External Dose Control - doe-std-1136-20040166

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

6.4.5 Dose to Lens of Eye

It is sometimes assumed that if the skin dose limit is not exceeded, the dose limit to the lens of the

eye will not be exceeded. Such assumptions should be well supported by calculations or (preferably) actual

measurements. See Figure 6-3 for data indicating significant uranium beta penetration of even face shields.

It is suggested and is a common practice in most fabrication areas to require the use of safety glasses, a

practice which tends to mitigate this concern.

6.5 EXTERNAL DOSE CONTROL

Reduction of personnel doses to levels that are ALARA is largely a matter of common sense

applied to the pr inciples of time, distance, and shielding. The first step in any dose control program is to

adequately identify, characterize, and measure the radiation fields. Only after this step has been performed

can optimum dose control be achieved for a given amount of time, money, and energy. However, other

considerations may be just as important. Good housekeeping practices are vital to keep dose rates low.

Even invisible dust layers on the interior surfaces of glove boxes can increase radiation fields. Storing

gloves inside the glove box when not in use and placing lightweight "pie plate" shields over the glove-port

openings are examples of practices that can significantly reduce dose rates.

6.5.1 Time

As a general rule, a reduction in exposure time will yield a reduction in doses. Any operation that

involves high dose rates (more than a few mrem/hour) or extended exposures should be reviewed for

possible reductions in a worker's exposure time. Traffic and material flow in proposed facilities should be

closely examined for opportunities to reduce exposure time.

6.5.2 Distance

Beta dose rates from uranium and its decay products decrease rapidly with distance from the source

due to geometry and air shielding while gamma and neutron radiation decrease less with distance due to

scattering buildup. Because uranium facilities usually involve a high percentage of contact work,

considerable dose reduction can result from simple techniques to make operations semi-remote and allow

workers to function. Even short distances can effect significant dose reductions.

6.5.3 Shielding

Shielding is probably the most widely used (and most effective) method of reducing beta doses

from uranium. Relatively lightweight, cheap, and flexible shielding (e.g., plastic or rubber mats) has been

used effectively. Figure 6-3 demonstrates the spectral basis for shielding and lists a few protective clothing

reduction factors. Table 6-10 lists the thicknesses of common shielding materials necessary to stop

essentially all of the beta particles from uranium (i.e., 234mPa). Generally, the less dense shielding materials

are used whenever possible to eliminate bremsstrahlung as well as beta radiation fields.

Protective clothing commonly worn in the nuclear industry can also afford beta dose reduction.

Figure 6-3 and Table 6-11 list approximate dose reduction factors provided by such clothing. Particular

attention should be paid to the use of gloves for "hands-on" work. Although lightweight rubber gloves

provide some reduction, consideration should be given to using heavy leather or even leaded gloves for

operations that do not require manual dexterity. Such gloves can be particularly effective in handling

materials emitting high beta fields from unsupported uranium decay products.

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