Modeling The Behavior of Uranium in the Body

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

5.5.4 Conditions for Adjustments of Actio n Levels

When workers are potentially exposed to other radiation sources or toxic agents, the action levels

should be reevaluated. Since uranium has both chemical and radiological toxicity characteristics,

urinalysis results should be interpreted both in terms of mass and radioactivity to ensure that the most

appropriate set of action levels is used (ANSI/HPS 1995).

5.6 MODELING THE BEHAVIOR OF URANIUM IN THE BODY

A key issue in uranium dosimetry is the modeling of how the material behaves in the body. Some of

the standard models are described below, with additional discussion of the biological behavior given in

Section 2.4. It is important that an internal dosimetry program establishes and documents the routine

models and assumptions used for dosimetry. Computer codes typically incorporate standard models but

may allow the flexibility to alter parameters. When altered on an individual specific basis, the revised

models need to be addressed in the pertinent case evaluations or the technical basis.

5.6.1 Respiratory Tract

The respiratory tract model of ICRP Publication 30 is commonly used for evaluating inhalation

intakes of radioactivity. The model has been widely published and included in reference books (e.g.,

Cember 1996; Shleien 1992) and internal dos imetry computer codes, hence it is not reproduced here. In

1994 a newer respiratory track model, ICRP Publication 66, was published (ICRP 1994)

Like all models, the ICRP respiratory tract model represents anticipated behavior. Once an

exposure has occurred and actual data become available, deviations from the model in light of the data

are appropriate.

In practice, the model has proved extremely valuable for calculating derived investigation levels and

estimating intakes from bioassay data, using standard D, W, and Y classes of material. Model interpretation

becomes more subjective when extensive data become available. Others (Carbaugh et al. 1991 and La Bone

et al. 1992) have provided excellent examples of two cases where the standard lung model assumptio ns did

not fit the data.

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 is also widely promulgated and used for

evaluating ingestion intakes. It is also 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, that is, the fraction of a stable element reaching the body fluids following

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