Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
similar to the fraction of 228 Th escaping by recoil, and therefore presumably similar to
the fraction of 220 Rn escaping by recoil, since the recoil ranges of 220 Rn and 228 Th are
similar (Griffith et al, 1980).
(164) Johnson and Peterman (1984) developed a model to describe the emanation of
220
Rn from ThO2 particles by alpha-particle recoil, and its exhalation from the lungs.
They calculated that the fraction of 220 Rn atoms produced that escaped from particles
(density 10 g cm –3 ) by recoil decreased from ~1.0 at 1 nm to ~0.5 at 10 nm and ~0.1
at 0.5 µm diameter. The average fraction for an aerosol of AMAD 1 µm was
calculated to be 0.2, which seems to be broadly consistent with the results derived by
Griffith et al (1980).
(165) Thus it seems that recoil is a mechanism that is at least as important as
diffusion for emanation of radon from particles. It seems possible that it is the
dominant mechanism, in which case for aerosols of AMAD about 1 µm there would
be a release to lung air of ~10% of 222 Rn, 220 Rn or 219 Rn formed in particles in the
lungs. Furthermore, alpha-particle recoil applies not only to radon formed as a decay
product, but also to other decay products formed by alpha emission. In the case of
decay chains, this will result in successively lower activities of members of the chain
compared to the parent retained in relatively insoluble particles. There is some
experimental evidence confirming this (see thorium inhalation section). However, it
was considered impractical to implement loss of decay products by alpha recoil in the
calculation of dose coefficients and bioassay functions in this series of documents.
Assessment of the fractional loss for representative workplace aerosols would be
complex, because it depends on the alpha decay energy, the size distribution of
deposited particles, and their shape and density: simplifying assumptions would be
needed for practical implementation. Investigations conducted at by the Task Group
into the effect of recoil on doses for inhaled 232 U and its decay products following
inhalation in relatively insoluble particles found, as expected, that doses to the
respiratory tract decreased and doses to tissues resulting from systemic uptake
increased. However, there was little impact on effective dose in this example. The
computational effort involved in identifying radionuclides formed by alpha decay, and
partitioning the decay product atoms between a fraction remaining in the particle and
a fraction escaping to lung fluids would be considerable, and was considered
disproportionate to the benefit gained on a routine basis as in these documents. For
calculation purposes the assumption that radon formed as a decay product within the
respiratory tract escapes from the body at a rate of 100 d –1 is retained in this series of
documents. Nevertheless, this phenomenon should be borne in mind, especially when
using decay products to monitor intakes and doses of the parent radionuclide.
Soluble (dissociated) material
(166) The behaviour of soluble or dissolved material (specifically the rate of uptake
to blood) of decay products formed in the respiratory tract can be expected to depend
on the element of which the decay product formed is an isotope. As discussed above,
for soluble (Type F) materials the rapid dissolution rate, sr, represents the overall
absorption from the respiratory tract to blood and is element-specific for many
elements. Hence, when a Type F material is deposited in the respiratory tract, the
value of sr for a decay product formed would be expected to be that of the element
formed ('independent kinetics'), rather than following that of the parent ('shared
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