Details of the Actual Dose Assessment

DOE-STD-1121-98

particle sizes may need to be characterized by their activity median thermodynamic diameter (AMTD) for

small diameters for which diffusion behavior predominates. Note that use of respiratory tract models

other than ICRP 30 must be justified in the program's technical basis documents.

Lung Solubility and Transportability. Decisions regarding transportability of radionuclides from the

lung should be documented and justified in the dose assessment. The transfer rate of a radionuclide from

the lung across to other regions of the body is dependent on the physical and chemical forms of the

radionuclide and its host aerosol, and on the biokinetic characteristics of the subject. These

characteristics of internally deposited radionuclides can be inferred from bioassay measurements, when

available. If bioassay measurements are not available or are not complete, these characteristics should be

estimated from the general chemical form of the radioactive material and information given in the ICRP

Publication 30 series (1979-1982). If there is no basis for specifying the chemical form, then

conservative estimates based on the range of values provided for the radionuclide in ICRP publications

should be used. For example, the choice of a class Y material for inhalation of uranium compounds

would result in a maximum committed effective dose equivalent per unit intake. In contrast, the choice of

a class D material for inhalation may result in a maximum value for the dose equivalent to bone surfaces.

It would be wise to study in advance the solubility classification to be assigned to radionuclides

commonly encountered in the workplace.

Transportability classes that differ from the ICRP models have been observed. At the Y-12 plant in

Oak Ridge, a combination of class W and class Y uranium has been observed (Forrest and Barber 1993;

Barber and Forrest 1995). This combination has been called "class Q," for "quarterly". Similarly,

material that clears more slowly than class Y has been observed and termed class Super-Y (Sula et al.

1991).

Occupational exposures may involve mixtures of radionuclides with various abundances and

physical and chemical compositions. These radionuclides may be contained in a host matrix with

characteristics that determine the actual solubility or transportability of most or all of the radionuclides in

the mixture. Prior experience or studies for specific exposure conditions are the best means for

determining the presence of and behavior of individual radionuclides in such mixtures.

Radioactive Progeny. Radioactive progeny produced by the decay of retained quantities should be

modeled separately from the parent if the systemic retention and biokinetics for the progeny radionuclide

are well known and if the physical half-life of the progeny is long enough to make a dosimetric

difference. Otherwise, the progeny should be assumed to be distributed and retained as the parent

radionuclide. It is particularly important to model radioiodines and noble gases separately from parent

radionuclides for internal dose assessment (ICRP 1979a).

7.3.3 Details of the Actual Dose Assessment

After the approach (direct or model) has been selected, intake or uptake can be assessed from various

bioassay results. An objective best fit of the predicted to observed bioassay measurement results should

be made. Documentation of the data, assumptions and methods used should be included in the dose

assessment. If alternate methods result in different results, the bases for reaching the decision on the

accepted result should be documented and should be reviewed by a second qualified dosimetrist (either

within the organization or outside).