Internal Dosimetry - doe-std-1128-98

DOE-STD-1128-98

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INTERNAL DOSIMETRY

Internal dosimetry is an essential part of a quality health physics program at every facility

where plutonium is handled or processed. The purpose of an internal dosimetry program is

to monitor workplace activities, to assess accidental or inadvertent intakes of radioactive

material, and to conduct internal dose assessments from bioassay measurement data.

It is DOE policy that facilities are designed, operated, and remediated to prevent intakes of

radioactive materials. Radiological controls for the workplace should ensure that

radionuclides are contained and handled properly, and that intakes, if they occur at all, are

negligible to the extent achievable with state-of-the-art technology. In spite of excellent

design and operation policies, inadvertent intakes of radioactive material can occur as a

result of equipment malfunction, failure to follow procedures, or the unanticipated presence

of radioactive material.

Experience has shown that the most common route for inadvertent plutonium intake is

inhalation. Intakes can also occur by accidental ingestion or by wound contamination.

Surveillance programs should be designed to rapidly detect a release in the event of a loss

of radioactive material containment. Internal dosimetry programs should be tailored to the

needs of each plutonium handling facility so that inadvertent intakes are discovered and

quantified and workers' dose equivalents are determined by appropriate methods.

When workers are inadvertently exposed to radioactive material, appropriate corrective

action should be taken to ensure that control and containment have been re-established.

Prompt detection by routine workplace monitoring practices is essential to regaining

control after any contamination spread or loss of containment. Prompt workplace

indications of potential intake are also crucial to start special bioassay monitoring for intake

and dose assessment. An early assessment of the probable severity of an intake and its

corresponding dose, preferably within the first two hours of the intake, is needed for

decisions on dose reduction therapy and event reporting. For plutonium and americium

intakes, the bioassay data necessary for final dose assessment may require long periods of

time (many months) to obtain. Until such data become available, ongoing preliminary

assessments of intake and dose may be necessary to provide guidance for the administrative

and medical management of the workers.

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INTERNAL DOSE EVALUATION PROGRAM

Internal doses are not directly measured but are estimated or calculated based on

knowledge of the material to which a worker may be exposed and it's known or assumed

biokinetic behavior. The common approach to internal dosimetry is to calculate an

occupational intake based on worker bioassay measurements or workplace air-sample data

and assumed breathing rates. Once an intake is calculated, appropriate internal dose

equivalents to organs and tissues of concern can be estimated by using fundamental

dosimetry principles, by various intake-to-dose conversion factors, which incorporate

assumed biokinetic models, or by an appropriate computer code. Intake-to-dose conversion

factors can be found in Federal Guidance Report No. 11 (EPA, 1988b) or ICRP Publication

30 (ICRP, 1979, 1988b). Further discussion on intake and dose assessment is provided in

Section 5.8. DOE is currently evaluating its internal dosimetry methodology and

considering updating it to adopt newer models.

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