Distance - STD-1128-98master0198

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

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

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