Distance - doe-std-1128-98

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.

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