Occupational Intakes of Radionuclides Part 1 - ICRP

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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
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