Measured Gamma Dose Rates - STD-1128-98master0171

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

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

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