Measured Gamma Dose Rates - doe-std-1128-98

DOE-STD-1128-98

The age and isotopic composition are very important in determining the dose rate

from plutonium because of the ingrowth of 241Am from the decay of 241Pu, which

has a half-life of only 15 years. (The growth of plutonium daughters was

discussed in detail in Section 2.1.1.) Old plutonium processing facilities can have

high gamma dose rates, particularly from nearly invisible dust layers containing

241Am, which has a 37% probability of emitting a 60-keV photon per alpha

disintegration. A surprising amount of plutonium oxide powder can be found in

dust layers on the interior surfaces of glove boxes because of the very high

density of plutonium. For example, a 0.001-in.-thick layer of plutonium oxide

dust on the 4-ft by 8-ft floor of a glove box can contain almost 200 grams of

plutonium. Even though a glove box has additional iron or lead shielding, high

gamma dose rates can persist because of the photons emitted by dust layers on

the surface of gloves. Covers must be placed over glove ports to reduce gamma

dose rates around plutonium processing lines.

Doses to the extremities are usually dominated by gamma rays in typical glove-

box operations. Extremity dosimeters must be used by all personnel who perform

hand contact operations with plutonium or who are involved in the manual

decommissioning of plutonium facilities. Extremities are defined as the hands

and forearms below the elbows and the feet and legs below the knees. In a

plutonium facility, the contact doses to the hands and forearms are the most

limiting cases. The extremity dose is more limiting than a whole body dose if the

dose gradient is greater than 10:1 over a distance of 1 meter, the maximum

distance from the fingers to the trunk of the body. In most cases, the source is not

at arm's length and the dose gradient needs to be 10:1 or 20:1 for the extremity

dose to be limiting (NUREG/CR-4297, Reece et al., 1985). But in highly

shielded glove boxes, it is possible to have very high extremity doses from dust

layers on gloves; the dose to the torso can be much lower because of shielding

applied to the glove box.

6.2.2.1 Measured Gamma Dose Rates

There is a considerable amount of experimental data for measured

photon dose rates from plutonium glove-box operations as recorded in

progress reports issued by the Hanford Engineering Development

Laboratory from the Personal Dosimetry and Shielding Program. For

example, the photon dose rates were measured on an anthropomorphic

Remab arm phantom inserted into gloves in a plutonium glove box. The

arm phantom contains a human skeleton surrounded by tissue equivalent

fluid inside a molded plastic "skin." Thermoluminescent dosimeters

(TLD-700s) were positioned at various locations along the surface of the

arm phantom and inside tubes inserted into the bones.

Measurements were first made in a "clean" glove box before it was

placed into service. The arm phantom was placed inside the glove and

positioned in contact with a 1-quart steel can (nominal wall thickness of

10 mil or 0.25 mm), containing 1 kg of plutonium dioxide with the

isotopic composition shown in Table 6.4. Measurements were made at

the various locations with the arm phantom inside 20-mil Neoprene

gloves (average thickness 0.021 in., 0.53 mm) and inside 37-mil (0.94-

mm) lead-loaded Neoprene gloves.

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