Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
(160) Generally the assumption applied to the slowly-dissolving fractions of Type M
and Type S materials is that the dissolution of a decay product is determined by
that of the particle matrix in which it is formed. Thus its dissolution parameter
values should be those of the inhaled material.
Emanation of radon and alpha recoil
(161) In applying the HRTM, general exceptions have been made for noble gases
formed as decay products (ICRP, 1994b, 1995c). Radioisotopes of xenon formed
from the decay of iodine are assumed to escape from the body without decay, as in
Publication 30 Part 1 (ICRP, 1979). This includes xenon formed in the respiratory
tract. For calculation purposes it is assumed that radon formed as a decay product
within the respiratory tract escapes from the body at a rate of 100 d -1 , in addition to
other routes of removal (ICRP, 1995c). This rate was set as a convenient, arbitrary,
rapid rate. The underlying assumption is that loss of radon (for example) is a
continuous process such as diffusion. The three radon isotopes in the natural decay
series: 222 Rn (radon), 220 Rn (thoron), and 219 Rn (actinon) have half-lives of about 3.8
days, 56 seconds and 4 seconds, and therefore decay rates of about 0.18, 1100 and
15,000 d –1 , respectively. Hence the assumption of a rate of loss of 100 d –1 implies that
nearly all 222 Rn escapes from the particles before it decays, about 10% of 220 Rn
escapes, and nearly all 219 Rn decays within the particles. As described in the thorium
inhalation section, studies which have compared thorium lung contents with exhaled
thoron seem broadly consistent with the assumption that about 10% of thoron formed
within particles in the lungs escapes, but measurements of emanation of radon ( 222 Rn)
from uranium ore dust give values much lower than 100%.
(162) Griffith et al (1980) developed a model to describe the retention of 232 U and
its decay products (which include 228 Th) in the lungs following inhalation of ThO2 or
UO2 particles. In addition to chemical dissolution, they considered emanation of 220 Rn
from particles by diffusion, and emanation of decay products, including 220 Rn, as a
result of the recoil of nuclei formed in alpha-particle decay. They presented equations
to calculate fractional losses by diffusion and recoil as functions of particle size (but
only for spherical particles). They calculated recoil ranges of about 0.05 µm for the
decay products, (assuming a particle density of 10 g cm –3 ) and fractional losses by
recoil emanation in the range 0.3 – 0.1, for aerosols with AMAD in the range 1 – 10
µm. The calculated loss of 220 Rn from particles by diffusion emanation was difficult
to predict, ranging from 0.03 to 0.7 depending on the assumed diffusion coefficient
(10 –15 - 10 –11 cm 2 s –1 ).
(163) Coombs and Cuddihy (1983) measured the fraction of 228 Th escaping by
recoil, and the fraction of 220 Rn escaping by diffusion, from size-fractionated samples
of ThO2 and uranium oxide (mixture of UO2.2 and U3O8) containing 1% 232 U. The
fraction of 228 Th escaping increased from ~0.07 for particles with AMAD 2.5 µm
(count median diameter, CMD, ~1 µm) to ~0.3 for particles with AMAD 0.65 µm
(CMD ~0.1 µm). This was in reasonable agreement with the model of Griffith et al
(1980). Calculated recoil range was expressed in terms of recoil range multiplied by
density, with values of ~20 µg cm –2 . The fraction of 220 Rn escaping by diffusion
increased from ~0.07 for particles with AMAD 2.5 µm, to ~0.35 for particles with
AMAD 0.65 µm, and gave a diffusion coefficient of ~3x10 –14 cm 2 s –1 . This was
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