Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
the absence of specific information, it would be appropriate to assume that intake was
by inhalation for an occupational exposure. The effect of assumed route of intake on
assessed doses can be large and should be investigated when assessed doses are
significant.
Source term
(368) Assumptions regarding the source term (i.e. the identity of the radionuclides
and their relative abundances) may represent major sources of uncertainty when
monitoring does not include the measurement of all the radioisotopes present in the
working environment. In many situations a worker is exposed to several isotopes of
the same radionuclide, but monitoring is accomplished through the measurement of
one of the isotopes. For example, lung monitoring of uranium through the
measurement of 235 U relies on assumptions on the level of enrichment. In other
circumstances, assessments of exposure to certain radionuclides are based on the
monitoring results of a progeny radionuclide in the lungs. For example, monitoring of
232
Th by measurement of a progeny radionuclide relies on assumptions about the
equilibrium of radionuclides in the 232 Th decay chain in the material to which the
worker is exposed. Also, exposure to some radionuclides may be based on
measurement of a surrogate radionuclide known to be present in the working
environment. For example, lung monitoring of 239 Pu may be based on the
measurement of 241 Am, using assumptions about the fraction of 241 Am, which grows
from 241 Pu.
(369) Information on the chemical form, or mixture of forms, of an inhaled
radionuclide is needed to help determine an appropriate dissolution model for activity
deposited in the lungs. The dissolution rate in the lungs can represent a major source
of uncertainty in a dose assessment, particularly when dose estimates are based on
excretion data alone. For example, if dose estimates are based on urinary excretion
data, then the dose to lungs can sometimes be underestimated by several orders of
magnitude if the material is incorrectly assumed to be highly soluble or overestimated
by several orders of magnitude if the material is incorrectly assumed to have low
solubility. When no direct information is available on the inhaled form of a
radionuclide, a combination of urinary and faecal data and, where feasible, in vivo
lung measurements may greatly reduce the uncertainty in dose estimates associated
with the chemical form of the radionuclide.
Particle size
(370) The particle size can be an important source of uncertainty because it
influences the assumed deposition in the respiratory tract. The urinary and faecal
excretion rates depend of the particle size because the size influences the transfer of
unabsorbed particles to the alimentary tract. In some working environments
multimodal aerosols exist within the respirable size range.
6.5.3 Uncertainties in Biokinetic Models
(371) Biokinetic models are used in radiation protection to predict the transfer and
bioaccumulation of a radionuclide in various organs and the rate of excretion of the
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