External Dose Control - STD-1128-98master0165

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