elemental 131IeI2 was generated by reaction of 131IeNaI solution with potassium chromate in hydrochloric acid solution in a roundbottomed flask at 50 C, and the vapour phase drawn by suction over the filter combination at 4.5 m3/h for 30 min. As a prototype of organically bound radioiodine, 131I-ethyl-iodide in alcoholic solution was generated by iodine exchange between ethyl-iodide and 131 IeNaI at 50 C and likewise drawn by suction over the filter combination at 4.5 m3/h for 30 min. The two round-bottomed flasks were connected in series to allow mixing of the vapour phases. The proportions of the two species of iodine were determined after freezing out the combined vapour phases with liquid nitrogen, extraction of the condensate
3
with absolute alcohol and measurement of radioactivity in the alcoholic and aqueous phases. The identical vapour mix was then drawn by suction over the filter combination. 2.2.3. Measurement of the containment capability of the fume hoods The generation of iodine species for testing the fume hood followed the description above (para. 2.2). The test were performed with the extractor function of the fume hood switched on and in the working position (front sash opened 15 cm). 10 MBq Na131I were used for the generation of elemental and organically bound 131 I for the test of the containment. The radioactivity in the two vessels foreseen for generation of elemental or organically bound iodine, respectively, was measured before and after the chemical reaction (60 min) by which iodine was released. The fraction of the released species (elemental and organically bound 131I) was determined by the separation technique. Air samples were also drawn during the chemical reaction (para. 2.1.1). Air sample measurements were carried out with the filter system positioned to the left, right and in the middle of the fume hood. The following distances were chosen: d1 ¼ 1 cm (Fig. 2) and d2 ¼ 5 cm from the front sash (Table 2). 2.3. Emission from the fume hood 2.3.1. Decontamination capacity of the fume hood The purpose of these measurements was to test the decontamination capacity of the activated carbon filter combination in the filter unit to the right of the fume hood. To this end, an air removal device (tubing joint) was fitted, diverting the air, after passage over the filters, into a further air system in which the radioiodine content was measured. Both elemental and organically bound iodine were released for these measurements. A long measuring period of over 990 min was taken to ensure that 131I species could still be detected even at the very low radioiodine concentrations expected at this measuring point after passage through the filters. By this procedure a total volume of 59.4 m3 exhaust air was collected after filtering (Fig. 3). 2.3.2. Ambient air measurements These measurements were taken solely in the location of the fume hood where markedly high concentrations of radioiodine were detected after passage through the activated carbon filter. They were taken in the respective radionuclide laboratory, first in front of the air outlet for exhaust air from the laboratory; with the air extractor unit of the fume hood running and with laminar air flow. Molecular and organically bound radioiodine were generated in the radionuclide hood as described above. The second measuring position was at an air outlet bringing the ambient room air directly into the circulating air stream for the radionuclide laboratory.
3. Results 3.1. Calibration of the filter combination The results of examination of the separating capability of the filter combination, as described above, are summarized in Table 1. They show that the apparatus is suitable for the separation of aerosolic, molecular and organically bound iodine. At the last filter (TEDA impregnated activated carbon) a mixture of molecular and organically bound iodine occurs that is not retained by the previous filter in the series.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
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Table 1 Test of separating capability of the filter combination. Vapour phase
I2 Ethyl-iodide Combined vapour phasea a
% total
131
I-radioactivity in
Glass fibre filter
CdI2-filter I
CdI2-filter II
Silver zeolite filter
Activated carbon-TEDA-filter
0.6 0.4 0.9
95.4 1.2 14.6
0.9 0.8 1.2
1.1 96.8 77.9
2.0 1.2 2.3
The radioactivity measurements of the alcoholic and aqueous phases gave an I2/ethyl-iodide ratio of 1: 4.5.
3.2. Immission in the laboratory
Table 3 Decontamination capacity of the fume hood.
3.2.1. Measurements of the containment capability of the hoods A1 and A2 These results are summarized in Table 2. Activities of 2e4 MBq were released. 131I-radioactivities were detected at 1 cm and 5 cm away from the front sash. The maximum fraction of radioiodine released into the exhaust air in this case were 0.4$103% of the 131I-radioactivity released per m3 exhaust air for Hood A1 (5 cm left). The corresponding value for Hood A2 was 0.63$103% of the 131Iactivity released per m3 exhaust air (5 cm right). For Hood A1, the radioiodine concentrations at a distance of 1 cm from the front sash in the left and right measuring positions were a factor of 2e3 times higher. The radioiodine concentrations for both samples were markedly lower in the middle position in comparison to the right and left. The relative proportions of the chemical forms of radioiodine differed depending on measuring position and the fume hood examined. 3.3. Emission from the fume hood 3.3.1. Decontamination capacity of the fume hood These results are summarized in Table 3. The radioiodine penetrated the activated carbon filter mostly in the organically bound form. Here a percentage for the 131I-activity released per m3 air for Hood A1 was reached that is 15 times higher than that obtained for Hood A2.
Measurements after passage through filters
A1
A2
Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103x%/m3] Organically bound [%] Aerosolic [%] Molecular [%]
34.70 12.97 1.30 96.3 0.5 3.2
5.35 0.40 0.084 96.0 4.0 LOD
3.3.2. Ambient air measurements These tests were performed only in the vicinity of Hood A1, as this is the point where a particularly high radioiodine concentration was found to have accumulated in the exhaust air after passage through the extraction filters. The results of the measurements at two different positions (P1, P2) at the fume hood A1 are summarized in Table 4. The laboratory air was found to contain chiefly organically bound radioiodine. At measuring point P2 the concentration of radioiodine was 23 times higher than at measuring point P1 (Table 4). 4. Discussion The release of radioiodine from a fume hood was assessed through radioactivity measurements at various distances from the front of the hood. The results are plausible. The radioiodine concentration fell with increasing distance to the sash pane, when
Table 2 Measurements of the containment capability of the hoods A1 and A2. A1
A2a
d1 (1 cm to front sash) Left Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Middle Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Right Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%]b Aerosolic [%]b Molecular [%]b a b c
A1
A2
1.57 0.39 0.40 28.2 9.3 63.5
2.70 0.65 0.031 27.1 41.4 31.5
2.603 1.48 0.00068 28.7 56.4 14.8
2.70 0.65 0.004 76.7 23.3 LODc
2.91 1.18 0.049 75.3 24.7 LODc
3.2 0.3 0.63 29.2 38.2 32.6
d2 (5 cm to front sash) 1.061 0.73 1.77 25.4 45.8 29.8
e e e e e e
1.014 0.738 0.148 63.4 6.4 30.1
e e e e e e
1.170 1.039 1.33 30.7 50.0 19.3
e e e e e e
Left Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Middle Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Right Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%]
For this hood, measurements were taken only at a distance of 5 cm. The percentage values in the table refer to total activity attributable to the various chemical forms (activity at the filters, excluding the activated carbon filter). Limit of detection.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
€cker et al. / Journal of Environmental Radioactivity xxx (2016) 1e6 K. Schoma Table 4 Ambient air measurements in the vicinity of Hood A1 at two different positions (P1, P2). Measurement of ambient air
P1a
P2b
Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103x%/m3] Organically bound [%] Aerosolic [%] Molecular [%]
33.36 5.26 3.56 107 100 LOD LOD
18.036 9.51 8.6 106 75.4 3.1 21.4
a Measurement at the air outlet for exhaust air from the laboratory into the external surroundings. b Measurement at the air outlet directly connected to the circulating air stream.
opened to 15 cm. The various forms of iodine occurring in the exhaust air of a fume hood (organically bound iodine, elemental iodine and aerosolic iodine) can be qualitatively and quantitatively determined with the aid of testing equipment through specific adsorption into the various filters. These tests are reproducible and easy to perform. The volatility of small, organically bound iodine compounds is most prominent compared to the other radioiodine species. Nevertheless, the fraction of airfloating aerosol or molecular radioiodine measured in front of the fume hoods were often comparatively higher. A possible cause for this depletion of highly volatile compounds might be the action of the fume hood itself. The detection limit, based on experience so far gained with the €cker et al., 2001), correprocedure, lies at 0.1 Bq/m3 (Schoma sponding to a level of ca. 1.66$1018 mol or 2.18$1017 g iodine per m3 exhaust air measured. The method used here is thus a factor of 1011 more sensitive than the recommended DIN-standard. The most surprising finding was the high level of radioiodine in the exhaust air after passage through the activated carbon filter in Hood A1. The active carbon filter is designed to extract almost 100% of the radioiodine passing through it. This reveals a particular failure of the radionuclide hood units investigated here, especially A1. The deficiency arose under quite normal working conditions. The finding led to a decision to collect and monitor air at the outlet opening in the laboratory in which Hood A1 was located. Surprisingly, incidences of marked radioiodine contamination were also detected in the normal ambient laboratory air. The levels of this contamination varied by a factor of 23 depending on the position of measuring point. The two fume hoods should be more or less comparable in terms of escape of radioiodine so that any difference would point to a malfunction of components. The underlying cause for the insufficient retention of radioiodine by A1 was a defective tubing that was detected on the occasion of our studies. The operation of the filters turned out to be defective while fume hood module was not impaired. This emphasizes the need of testing a fume hood with a better sensitivity than provided by tests with SF6 according to the DIN/EN procedure. The respective test had been carried out by a specialized institution beforehand and gave no reasons for an objection. The levels measured in the laboratory air can now be evaluated from two different points of view. The ICRP publication 119 gives a dose-coefficient for organically bound 131I of 1.8$108 Sv/Bq and for [131I]-I2 of 2.0$108 Sv/Bq (Eckerman et al., 2012). Assuming a mixture of the two, the mean value would be 1.9$108 Sv/Bq. Taking the maximal permissible dose for the thyroid to be 300 mSv/year in a working year of 200 days (German Radiation Protection Ordinance, StrlSchV x 55), then the maximal permissible radioactivity per working year would be 1.58$107 Bq (~16 MBq). Assuming a respiratory minute volume of 1 m3/h (adult on moderate exertion over a 4 h period) and applying
5
the yearly threshold value for inflow of activity from 131I-iodine into the thyroid of 1.58$107 Bq and a percentage value of radioiodine in the ambient air of 8.6$106% pro m3 (as measured at the most exposed measuring point), then the maximal permissible dose that could be released from a fume hood under these conditions would be 230 GBq 131I per year or 115 MBq/working day. It should, however, be emphasized that in responsible handling of radionuclides, efforts should be made never to reach this threshold. A safety factor of 10 should therefore be factored in. Furthermore, the probability of release of radioactivity needs to be estimated for any planned work. Thus the activity-limiting considerations laid down in the German Ordinance for Radiation Protection must also be taken into account. For facilities with an exhaust air flow of 104 m3/h, the exemption limit concentration is 5 Bq/m3 while for those with higher exhaust air flows the stipulated value is 0.5 Bq/m3. Taking the above considerations and this threshold into account, the percentage fractions measured in relation to the radioiodine activities released in the radionuclide fume hood after passage over the active carbon filter (1.3$103%/m3), give a maximal permissible release of 0.38 MBq of 131I-activity per experiment. 5. Conclusion The method of testing the retention of radioiodine in fume hoods presented in this study is more effective than the test with SF6. The respective European standard is therefore not suitable for assessing radioactivity in fume hoods. For one fume hood (A1) the retention was insufficient and measurable amounts of radioiodine were found in the laboratory. The German legislation defines an exemption level of 5 Bq/m3. Based on this level, the maximum activity that can legally be handled in this fume hood (A1) would not exceed 0.38 MBq. References DIN 25466, 2012. Fume Hoods for Radioactive Materials e Rules for Construction and Tests. DIN EN 14175-1, 2003. Fume Cupboards e Part 1: Vocabulary; German Version. DIN EN 14175-2, 2003. Fume Cupboards e Part 2: Safety and Performance Requirements. DIN EN 14175-3, 2014. Fume Cupboards e Part 3: Type Test Methods; German Version. DIN EN 14175-4, 2004. Fume Cupboards e Part 4: On-site Test Methods. Eckerman, K., Harrison, J., Menzel, H.G., Clement, C.H., 2012. ICRP Publication 119. Compendium of Dose Coefficients based on ICRP Publication 60. Ann. ICRP 41 (s). Emel, W.A., Hetzer, D., Pelletier, C.A., Barefoot, E.D., Cline, J.E., 1977. An airborne radioiodine species sampler and its application for measuring removal efficiencies of large charcoal adsorbers for ventilation exhaust air. ERDA Air Clean. Conf. 14, 389e431. Evans, G.J., Jervis, R.E., 1992. Radiochemical studies of iodine behaviour under conditions relevant to nuclear reactor accidents. J. Radioanal. Nucl. Chem. 161, 121e133. German Radiation Protection Ordinance 2001, StrlSchV x 55. Giraud, V., 1985. Retention of iodine by iodine filters in nuclear power plants in fires (a literature review). Kernforsch. Karlsr. KfK Berichte. KfK 3867, 49 pp. Huang, Y., Zhang, H., Li, W., 1987. Adsorption properties of selective adsorbent for differentiating airborne radioiodine species. Fushe Fanghu 35e40. Keller, J.H., Duce, F.A., Pence, D.T., Moeck, W.J., 1970. Hypoiodous acid: an airborne inorganic iodine species in steam-air mixtures. From 11. AEC air cleaning conference; Richland, Wash. (31 Aug 1970). In: Proceedings of the Eleventh AEC Air Cleaning Conference. UNCL. Orig. Receipt Date: 31-Dec-71. Kovach, J.L., 1992. Parametric studies of radioactive iodine, hydrogen iodine and methyl iodide removal. In: Proceedings of the 22nd DOE/NRC Nuclear Air Cleaning Conference, NUREG/CP-0130, vol. 2, pp. 646e660. Lee, B.S., Jester, W.A., Olynyk, J.M., 1991. Radioiodine speciation in the hot cell effluent gases of a radiopharmaceutical production facility. Health Phys. 61, 255e258. Lothar, P., Jurgen, W., 1990. Process for the Removal of Iodine and Organic Iodine Compounds from Gases and Vapors Using Silver-Containing Zeolite of the Faujasite Type. Unit States Patent 4, 913,850, Application Number 318841, Apr. 3, 1990. Maeck, W.J., Pence, D.T., Keller, J.H., 1969. Highly efficient inorganic adsorber for
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€cker et al. / Journal of Environmental Radioactivity xxx (2016) 1e6 K. Schoma
airborne iodine species (silver zeolithe development studies). U.S. Atomic Energy Commission Report IN-1224, 1968; 19pp Nucl. Sci. Abstr. 23, 2372. €cker, K., Sudbrock, F., Fischer, T., Dietlein, M., Kobe, C., Gaidouk, M., Schoma Schicha, H., 2011. Exhalation of 131I after radioiodine therapy: measurements in exhaled air. Eur. J. Nucl. Med. Mol. Imaging 38, 2165e2172. €cker, K., Fischer, T., Eschner, W., Gaidouk, M.I., Schicha, H., 2001. Exhalation Schoma of I-131 after radioiodine therapy (RIT): time dependence and chemical form. Nuklearmedizin 40, 15e22.
Wilhelm, J.G., 1977. Iodine Filters in Nuclear Power Stations. Report 1977, KFK-2449, 139 pp. Wilhelm, J.G., 1982. Iodine Filters in Nuclear Installations. Report of the Commission of European Communities 1982, pp. 37e56. Wren, J.C., Long, W., Moore, C.J., 1999. Modelling the removal of radioiodine by TEDA-impregnated charcoal under reactor accident conditions. Nucl. Technol. 125, 13e27.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
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Retention efficacy and release of radioiodine in fume hoods €cker a, *, T. Fischer a, B. Zimmermanns a, J. Bregulla a, F. Sudbrock a, O. Prante b, K. Schoma A. Drzezga a a b
Department of Nuclear Medicine, University Hospital of Cologne, Germany Department of Nuclear Medicine, University Hospital Erlangen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2015 Received in revised form 18 December 2015 Accepted 12 January 2016 Available online xxx
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$107 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 0.148 103% 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 (1.3 103%/m3 air) was measured after the activated carbon filter. In the ambient air, values of up to 8.6 106% pro m3 laboratory air sampled were measured, despite a 6e8-fold air exchange. €cker et al., 2001) more sensitive than the standard The selected procedure is a factor of 1011 (Schoma 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. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Radioiodine Fume hood Radioiodine retention DIN standards
1. Introduction Fume hood extraction units are installed in nuclear medical and radiopharmaceutical facilities in order to maintain a working environment with a low level of microbacterial contamination for the handling of unsealed, radioactive materials. These units should be suitable for the production of ready-to-use radioactive pharmaceuticals, in accordance with GMP (GMP ¼ Good Manufacturing Practice) conditions. Those generally available are a combination of a laminar-flow cabinet and a radionuclide hood. Special construction and operating modifications guarantee protection of personnel from incorporation, contamination and external radiation exposure. Here the retention capability of the combined fume hood unit
* Corresponding author. Department of Nuclear Medicine, University Hospital of Cologne, Kerpener Str. 62, 50937 Cologne, Germany. E-mail address: [email protected] (K. Schom€ acker).
assures that no dangerous levels of airborne radionuclide are incorporated by the operator standing in front of the fume hood. This is particularly important in the case of radioactive iodine. The volatility of radioiodine depends on its chemical form. In order to prevent release of 131I-iodine, with its half-life of ca. 8 days, into the environment, radionuclide hoods are fitted with additional, activated carbon filters. This should guarantee a reliable decontamination of air leaving the fume hood unit. To guarantee that the fume hood unit offers full protection, safety equipment must be regularly maintained and monitored to ensure that it functions effectively. The regulations for the design and testing of radionuclide hoods are laid-down in the form of DIN/ EN standards. These DIN/EN standards can be national, European or international. They apply to the performance of standard technical tasks and frequently provide a basis for regulatory measures. The behaviour of released radioactive substances and of gaseous 131 I-iodine in particular is currently assessed in the respective DIN/
http://dx.doi.org/10.1016/j.jenvrad.2016.01.006 0265-931X/© 2016 Elsevier Ltd. All rights reserved.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
€cker et al. / Journal of Environmental Radioactivity xxx (2016) 1e6 K. Schoma
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EN standards as equivalent to non-radioactive chemically hazardous substances. Thus the German DIN standard for radionuclide hoods (DIN 25466) stipulates that the air technology type examination should follow recommendations set down in DIN EN 14175 (DIN EN 14175-1eDIN EN 14175-4). The regulations for type testing laid down in the standard EN 14175-3 (DIN EN 14175-3) in particular call for test measurements with the tracer gas sulphur hexafluoride. This requires a detection limit of at least 108 volume fractions (0.01 ppm). Assuming the gas density of SF6 to be ca. 6 kg/ m3, this is the equivalent of a molar concentration of ca. 4$107 mol/ m3 air. With regard to volatile radioiodide compounds, in the case of 131I this amount would correspond to a radioactivity concentration in the order of magnitude of Terabequerel per cubic meter. A theoretical exposure distributed over 1000 individuals with average thyroid retention of 55% to such a radioactivity of 131I would lead to an effective dose of ca. 6 Sv. For this reason, the current test specifications, based on DINstandards, are unsatisfactory. Below is a proposed method of high sensitivity for measurement of radioiodine concentrations in the air. This offers a suitable procedure for testing the protective function of radionuclide extraction units with regard to protection of personnel (retention of radioiodine in the area around the fume hood) and environmental protection (decontamination of exhaust air from the radionuclide hood).
Fig. 2. Measurement of the containment capability of the fume hoods. For air sample collection the filter system was positioned to the left of the fume hood at a distance of 1 cm from the front sash.
2. Materials and methods 2.1. The fume hoods The fume hoods investigated in this study are shielded hoods for manipulations at medium activity, with frontal sliding shield and laminar flow. The workstation should be suitable for the breakdown of air-hanging radioisotopes coming from the manipulation of liquid or volatile gaseous substances. The interior is made of stainless steel (AISI 304). An integrated ventilator draws the process air from the working area through an activated carbon filter combination within a separate filter cabinet that is also made of stainless steel. The bag change technology, which is based on research from the field of nuclear technology, guarantees a safe filter change (according to the manufacturer). 2.2. Immission in the laboratory 2.2.1. Description of the measuring equipment For the determination of 131I-iodine concentrations in air we developed a test using a filter system (Figs. 1e3) for selective adsorption of chemically different radioiodine species as described by Schom€ acker et al. (2001, 2011). This filter system was mounted in two radionuclide laboratories in separate locations within the area around the radionuclide hood at various measuring positions.
Fig. 1. Sketch of the air sampling system. 0: GF50 ¼ aerosole filtre, 1: BE 110 ¼ cadmium iodide, 2: BE 110 ¼ cadmium iodide, 3: Silverzeolithe filtre, 4: TEDAcoated charcoal.
Fig. 3. Test of the decontamination capacity of the activated carbon filter combination in the filter unit to the right of the fume hood via a removal device (tubing joint) was fitted.
The air was drawn in by a portable constant flow air-sampler Model AVS-28 A from the company SAIC/RADeCO (USA) through a filter system consisting of an aluminium mounting bracket with 5 tightly packed individual filters. The filters were serially arranged in the following order (Fig. 1): one circular glass fibre filter GF50 (Schleicher & Schuell), two BE-110 cadmium iodide filters (SAIC), one GY-130 silver zeolite filter (SAIC) and one CP-100 filter (TEDAimpregnated activated carbon, SAIC, USA). The diameter of the filters was 5.77 cm, with a thickness of 2.54 cm (except for the round glass-fibre filter). The portable constant flow air-sampler and the filters were supplied by the company “Technisches Büro Schütz”, Rimbach, Germany. The listed adsorption materials have been used for many years to contain iodine released during accidents in nuclear power stations. The arrangement of filters used here, or similar combinations of such filters, often serve in nuclear reactor operations or in hot cells to establish the chemical composition of the iodine-radioactivity contained in the exhaust air (Emel et al., 1977; Giraud, 1985; Lee et al., 1991; Wilhelm, 1977, 1982).
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
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The circular glass-fibre filter is composed of borosilicate glass fibres. According to the manufacturer's information, aerosol particles of <1 mm can be separated out, particularly high separation rates (99,993%) being achieved for aerosol particle diameters of 0.5e0.3 mm. The two cadmium-iodide filters contain CdI2-impregnated Chromosorb-P (grain size ca. 30 mesh, mass: ca. 45 g, ca. 15 mass% CdI2). These filters are highly effective for retaining molecular I2. Retention occurs through a combination of isotope exchange and the formation iodine containing cadmium compounds (Keller et al., 1970; Huang et al., 1987). The silver zeolite filters contain ca. 75 g of the molecular sieve 13x-Ag (grain size: 16e40 mesh, pore size: 0.6e0.7 nm), impregnated with 37% (percent by weight) silver in the form of silver nitrate. Organically bound iodine reacts with the highly reactive silver surface to form silver iodide (Maeck et al., 1969; Evans and Jervis, 1992; Lothar and Jurgen, 1990). The last of the five serially arranged filters contains TEDA (triethylene-diamine, 5% by weight), impregnated with activated carbon (ca. 50 g, grain size 40e50 mesh). This type of filter can retain molecular as well as organically bound iodine. The iodine retention is brought about by primary adsorption and the formation of quaternary ammonium salts on the activated carbon surface (Kovach, 1992; Wren et al., 1999). Depending on its chemical form, airborne radioiodine can thereby be retained, quantified and differentiated into molecular I2, organically bound iodine or iodine as an aerosol. The air volume flow through the filter combination was ca. 4.5 m3/h. The activity of the individual filters was measured with a NaI(Tl) scintillation detector (Scintibloc 8SF/2A-X, Crismatec, Photomultiplier ScintiPack, model 296 EG&G ORTEC, Software UMS) from Berthold Technologies GmbH. The total activity was calculated as the sum of the activities in all the filters. The scintillation counter was calibrated with a “standard filter” containing a defined activity of 131I. This calibration was carried out in precisely the same geometry as that used for exhalation measurements. The “standard filter” was prepared using an aliquot of a 131 I sample for which the activity had been measured beforehand in a calibrated ionisation chamber (Physikalisch-Technische Bundesanstalt PTB, Braunschweig, Germany). Our scintillation detector was calibrated to convert count rates into activities by means of the above-mentioned calibration and was recalibrated 5 times. The mean value for the efficiency was taken as 11.6 Bq/cps. Each count rate (in cps) measured in the energy window between 320 and 400 keV was thereby converted into a radioiodine activity (in Bq). As typically low count rates were expected, the background radiation was determined prior to measurements on the filters and for air-flow samples. 2.2.2. Calibration of the filter combination Before commencement of air measurements, the effectiveness of the filter combination in separating the various iodine species was tested as follows: Elemental 131IeI2 was generated by reaction of 131IeNaI solution with potassium chromate in hydrochloric acid solution in a roundbottomed flask at 50 C, and the vapour phase drawn by suction over the filter combination at 4.5 m3/h for 30 min. As a prototype of organically bound radioiodine, 131I-ethyl-iodide in alcoholic solution was generated by iodine exchange between ethyl-iodide and 131 IeNaI at 50 C and likewise drawn by suction over the filter combination at 4.5 m3/h for 30 min. The two round-bottomed flasks were connected in series to allow mixing of the vapour phases. The proportions of the two species of iodine were determined after freezing out the combined vapour phases with liquid nitrogen, extraction of the condensate
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with absolute alcohol and measurement of radioactivity in the alcoholic and aqueous phases. The identical vapour mix was then drawn by suction over the filter combination. 2.2.3. Measurement of the containment capability of the fume hoods The generation of iodine species for testing the fume hood followed the description above (para. 2.2). The test were performed with the extractor function of the fume hood switched on and in the working position (front sash opened 15 cm). 10 MBq Na131I were used for the generation of elemental and organically bound 131 I for the test of the containment. The radioactivity in the two vessels foreseen for generation of elemental or organically bound iodine, respectively, was measured before and after the chemical reaction (60 min) by which iodine was released. The fraction of the released species (elemental and organically bound 131I) was determined by the separation technique. Air samples were also drawn during the chemical reaction (para. 2.1.1). Air sample measurements were carried out with the filter system positioned to the left, right and in the middle of the fume hood. The following distances were chosen: d1 ¼ 1 cm (Fig. 2) and d2 ¼ 5 cm from the front sash (Table 2). 2.3. Emission from the fume hood 2.3.1. Decontamination capacity of the fume hood The purpose of these measurements was to test the decontamination capacity of the activated carbon filter combination in the filter unit to the right of the fume hood. To this end, an air removal device (tubing joint) was fitted, diverting the air, after passage over the filters, into a further air system in which the radioiodine content was measured. Both elemental and organically bound iodine were released for these measurements. A long measuring period of over 990 min was taken to ensure that 131I species could still be detected even at the very low radioiodine concentrations expected at this measuring point after passage through the filters. By this procedure a total volume of 59.4 m3 exhaust air was collected after filtering (Fig. 3). 2.3.2. Ambient air measurements These measurements were taken solely in the location of the fume hood where markedly high concentrations of radioiodine were detected after passage through the activated carbon filter. They were taken in the respective radionuclide laboratory, first in front of the air outlet for exhaust air from the laboratory; with the air extractor unit of the fume hood running and with laminar air flow. Molecular and organically bound radioiodine were generated in the radionuclide hood as described above. The second measuring position was at an air outlet bringing the ambient room air directly into the circulating air stream for the radionuclide laboratory.
3. Results 3.1. Calibration of the filter combination The results of examination of the separating capability of the filter combination, as described above, are summarized in Table 1. They show that the apparatus is suitable for the separation of aerosolic, molecular and organically bound iodine. At the last filter (TEDA impregnated activated carbon) a mixture of molecular and organically bound iodine occurs that is not retained by the previous filter in the series.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
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Table 1 Test of separating capability of the filter combination. Vapour phase
I2 Ethyl-iodide Combined vapour phasea a
% total
131
I-radioactivity in
Glass fibre filter
CdI2-filter I
CdI2-filter II
Silver zeolite filter
Activated carbon-TEDA-filter
0.6 0.4 0.9
95.4 1.2 14.6
0.9 0.8 1.2
1.1 96.8 77.9
2.0 1.2 2.3
The radioactivity measurements of the alcoholic and aqueous phases gave an I2/ethyl-iodide ratio of 1: 4.5.
3.2. Immission in the laboratory
Table 3 Decontamination capacity of the fume hood.
3.2.1. Measurements of the containment capability of the hoods A1 and A2 These results are summarized in Table 2. Activities of 2e4 MBq were released. 131I-radioactivities were detected at 1 cm and 5 cm away from the front sash. The maximum fraction of radioiodine released into the exhaust air in this case were 0.4$103% of the 131I-radioactivity released per m3 exhaust air for Hood A1 (5 cm left). The corresponding value for Hood A2 was 0.63$103% of the 131Iactivity released per m3 exhaust air (5 cm right). For Hood A1, the radioiodine concentrations at a distance of 1 cm from the front sash in the left and right measuring positions were a factor of 2e3 times higher. The radioiodine concentrations for both samples were markedly lower in the middle position in comparison to the right and left. The relative proportions of the chemical forms of radioiodine differed depending on measuring position and the fume hood examined. 3.3. Emission from the fume hood 3.3.1. Decontamination capacity of the fume hood These results are summarized in Table 3. The radioiodine penetrated the activated carbon filter mostly in the organically bound form. Here a percentage for the 131I-activity released per m3 air for Hood A1 was reached that is 15 times higher than that obtained for Hood A2.
Measurements after passage through filters
A1
A2
Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103x%/m3] Organically bound [%] Aerosolic [%] Molecular [%]
34.70 12.97 1.30 96.3 0.5 3.2
5.35 0.40 0.084 96.0 4.0 LOD
3.3.2. Ambient air measurements These tests were performed only in the vicinity of Hood A1, as this is the point where a particularly high radioiodine concentration was found to have accumulated in the exhaust air after passage through the extraction filters. The results of the measurements at two different positions (P1, P2) at the fume hood A1 are summarized in Table 4. The laboratory air was found to contain chiefly organically bound radioiodine. At measuring point P2 the concentration of radioiodine was 23 times higher than at measuring point P1 (Table 4). 4. Discussion The release of radioiodine from a fume hood was assessed through radioactivity measurements at various distances from the front of the hood. The results are plausible. The radioiodine concentration fell with increasing distance to the sash pane, when
Table 2 Measurements of the containment capability of the hoods A1 and A2. A1
A2a
d1 (1 cm to front sash) Left Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Middle Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Right Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%]b Aerosolic [%]b Molecular [%]b a b c
A1
A2
1.57 0.39 0.40 28.2 9.3 63.5
2.70 0.65 0.031 27.1 41.4 31.5
2.603 1.48 0.00068 28.7 56.4 14.8
2.70 0.65 0.004 76.7 23.3 LODc
2.91 1.18 0.049 75.3 24.7 LODc
3.2 0.3 0.63 29.2 38.2 32.6
d2 (5 cm to front sash) 1.061 0.73 1.77 25.4 45.8 29.8
e e e e e e
1.014 0.738 0.148 63.4 6.4 30.1
e e e e e e
1.170 1.039 1.33 30.7 50.0 19.3
e e e e e e
Left Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Middle Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%] Right Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103%/m3] Organically bound [%] Aerosolic [%] Molecular [%]
For this hood, measurements were taken only at a distance of 5 cm. The percentage values in the table refer to total activity attributable to the various chemical forms (activity at the filters, excluding the activated carbon filter). Limit of detection.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006
€cker et al. / Journal of Environmental Radioactivity xxx (2016) 1e6 K. Schoma Table 4 Ambient air measurements in the vicinity of Hood A1 at two different positions (P1, P2). Measurement of ambient air
P1a
P2b
Radioiodine released (organic) [MBq] Radioiodine released (molecular) [MBq] Fraction of airborne radioiodine [103x%/m3] Organically bound [%] Aerosolic [%] Molecular [%]
33.36 5.26 3.56 107 100 LOD LOD
18.036 9.51 8.6 106 75.4 3.1 21.4
a Measurement at the air outlet for exhaust air from the laboratory into the external surroundings. b Measurement at the air outlet directly connected to the circulating air stream.
opened to 15 cm. The various forms of iodine occurring in the exhaust air of a fume hood (organically bound iodine, elemental iodine and aerosolic iodine) can be qualitatively and quantitatively determined with the aid of testing equipment through specific adsorption into the various filters. These tests are reproducible and easy to perform. The volatility of small, organically bound iodine compounds is most prominent compared to the other radioiodine species. Nevertheless, the fraction of airfloating aerosol or molecular radioiodine measured in front of the fume hoods were often comparatively higher. A possible cause for this depletion of highly volatile compounds might be the action of the fume hood itself. The detection limit, based on experience so far gained with the €cker et al., 2001), correprocedure, lies at 0.1 Bq/m3 (Schoma sponding to a level of ca. 1.66$1018 mol or 2.18$1017 g iodine per m3 exhaust air measured. The method used here is thus a factor of 1011 more sensitive than the recommended DIN-standard. The most surprising finding was the high level of radioiodine in the exhaust air after passage through the activated carbon filter in Hood A1. The active carbon filter is designed to extract almost 100% of the radioiodine passing through it. This reveals a particular failure of the radionuclide hood units investigated here, especially A1. The deficiency arose under quite normal working conditions. The finding led to a decision to collect and monitor air at the outlet opening in the laboratory in which Hood A1 was located. Surprisingly, incidences of marked radioiodine contamination were also detected in the normal ambient laboratory air. The levels of this contamination varied by a factor of 23 depending on the position of measuring point. The two fume hoods should be more or less comparable in terms of escape of radioiodine so that any difference would point to a malfunction of components. The underlying cause for the insufficient retention of radioiodine by A1 was a defective tubing that was detected on the occasion of our studies. The operation of the filters turned out to be defective while fume hood module was not impaired. This emphasizes the need of testing a fume hood with a better sensitivity than provided by tests with SF6 according to the DIN/EN procedure. The respective test had been carried out by a specialized institution beforehand and gave no reasons for an objection. The levels measured in the laboratory air can now be evaluated from two different points of view. The ICRP publication 119 gives a dose-coefficient for organically bound 131I of 1.8$108 Sv/Bq and for [131I]-I2 of 2.0$108 Sv/Bq (Eckerman et al., 2012). Assuming a mixture of the two, the mean value would be 1.9$108 Sv/Bq. Taking the maximal permissible dose for the thyroid to be 300 mSv/year in a working year of 200 days (German Radiation Protection Ordinance, StrlSchV x 55), then the maximal permissible radioactivity per working year would be 1.58$107 Bq (~16 MBq). Assuming a respiratory minute volume of 1 m3/h (adult on moderate exertion over a 4 h period) and applying
5
the yearly threshold value for inflow of activity from 131I-iodine into the thyroid of 1.58$107 Bq and a percentage value of radioiodine in the ambient air of 8.6$106% pro m3 (as measured at the most exposed measuring point), then the maximal permissible dose that could be released from a fume hood under these conditions would be 230 GBq 131I per year or 115 MBq/working day. It should, however, be emphasized that in responsible handling of radionuclides, efforts should be made never to reach this threshold. A safety factor of 10 should therefore be factored in. Furthermore, the probability of release of radioactivity needs to be estimated for any planned work. Thus the activity-limiting considerations laid down in the German Ordinance for Radiation Protection must also be taken into account. For facilities with an exhaust air flow of 104 m3/h, the exemption limit concentration is 5 Bq/m3 while for those with higher exhaust air flows the stipulated value is 0.5 Bq/m3. Taking the above considerations and this threshold into account, the percentage fractions measured in relation to the radioiodine activities released in the radionuclide fume hood after passage over the active carbon filter (1.3$103%/m3), give a maximal permissible release of 0.38 MBq of 131I-activity per experiment. 5. Conclusion The method of testing the retention of radioiodine in fume hoods presented in this study is more effective than the test with SF6. The respective European standard is therefore not suitable for assessing radioactivity in fume hoods. For one fume hood (A1) the retention was insufficient and measurable amounts of radioiodine were found in the laboratory. The German legislation defines an exemption level of 5 Bq/m3. Based on this level, the maximum activity that can legally be handled in this fume hood (A1) would not exceed 0.38 MBq. References DIN 25466, 2012. Fume Hoods for Radioactive Materials e Rules for Construction and Tests. DIN EN 14175-1, 2003. Fume Cupboards e Part 1: Vocabulary; German Version. DIN EN 14175-2, 2003. Fume Cupboards e Part 2: Safety and Performance Requirements. DIN EN 14175-3, 2014. Fume Cupboards e Part 3: Type Test Methods; German Version. DIN EN 14175-4, 2004. Fume Cupboards e Part 4: On-site Test Methods. Eckerman, K., Harrison, J., Menzel, H.G., Clement, C.H., 2012. ICRP Publication 119. Compendium of Dose Coefficients based on ICRP Publication 60. Ann. ICRP 41 (s). Emel, W.A., Hetzer, D., Pelletier, C.A., Barefoot, E.D., Cline, J.E., 1977. An airborne radioiodine species sampler and its application for measuring removal efficiencies of large charcoal adsorbers for ventilation exhaust air. ERDA Air Clean. Conf. 14, 389e431. Evans, G.J., Jervis, R.E., 1992. Radiochemical studies of iodine behaviour under conditions relevant to nuclear reactor accidents. J. Radioanal. Nucl. Chem. 161, 121e133. German Radiation Protection Ordinance 2001, StrlSchV x 55. Giraud, V., 1985. Retention of iodine by iodine filters in nuclear power plants in fires (a literature review). Kernforsch. Karlsr. KfK Berichte. KfK 3867, 49 pp. Huang, Y., Zhang, H., Li, W., 1987. Adsorption properties of selective adsorbent for differentiating airborne radioiodine species. Fushe Fanghu 35e40. Keller, J.H., Duce, F.A., Pence, D.T., Moeck, W.J., 1970. Hypoiodous acid: an airborne inorganic iodine species in steam-air mixtures. From 11. AEC air cleaning conference; Richland, Wash. (31 Aug 1970). In: Proceedings of the Eleventh AEC Air Cleaning Conference. UNCL. Orig. Receipt Date: 31-Dec-71. Kovach, J.L., 1992. Parametric studies of radioactive iodine, hydrogen iodine and methyl iodide removal. In: Proceedings of the 22nd DOE/NRC Nuclear Air Cleaning Conference, NUREG/CP-0130, vol. 2, pp. 646e660. Lee, B.S., Jester, W.A., Olynyk, J.M., 1991. Radioiodine speciation in the hot cell effluent gases of a radiopharmaceutical production facility. Health Phys. 61, 255e258. Lothar, P., Jurgen, W., 1990. Process for the Removal of Iodine and Organic Iodine Compounds from Gases and Vapors Using Silver-Containing Zeolite of the Faujasite Type. Unit States Patent 4, 913,850, Application Number 318841, Apr. 3, 1990. Maeck, W.J., Pence, D.T., Keller, J.H., 1969. Highly efficient inorganic adsorber for
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airborne iodine species (silver zeolithe development studies). U.S. Atomic Energy Commission Report IN-1224, 1968; 19pp Nucl. Sci. Abstr. 23, 2372. €cker, K., Sudbrock, F., Fischer, T., Dietlein, M., Kobe, C., Gaidouk, M., Schoma Schicha, H., 2011. Exhalation of 131I after radioiodine therapy: measurements in exhaled air. Eur. J. Nucl. Med. Mol. Imaging 38, 2165e2172. €cker, K., Fischer, T., Eschner, W., Gaidouk, M.I., Schicha, H., 2001. Exhalation Schoma of I-131 after radioiodine therapy (RIT): time dependence and chemical form. Nuklearmedizin 40, 15e22.
Wilhelm, J.G., 1977. Iodine Filters in Nuclear Power Stations. Report 1977, KFK-2449, 139 pp. Wilhelm, J.G., 1982. Iodine Filters in Nuclear Installations. Report of the Commission of European Communities 1982, pp. 37e56. Wren, J.C., Long, W., Moore, C.J., 1999. Modelling the removal of radioiodine by TEDA-impregnated charcoal under reactor accident conditions. Nucl. Technol. 125, 13e27.
€cker, K., et al., Retention efficacy and release of radioiodine in fume hoods, Journal of Environmental Please cite this article in press as: Schoma Radioactivity (2016), http://dx.doi.org/10.1016/j.jenvrad.2016.01.006