Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
Figure 9. Alternative compartment models representing time-dependent absorption to body
fluids (dissolution and uptake). In the model shown in Figure 9(a) a fraction fr of the deposit
is initially assigned to the compartment labelled ‘Rapid dissolution’, and the rest of the
deposit (1 – fr) is initially assigned to the compartment labelled ‘Slow dissolution’. In the
model shown in Figure 9(b) all the deposit is initially assigned to the compartment labelled
‘Particles in initial state’, and material in the compartment labelled ‘Particles in transformed
state’ is subject to particle transport at the same rate as material in the compartment labelled
‘Particles in initial state’. Material in the compartment labelled ‘Bound material’ is not
subject to particle transport and is cleared only by uptake into body fluids. For definition of
symbols, see text.
(132) If the dissolution rate decreases with time, as is usually the case, either system
could be used, and would give the same results, with the following values:
sp = ss + fr (sr – ss)
spt = (1 – fr) (sr – ss)
st = ss
(133) The system shown in Figure 9(b) was applied by default in earlier
Publications (ICRP, 1994b, 1995c, 1997b). The additional flexibility it provides is,
however, rarely required in practice, and it is more complex (and less intuitive) to
present. The simpler approach is therefore adopted now as the default, with the more
flexible approach retained as an alternative. Examples of materials that show
dissolution rates that increase with time, which have been represented by ‘particles in
initial state’ and ‘particles in transformed state’, including uranium aluminide, are
given in the element sections in subsequent reports of this series.
(134) Uptake: uptake to body fluids of dissolved material is usually assumed to be
instantaneous. For some elements, however, part of the dissolved material is absorbed
rapidly into body fluids, but a significant fraction is absorbed more slowly because of
binding to respiratory tract components. To represent time-dependent uptake, it is
assumed that a fraction (fb) of the dissolved material is retained in the ‘bound’ state,
from which it goes into body fluids at a rate sb, while the remaining fraction (1 – fb)
goes to body fluids instantaneously (Figure 9). In the model, material in the ‘bound’
state is not cleared by particle transport processes, but only by uptake to body fluids.
Thus, only one ‘bound’ compartment is required for each region.
(135) The system shown in Figure 9 applies to each of the compartments in the
particle transport model shown in Figure 6. It is assumed that no absorption takes
place from ET1, but if the model in Figure 9 (a) is used the ET1 deposition still has to
be partitioned between fast and slow compartments because material is cleared from
ET1 to ET2, from which absorption does take place.
(136) For all elements, default values of parameters are recommended, according to
whether the absorption is considered to be fast (Type F), moderate (M) or slow (S).
The original reference values, given in Publication 66 (ICRP, 1994a) and reproduced
in Table 5, were specified in terms of the parameters initial dissolution rate, sp;
transformation rate, spt; and final dissolution rate, st (Figure 9 (b)), rather than fr, sr
and ss (Figure 9 (a)), for which approximate values were given. For gases or vapours,
instantaneous uptake to body fluids has also been recommended, as in Publication 68
(ICRP, 1994b), and defined as Type V (very fast), in Publication 71 (ICRP, 1995).
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