Gastrointestinal Tract - doe-std-1128-98

DOE-STD-1128-98

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.

5.6.2

Gastrointestinal Tract

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.

5.6.3

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

appropriate.

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.

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