Neutron Shielding - doe-std-1128-98

DOE-STD-1128-98

Table 6.11 gives examples of how effective various gamma shielding

materials are in reducing the dose rates from low-exposure (6% 240Pu)

and high-exposure (19% 240Pu) sources. The sources consist of cylinders

containing 1 kg of plutonium oxide; the dose rates are given at a distance

of 2 m from the source. This example is typical of the shielding

effectiveness for cans of plutonium containing kilogram quantities of

plutonium oxide, as might be found in storage vaults.

In contrast, Figure 6.6 shows the reduction in photon dose rates from a

small sample of plutonium oxide power weighing about 100 grams. The

dose rates were measured at a distance of 3 cm from the surface of the

plutonium, which was contained in polyvinyl chloride plastic bags (a

total thickness of 33 mil or 0.85 mm) for radiation measurements. The

isotopic composition of the plutonium was similar to that given in Table

6.12.

Photon radiation is a significant source of exposure, especially during

D&D activities, when most of the plutonium has already been removed.

Much of the photon exposure problem originates from thin dust layers, as

described in the preceding paragraphs. High photon doses often originate

from "streaming" through glove ports from dust layers on gloves. But

there also can be appreciable neutron dose rates, even in supposedly

"empty" glove boxes, from plutonium hold-up, especially in fluorinator

glove boxes where there is a high neutron emission rate from alpha-

neutron reactions. Wearing lead-loaded aprons can reduce dose rates by a

factor of 2 in plutonium fuel manufacturing. High-exposure plutonium

(>10% 240Pu) should be handled in glove boxes with lead-loaded

Neoprene gloves although some loss of mobility and dexterity may

result. The photons from plutonium are easily shielded by several

millimeters of lead or iron, but it requires almost 15 cm (6 in.) of

polyethylene or hydrogenous moderator to reduce neutron doses by a

factor of 10. Simplistically stated, the gamma dose rate is a function of

surface area, while neutron dose rate is a function of the mass of the

plutonium and its chemical form.

6.4.3.2 Neutron Shielding

The neutron radiations from plutonium are much more difficult to shield

than the photon radiations. As a rule of thumb, it requires about 15 cm of

hydrogenous shielding to reduce the neutron dose rate by an order of

6-35