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Gamma Doses - doe-std-1128-98_ch10158
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Table 6-4. Isotopic Composition of the Plutonium Used in the Extremity Dosimetry Measurments


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.
6-7


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