Neutron Shielding - STD-1128-98master0199

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

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.

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 magnitude.

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