Internal Dose Determination

DOE-HDBK-1184-2004

5.2

Internal Dose Determination

The first step in determining an individual's dose resulting from a radionuclide

intake is the intake assessment (i.e., determining the amount of radioactive material

present in the body). Intake assessments for tritium exposure usually rely on

radiobioassay. Common radiobiossay techniques for tritium (urinalysis) are

rendered ineffective for some forms of STC intakes (especially insoluble

particulates) because of the difficulties associated with relating the results of these

analyses to specific intake levels. Therefore, representative air monitoring results

are often used for assessment of doses resulting from exposure to particulate

STCs. Variations in individual biological characteristics and statistical uncertainty

associated with any measurement make dose assessment a process that must be

approached with reasonable assumptions and documentation that support the

calculation methodology.

Personal intake and dose assessments can be based on data from representative

air monitoring using any appropriate technique, most commonly lapel sampling.

Fixed air sample heads and portable air samplers may be used for individual dose

assessment if one can ensure the sample is representative of the air inhaled. But,

they are primarily used to verify the adequacy of radiological controls and postings

and to document radiological conditions in the area of interest. See DOE-STD-

1211-98 (DOE 1999h) Internal Dosimetry.

5.2.1

Intake Determination Methodology for Tritiated Particulates

Determinations of individual radioactive material intakes generally fall under three

different methods: 1) in-vivo analyses, such as whole body or organ counting; 2) in-

vitro analyses, using analyses of excreta (urine or fecal analyses); and 3) area

monitoring, using results of area surface and air contamination monitoring

programs. The actual method used depends on a number of factors, including the

characteristics of the material to be analyzed, results of prior scientific analyses to

develop applicable protocols, and the equipment available. This section discusses

the applicability of these various methods to evaluating intakes of particulate STCs.

5.2.1.1 Air Monitoring

Air monitoring can be used to estimate intake directly, as opposed to indirect

bioassay methods. Intake is considered to be proportional to the actual activity

captured on the filter of an air-sampling device (assuming that the sample is

representative of the breathing air for the individual in question). Particulates tend

to shield their tritium beta activity by self-absorption of the beta radiation within the

mass of the particle. Observed activity on a filter sample, measured by suspending

particulates from the filter into liquid scintillation counting (LSC) solution, therefore

under-represents the actual activity available for deposition to the lung, i.e., intake.

Self-absorption factors (SAFs) vary, as a function of respirable (<10 m AMAD)

particulate size and material, by a factor of approximately 10. However, when

tritiated particulate intake is defined in terms of observed activity (and when DCF is

correspondingly defined in terms of observed activity intake), the uncertainty in the

observed intake essentially disappears, since self-absorption is accounted for.