Occupational Intakes of Radionuclides Part 1 - ICRP

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4186 4187 4188 4189 4190 4191 4192 4193 4194 4195 4196 4197 4198 4199 4200 4201 4202 4203 4204 4205 4206 4207 4208 4209 4210 4211 4212 4213 4214 4215 4216 4217 4218 4219 4220 4221 4222 4223 4224 4225 4226 4227 4228 4229 4230 DRAFT REPORT FOR CONSULTATION knowledge about the exposure before bioassay data are obtained. Each sample is weighted by the appropriate likelihood function for a given intake to produce a quantity termed the ‘weighted likelihood’. The probability of each intake and time of intake, given the observed measurement data, is calculated from the weighted likelihoods using simple numerical integration techniques. These methods, although computationally intensive, obviate the need to assume intake times when other types of circumstantial information are absent. (320) In Publications 54 and 78 (ICRP, 1988a, 1997b), it is argued that in the absence of any information, the time of intake is equally likely to have occurred before the mid-point of the monitoring interval, than after it, and therefore suggests that in these situations, a value of t=T/2 should be used, i.e. the intake is assumed to have occurred at the mid-point of the monitoring interval. Alternative approaches have been suggested (Strom 2003; Puncher et al, 2006; Birchall et al, 2007; Marsh et al, 2008). However the results of these alternative approaches, in most circumstances, differ very little from the mid-point method. The mid-point method is recommended here for reference evaluations (Section 6.2.1). 6.3.2 Route of Intake (321) Although intakes by inhalation alone are the most frequent in the workplace, intakes by ingestion and uptake through wounds and intact skin cannot be excluded. If the route of intake is not known and several bioassay results are available, including different types of bioassay measurements, a comparison of these results may help in determining it. In some facilities simultaneous intakes by several routes can occur. (322) If the radionuclide activity can be assessed by direct measurements, lung counting can be used to differentiate between inhaled and ingested material. However, if this is not possible and the radionuclide is in an insoluble form, interpretation of activities excreted in faecal and urine samples in terms of intake is quite problematic. Both the ingested material and the inhaled material deposited in the upper respiratory tract will clear through the faeces in the first few days after intake. Consequently, it is important to initiate excreta sampling as soon as possible after an acute intake, continuing for an extended period. Material in the faeces after the second week will originate mainly from the respiratory tract, and so later measurements can be used to correct the earlier faecal sample measurements for this component. In the monitoring of workers chronically exposed to long-lived, insoluble radionuclides, activities in the faeces after a 15 days absence from work will mostly reflect the delayed clearance from inhaled material, which dominates the dose (IAEA, 1999, 2004). Intakes of radioactive materials through wounds may occur as a result of accidents. A summary of the wound model developed by NCRP (NCRP, 2007) is presented in Section 3.4. 6.3.3 Particle Size (323) Radionuclides can become airborne through numerous processes and can be present in various physical forms such as gases, vapours, and particles with a wide range of sizes, shapes and densities. Most aerosols are composed of particles with complex shapes and varying particle sizes (NCRP, 2010). For modelling purposes in dose calculations, ICRP advises the use of the activity median aerodynamic diameter 118
4231 4232 4233 4234 4235 4236 4237 4238 4239 4240 4241 4242 4243 4244 4245 4246 4247 4248 4249 4250 4251 4252 4253 4254 4255 4256 4257 4258 4259 4260 4261 4262 4263 4264 4265 4266 4267 4268 4269 4270 4271 4272 4273 4274 4275 DRAFT REPORT FOR CONSULTATION (AMAD) which, together with the geometric standard deviation, describes the particle size distribution of the inhaled aerosol (ICRP, 2002b). The AMAD influences deposition in the respiratory tract and as a consequence the transfer of unabsorbed particles to the GI tract. (324) The AMAD of the airborne contamination in the workplace may be characterised as part of a workplace monitoring programme. In some working environments more than one particle size distribution mode may be detected. In cases of accidental releases of material, information on the particle size distribution of the airborne fraction of the release should be obtained whenever possible. When the size distribution of the radioactive aerosol is not known, then default value of 5 μm AMAD for occupational exposures should be used (ICRP, 1994a, 2002a). 6.3.4 Chemical Composition (325) The chemical form of the intake can have a significant effect on the behaviour of the radionuclide that has entered the body. Chemical forms commonly encountered in the working environment are given in subsequent reports in this series for selected radionuclides. Where there is adequate experimental data, chemical forms are assigned to one of the default absorption Types (F, M or S), and a value for the alimentary tract transfer factor, fA, is assigned. In some special cases, material-specific values for the parameters describing absorption to blood are provided (Section 3.2.3). (326) A compound might have absorption characteristics slightly or considerably different from those of the default. The interpretation of bioassay measurements is sensitive to the choice of absorption parameter values of the inhaled radioactive material. In cases of significant intakes of radionuclides, and in an accident situation, it may be necessary to obtain specific data on the chemical form of the radionuclide(s) involved to obtain a more realistic assessment of the intake and committed effective dose. However the gathering of additional source-term information takes time, and often will not be available soon after the incident/accident. The specific/reference ICRP lung absorption parameter and fA of the chemical form that most closely describes the release material should be used in the first dose calculations, following the first monitoring results. Follow-up bioassay monitoring and further investigations of the accident should be used to confirm or modify the results of the first dose calculations. (327) In many situations the worker is exposed to several chemical forms of the same radionuclide. Workers exposed in different areas of a uranium enrichment facility, for example, might be exposed to different chemical forms of uranium. Interpretation of bioassay results, excreta results in particular, will rely heavily on the assumptions related to the contributions of the different chemical forms to these results. 6.3.5 Influence of Background (328) Radionuclides from the three natural radioactive decay series and other natural and anthropogenic sources are present in all environmental media, and thus are also contained in foodstuffs, drinking water and in the air, leading to intakes by human populations. Their presence should be taken into account when interpreting bioassay 119
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Occupational Intakes of Radionuclides Part 1 - ICRP

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