Most internal dosimetry computer codes allow adjustment of particle size and
selection of solubility classes. Some codes also permit detailed adjustment of the
model's individual compartment parameters; with these codes, it may be possible to
arrive at various subjective interpretations to explain the same data. When
adjustments are made to the standard assumptions, it is important to explain what
those adjustments are and why they were made.
The gastrointestinal tract model of ICRP Publication 30 (1979 and 1988b) is also
widely promulgated and used for evaluating ingestion intakes and as well as being
coupled to the respiratory tract for inhalation intakes. The model is particularly
subject to individual variations in fecal voiding frequency, so judgment must be used
in its application to human data.
A key parameter of the model for internal dosimetry is the f1 factor for absorption to
blood of material in the small intestine. The f1 factor varies from 10-5 for plutonium
oxides to 10-4 for plutonium nitrates and to 10-3 for other compounds and americium.
Systemic Retention and Excretion of Plutonium
Standard models for the systemic retention of plutonium are commonly used for
internal dosimetry because in vivo detection of plutonium within the individual
systemic compartments is not usually possible. Three models proposed by the ICRP
over a 10-year period are described in Section 2.4.2 of this document. Each of them
has had a wide application, and ICRP has suggested that results derived using one
model do not need to be rederived for compliance purposes using the newest model.
Studies by the U.S. Transuranium Registry and summarized by Kathren (1994) have
indicated that alternate compartments and clearance half-times may be more
For convention, this document will use the ICRP 30, Part 4 (1988b) systemic
retention parameters for plutonium internal dosimetry. That model assumes that for
plutonium reaching the blood, 45% is deposited on bone surfaces from which it
clears with a 50-year half-time, 45% is deposited in the liver with a 20-year clearance
half-time, and a very small fraction (3.5 x 10-4 for males and 1.1 x 10-4 for females) is
permanently retained in gonadal tissue. The remaining 10% goes uniformly to all
other tissues and direct excretion.
Excretion models for plutonium include the empirical models of Langham (1956) and
Langham et al. (1980), Durbin (1972), Jones (1985), and Tancock and Taylor (1993),
as well as study models such as Leggett (1984). This technical document does not
take a position on the "best" model. Site choices of dosimetry tools such as reference
tabulations (Lessard et al., 1987; ICRP, 1988a) and computer codes may dictate one
model over another. The choice of model and explanation of its selection are among
the technical bases of the site internal dosimetry program.