Fisher-Modified Wrenn-Lipsztein Uranium Urinary Excretion Model cont'd

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

While Equation 5.4 represents a replacement function for the Fisher-Modified Wrenn-Lipsztein

model, it does not have output values that are identical to the model given by Equation 5.2, especially

between 1 and 5 d.

Standard models for the systemic retention of uranium are commonly used for internal dosimetry

because in vivo detection of uranium within the individual systemic compartments is not usually possible.

Three models were proposed by the ICRP over a 10-year period. Each of them has had a wide application,

and the 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 (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 systemic retention parameters for

uranium internal dosimetry. The ICRP model (ICRP 1979) for uranium is a "once-through" model:

53.6% of uranium entering the transfer compartment (the blood) is assumed to be excreted directly in

urine; the remainder is distributed among the bone (22.3%), kidney (12%), and other soft tissue (12%).

Others (Durbin 1986 and Wrenn et al. 1985) have described a recycling model based on extensive

review of available data. In this model, 67.3% is excreted in urine, 17% taken up by bone volume, 7%

by soft tissue, and 8% by the kidney.

These two models are the only ones to have been widely applied to evaluate exposure to uranium in

recent years. The recycling model is a more physiological representation, but the ICRP model is more

widely accepted.

The appropriate toxicokinetic model for uranium entering the blood after exposure is the recycling

model. The different physical and chemical forms of the uranium are unimportant. The parameters of the

model may not be appropriate at high uranium concentrations.

Uranium is transported through the bloodstream as a carbonate ion (UO2[CO3]2 2-). The chemical form

of uranium that enters the blood is dependent on the chemical form of the uranium that was inhaled. A

substantial portion of uranium filtered by the kidneys is temporarily retained in the renal tubules before

passing to the urinary bladder contents (ICRP 1995).

The skeletal behavior of uranium is in some ways qualitatively similar to that of the alkaline earths,

with UO2 2+ exchanging with Ca2+ on the bone mineral surfaces. There remain substantial uncertainties

regarding the long-term retention of uranium in bone, as well as soft tissues (ICRP 1995).

Urinary excretion of uranium is assumed to arise from:

uranium moving directly from plasma to the urinary bladder contents, accounting for 63% of

uranium leaving the circulation or

uranium moving to the urinary bladder contents after temporary residence in the renal tubules,

accounting for 12% of uranium leaving the circulation, with a half- life of 7 days (ICRP 1995).

5-27