Biological Indicators

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

energy, knowledge of the neutron energy spectrum is important in accident dosimetry. In criticality

accidents, neutrons with energies greater than 100 keV contribute most of the dose; therefore,

measurement of the fast neutron dose is of the most importance. See Delafield (Delafield 1988) for

a review of the different types of neutron dosimeters available for accidents.

Gamma-Measuring Component of Dosimeter. Delafield noted the ratio of the gamma rays to

neutron dose will vary according to the type of critical assembly and whether or not additional

shielding is present. For unshielded assemblies, the gamma-to-neutron ratio can range from 0.1 for

a small heavy-metal system up to approximately 3 for a small hydrogen-moderated solution

system. A concrete or hydrogenous shielding material will increase the gamma-to-neutron ratio.

Gamma dose can be determined by TLD, film, or radiophotoluminescent glass.

Dosimeter Comparison Studies. Sims and Dickson (Sims and Dickson 1979; Sims 1989) present a

summary of nuclear accident dosimetry intercomparison studies performed at the Oak Ridge

National Laboratory Health Physics Research Reactor. The more recent summary showed that of

the 22 studies conducted over 21 years, 68% of the neutron dosimeter results were within the 25%

accuracy standard and 52% of the gamma dosimeter results were within the 20% accuracy

standard. Most measurements that failed to meet the accuracy standards overestimated the actual

dose. Some of their other findings include the following:

Doses from hard neutron energy spectra are more accurately measured than those from

soft energy spectra.

The threshold detector unit (TDU) is the most accurate type of nuclear accident neutron

dosimeter; however, its use is declining due to increasingly strict control of small

quantities of fissionable materials.

Activation foils (ACT) are the most popular nuclear accident neutron dosimeter.

For gamma dosimeters, TLDs are the most popular and the least accurate, and film is the

least popular and the most accurate.

7.4.2.3 Biological Indicators

Earlier in this section, a quick-sort method was described that uses neutron activation of sodium in

the blood as an indicator of worker exposure. More sophisticated laboratory analysis of blood samples

can be performed to obtain a more accurate estimate of worker dose (Delafield 1988; Hankins 1979). The

use of neutron activation of sulfur in hair (32S(n,p)32P) is another method to estimate absorbed dose for

workers involved in a criticality accident. The orientation of the subject can also be determined by taking

samples of hair from the front and back of the person. Hankins described a technique for determining

neutron dose to within 20-30% using a combination of blood and hair activations. The evaluation was

independent of the worker's orientation, of shielding provided by wall and equipment, and of neutron

leakage spectra.

7.5 RESPONSIBILITIES OF RADIOLOGICAL CONTROL STAFF

The radiological control staff should have a basic understanding of program structure, engineering

criteria, and administrative controls as related to nuclear criticality safety and reviewed in earlier sections

of this chapter. However, the health physicist's primary responsibilities with regard to nuclear criticality

safety include emergency instrumentation and emergency response actions.

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