External Dose Reduction - STD-1128-98master0197

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

proportional counters filled with gasses at different pressures. Each counter

measures a different portion of the neutron spectrum, and quite accurate spectra

can be obtained by "linking" all the measurements together. Unfortunately, this

type of spectrometer is quite expensive, bulky, and difficult to use for workplace

measurements.

Recently, neutron energy spectrometers have been fabricated from superheated

drop detectors. Each type of superheated drop detector responds to neutrons

with energies above a certain threshold. By using a combination of detectors

with different thresholds and appropriate unfolding algorithms, it is possible to

determine approximate neutron energy spectra from simple measurements.

Commercially available units currently tend to suffer from quality control

problems; i.e, the sensitivity per unit neutron fluence varies too much to make

highly accurate spectral measurements. (The sensitivity varies with the degree

of superheat and ambient temperature.) However, this technique offers great

promise as a very simple, relatively inexpensive method for neutron

spectrometry and dose determinations, particularly in facilities with a constant

temperature.

6.4

EXTERNAL DOSE REDUCTION

The traditional methods of using time, distance, and shielding are typically employed in

plutonium facilities to reduce exposures to ALARA levels. 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 create gamma radiation fields of

10 mrem/h or more, especially through lightly shielded glove ports. The practice of pulling

gloves outside for storage should not be condoned in operations that generate dust or powders.

Dose rates of 30 mrem/h have been measured in facilities processing high-exposure oxide

powders. A factor of 30 reduction in dose rate was achieved by merely storing the gloves inside

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

openings.

6.4.1

Time

Obviously, reducing the time a worker is exposed in a radiation field will reduce the

dose. Any operation which involves high dose rates (more than a few mrem/hour) or

long exposures should be reviewed for possible reductions in a worker's exposure time.

For example, a worker should minimize the time spent near a fluorination operation.

After the equipment has been set up, the worker should leave the area during the actual

fluorination step.

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