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DOE-STD-1128-98
Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities
6.0
EXTERNAL DOSE CONTROL
The purpose of an external dose control program is to protect the individual radiation worker by
minimizing dose to levels as low as reasonably achievable (ALARA) and preventing exposures
above prescribed limits. This also implies minimizing the collective dose by summing all the
individual total effective dose equivalents in a specified population. This section discusses methods
to minimize exposures by characterizing the radiations emitted by plutonium and effective methods
to shield or otherwise reduce exposures.
The Department of Energy provides a detailed explanation of the recommendations for external
dosimetry in the Implementation Guide. External Dosimetry Program (DOE, 1999e). Specific
applicable documents for external dosimetry are listed in the reference list of that Implementation
Guide. Because the requirements and recommendations are explicitly given in these documents,
they will not be discussed in any great detail in this section. Rather, the emphasis will be given to
items that are unique to plutonium facilities and the radiological aspects for safe handling of
plutonium.
Measuring the external radiation exposure and the resultant dose for personnel handling plutonium
is a difficult task because of the many radiations involved. Examples of the radioactive decay
schemes and radiations emitted were presented in Section 2.0 for the various plutonium isotopes
and radioactive progeny. Plutonium has a wide distribution of gamma energies; literally hundreds
of different photon energies are present. Fortunately, plutonium emits few high-energy photons, so
photon dose rates are low. But plutonium also emits highly penetrating neutrons from spontaneous
fission and alpha-neutron reactions from compounds and alloys.
In the past, most of the dose in plutonium facilities was the result of plutonium production and
fabrication operations. Most of these operations involved physical contact with freshly separated
plutonium in glove boxes during fabrication and assembly operations. With the reduction in
weapons production, emphasis has shifted to dismantlement and storage operations and to D&D of
plutonium facilities. Much of the material in these facilities is low-exposure plutonium containing
6% 240Pu that is at least 20-years-old, so a significant fraction of the 241Pu has decayed into 241Am.
The radioactive progeny have increased gamma dose rates, making dismantlement of plutonium
facilities more difficult. Although many of the examples in this section involve higher-exposure
plutonium, it is expected that most dosage in plutonium facilities will originate from clean-up and
storage of weapons-grade plutonium.
6.1
DOSE LIMITS
Limits of interest used for control of external radiations are specified at various depths by 10 CFR
835 (DOE, 1998a) as well as the ICRP and the National Council on Radiation Protection and
6-1


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