Section III - Discussion
III. DISCUSSION
Internal dosimetry is the analysis and measurement of radionuclides in
humans or bioassay samples and the evaluation of intakes and doses from
those measurements. It involves evaluation of bioassay data, evaluation
of the intake, distribution, retention, and elimination of
radionuclides, and evaluation of various absorbed doses and dose
equivalent quantities. Internal dosimetry is inherently indirect in
nature. It is not possible to determine the exact organ absorbed dose,
dose equivalent, or effective dose equivalent in a living human being
resulting from an intake of radioactive materials. Internal dose is
usually a derived or inferred quantity, obtained by evaluation of
indirect measurements and computational models. This is particularly
true for alpha- and beta-emitting radionuclides in the body which have
low photon emission abundances. Direct measurements of internalized
photon-emitting radionuclides in organs also may be difficult because of
attenuation and scattering by overlying tissues.
10 CFR 835 contains requirements affecting internal dosimetry programs
throughout DOE and contractor facilities. The salient features are:
-- Internal doses to workers "shall" be evaluated as specified in 10
CFR 835.402 (c).
-- internal doses are added to external doses. The total effective
dose equivalent during a year (TEDE) "shall" be determined by summing
the effective dose equivalent from external exposures (or deep dose
equivalent) and the committed effective dose equivalent from
intakes during the year (10 CFR 835.203(a)). This is done for the
purposes of demonstrating compliance with occupational dose limits
and keeping worker doses as low as reasonably achievable (ALARA).
The quantity committed effective dose equivalent is the mechanism
by which internal doses are added to external doses on the basis of
equal risk.
-- exposures from background, therapeutic, and diagnostic medical
radiation, and voluntary participation in medical research programs
"shall" not be included in the assessment of compliance with the
occupational exposure limits or in dose records (10 CFR
835.202(c)).
10 CFR 835.202 requires that the occupational exposure of a general
employee to radiation or radioactive material resulting from routine DOE
activities "shall" not cause the following limits to be exceeded:
-- Stochastic effects. The annual limit for the TEDE received from
both internal and external sources is 5 rems (0.05 Sv); and
-- Nonstochastic effects. The annual limits are: (i) Lens of the
eye, dose equivalent of 15 rems (0.15 Sv); (ii) Extremity or skin,
shallow dose equivalent of 50 rems (0.5 Sv); and (iii) Any organ
or tissue, a total dose equivalent of 50 rems (0.5 Sv) (10 CFR
835.202(a) and RCM 213).
10 CFR 835.402(c) requires that monitoring of individual exposures to
internal radiation "shall" be performed in accordance with the following:
Internal dose evaluation programs (including routine bioassay programs)
"shall" be conducted for (1) radiological workers who, under typical
conditions, are likely to receive 0.1 rem (0.001 Sv) or more committed
effective dose equivalent and/or 5 rems (0.05 Sv) or more committed dose
equivalent to any organ or tissue, from all occupational radionuclide
intakes in the year; (2) declared pregnant workers likely to receive an
intake resulting in a dose equivalent to the embryo/fetus in excess of
0.05 rem (0.0005 Sv), (3) minors and members of the public who are
likely to receive, in one year, an intake resulting in a committed
effective dose equivalent in excess of 0.05 rem (0.0005 Sv) and (4)
internal dose evaluation programs "shall" be adequate to demonstrate
compliance with 10 CFR 835.202.
The RCM introduces:
-- Administrative Control Levels (RCM 211) that are below dose limits
of the RCM and 10 CFR 835;
-- the Lifetime Control Level (RCM 212) of N rems (N times 0.01 Sv),
where N is the individual's age in years; and
-- Special Control Levels for individualized exposure control (RCM
216).
The administrative control level values apply to TEDE, and the lifetime
control level applies to lifetime occupational dose.
For individuals with a lifetime occupational dose, in rems, exceeding
their age, in years, a Special Control Level of less than 1 rem (0.01
Sv) TEDE shall be established for the present and subsequent years of
the individual's employment (RCM 216). The Special Control Level should
allow the individual's lifetime occupational dose, in rems, to approach
their age, in years, as additional occupational exposure is received
over a period of time. Once established, the Special Control Level may
be discontinued after the criterion of lifetime occupational dose
exceeding an individual's age is no longer met (RCM 216.1).
Several limits besides those for general employees shall be considered
in the design of an internal dosimetry program. A dose equivalent limit
of 0.5 rem (0.005 Sv) from conception to birth is specified for the
embryo/fetus of a declared pregnant worker (10 CFR 835.206) and efforts
should be made to avoid exceeding 0.05 rem (0.0005 Sv) per month to the
declared pregnant worker (RCM 215 and RCM Table 2-1). Annual limits are
0.1 rem (0.001 Sv) TEDE for minors and students under age 18 (10 CFR
835.207 and RCM Table 2-1), members of the public entering a controlled
area (10 CFR 835.208 and RCM Table 2-1), and visitors (RCM Table 2-1 and
RCM 214).
Radiation protection programs for limiting internal exposures are based
on the DOE policy of controlling radioactive material at the source.
One key element in an effective program for minimizing internal
exposures is the control and minimization of contaminated equipment and
contaminated areas. 10 CFR 835.1002(c) requires that the design
objective "shall" be to avoid releases of airborne radioactive material to
the workplace atmosphere under normal conditions and, under any
situation, to control the inhalation of such materials ALARA. It is
nonetheless recognized that low-level, chronic occupational exposures to
some materials are difficult to avoid due to the types of material
handled or processed, their chemical or physical forms, and the nature
of operations, and that incidents may cause unplanned releases of
radioactive material. Either or both of these conditions necessitate an
internal dosimetry program at most DOE and DOE contractor facilities.
10 CFR 835 reflects many of the scientific recommendations of the
National Council on Radiation Protection and Measurements (NCRP) and the
International Commission on Radiological Protection (ICRP). 10 CFR 835
contains both primary and secondary limits for exposure of workers to
external and internal sources of ionizing radiation in the workplace.
The primary limits on internal exposures are expressed in terms of
committed effective dose equivalent. Secondary limits for radiation
protection include annual limits on intake (10 CFR 835.403(a)(1)) and
derived air concentrations (10 CFR 835, Appendices A and C). The ALI
and DAC values are similar to those recommended in ICRP Publication 30,
"Limits for Intakes of Radionuclides by Workers: Design and
Interpretation" (ICRP, 1979); ICRP Publication 48, "The Metabolism of
Plutonium and Related Elements" (ICRP, 1986); and ICRP Publication 54,
"Individual Monitoring for Intakes of Radionuclides by Workers: Design
and Interpretation" (ICRP, 1988). The values were chosen to provide a
safe working environment and to maintain exposures to within the primary
exposure limits.
Air monitoring is the primary method for demonstrating compliance with
workplace control limits. Bioassay measurements that indicate a failure
to control the workplace may indicate the need for improvements in
workplace air monitoring.
Internal dose evaluations, along with associated bioassay measurements,
are the primary methods for demonstrating compliance with dose limits
for protecting workers. 10 CFR 835 states that the estimation of
internal dose "shall" be based on bioassay data rather than air
concentration values unless bioassay data are unavailable, inadequate,
or internal dose estimates based on representative air concentration
values are demonstrated to be as or more accurate (10 CFR 835.209(c)
and RCM 521.2). This is discussed below under Section IV.D.7.
"Supplementing Routine Bioassay Program in Cases Where the DIL is Less
Than the MDA."
Both air monitoring and bioassay results "shall" be used by a facility's
radiation protection organization in managing worker exposures to
maintain them below the limits and as low as reasonably achievable
(ALARA; 10 CFR 835.2, 835.202, and 835.1001; RCM 111).