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.
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.