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External Dose Reduction - doe-std-1128-98_ch10185
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Neutron Shielding - doe-std-1128-98_ch10187


DOE-STD-1128-98
6.4.2
Distance
Because of the inverse square relationship with discrete radiation sources,
significant dose reductions can be achieved by increasing the distance between
the worker and the plutonium source. Also, the low-energy photons emitted
through glove ports and bag-out ports can be attenuated by several feet of air.
Most plutonium operations involve contact work, so increasing the distance may
not always be practical. But significant reductions in doses can be achieved by
reducing plutonium inventories in glove boxes. It is good practice not to store
plutonium samples in glove boxes, but to remove them to storage vaults or other
shielded locations. In many cases, the plutonium samples can be stored in the
glove box in "wells" or specially shielded areas at some distance from the work
areas where the plutonium technicians spend most of their time. The best method
of reducing neutron dose is simply to remove the plutonium from the glove box
and minimize inventories in the glove box.
6.4.3
Shielding
The most practical method of reducing doses in plutonium operations is to apply
shielding. Plutonium emits both neutrons and photons, which require different
types of shielding materials to be effective. There are also additional constraints
that must be met, such as the maximum thickness of shielding that can be placed
on glove boxes and still retain worker mobility. It has been found that more than
about 8 cm (4 in.) of shielding on the exterior surface of a glove box greatly
reduces the worker's manual dexterity and efficiency. It is also important to place
the shielding close to the plutonium source and not to try to shield personnel.
Because neutrons scatter around shadow shields, it is usually best to shield all
surfaces of glove boxes or storage areas. The following sections describe the
shielding effectiveness of common photon and neutron shielding materials.
6.4.3.1 Photon Shielding
Because of the preponderance of low-energy photons, significant
reductions in gamma doses can be achieved by even modest shielding. It
is important to note that there is a significant amount of self-shielding in
plutonium samples. A 1- mm-thick plutonium metal sample is "infinitely
thick" and additional thicknesses will not appreciably increase the dose
rate. For this reason, the photon dose is more dependent on the surface
area rather than on the mass of plutonium. Invisible dust layers on gloves
and interior surfaces of glove boxes can produce high exposure rates,
especially if the gloves are pulled outside the glove box for storage to
prevent them from being caught in machinery. Simple iron or lead
shields placed over the glove ports can reduce the dose rates near the
glove box by an order of magnitude. Modest gamma shields of 6 mm
(0.25 in.) of lead and 13 mm (0.5 in.) of lead-loaded x-ray glass are
usually sufficient to reduce photon dose rates from plutonium to
acceptable levels.
6-34


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