Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
(209) A model that describes the time-dependent distribution and excretion of a
radionuclide in the body after it reaches the systemic circulation is referred to here as
a systemic biokinetic model. In contrast to ICRP’s current and past biokinetic models
describing the behaviour of radionuclides in the respiratory and alimentary tracts,
ICRP’s systemic biokinetic models generally have been element-specific models with
regard to model structure as well as parameter values. A generic model structure that
depicts all potentially important systemic repositories and paths of transfer of all
elements of interest in radiation protection would be too complex to be of much
practical use. However, generic model structures have been used in previous ICRP
documents to describe the systemic biokinetics of small groups of elements, typically
chemical families, known or expected to have qualitatively similar behaviour in the
body. For example, Publication 20 (ICRP, 1973) introduced a generic model
formulation for the alkaline earth elements calcium, strontium, barium, and radium,
but provided element-specific values for most model parameters. In Parts 1-3 of
Publication 30 (ICRP, 1979, 1980, 1981) a model developed for plutonium, including
parameter values as well as model structure, was applied to most actinide elements.
The biokinetic models for several of these actinide elements were modified in Part 4
of Publication 30 (ICRP, 1988), where the model structure for plutonium was used as
a generic structure; a common set of parameter values was applied to plutonium,
americium, and curium; and element-specific values were applied to selected
parameters in the models for other elements. The use of generic systemic model
structures was increased in ICRP’s reports on doses to members of the public from
intake of radionuclides (ICRP, 1993, 1995a, 1995b) and is further expanded in the
present document because it facilitates the development, description, and application
of systemic biokinetic models.
3.5.2 Formulation of systemic models in modern ICRP reports
(210) Publication 30 (ICRP, 1979, 1980, 1981, 1988) provided a comprehensive set
of systemic biokinetic models for radionuclides commonly encountered in
occupational settings. The models were generally in the form of retention functions
(e.g., sums of exponential terms) that may be interpreted as first-order compartmental
models with one-directional flow. These models were designed mainly to estimate the
cumulative activities of each radionuclide in its main repositories in the body. They
do not depict realistic paths of movement of radionuclides in the body but describe
only the initial distribution of elements after uptake to blood and the net biological
half-times of elements in source organs. Activity absorbed from the gastrointestinal or
respiratory tract or through wounds was assumed to enter a transfer compartment,
from which it transfers to source organs with a specified half-time, typically 0.25 d or
longer. Retention in a source organ was usually described in terms of 1 - 3 first-order
retention components, with multiple biological half-times representing retention in
multiple hypothetical compartments within a source organ. Feedback of activity from
tissues to blood was not treated explicitly in Publication 30 with the exception of the
model for iodine. It was generally assumed that activity leaving an organ moves
directly to a collective excretion compartment, i.e., radioactive decay along actual
routes of excretion is not assessed. Relatively short-lived radionuclides (half-lives up
to 15 d) depositing in bone were generally assigned to bone surface and longer-lived
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