External Dose Control - doe-std-1128-98

DOE-STD-1128-98

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 Measurements (NCRP). The limits are given in Table 6.1 for the appropriate

depths in tissue for the whole body, lens of the eye, "unlimited areas of skin," forearms,

and hands and feet.

6-1