Initial Screening Evaluation - doe-std-1128-98

DOE-STD-1128-98

7.4.2.1

Initial Screening Evaluation

A nuclear accident dosimetry program should provide absorbed dose

information within 24 hours after the incident. A nuclear accident

dosimetry program shall include a method to conduct initial screening of

personnel involved in a nuclear accident to determine whether significant

exposures to radiation have occurred (10 CFR 835.1304)[also see ANSI

N13.3 (ANSI, 1969b)]. Discussions on initial screening evaluations to

segregate exposed from unexposed individuals (sometimes referred to as

"quick sort techniques") are found in several references (Moe, 1988;

Delafield, 1988; Petersen and Langham, 1966; Hankins, 1979; Swaja and

Oyan, 1987).

A common initial screening method is to provide all workers in areas

requiring nuclear accident dosimetry with an indium foil in their personnel

dosimeter or security badge. During a criticality excursion the foil will

become activated by neutrons per the 115In (n, gamma) 116mIn reaction and

can be measured with a portable beta-gamma survey instrument or ion

chamber. The 116mIn has a 54-min half-life and releases a 1-MeV beta

(maximum energy) and a 1.3-MeV gamma (80% of the time).

An alternate screening is to measure body activity due to neutron activation

of the sodium in the blood via the 23Na (n, gamma) 24Na reaction. Sodium-

24 has 15-hour half-life and releases a 1.4-MeV beta (maximum energy)

and two gammas (1.37 MeV and 2.75 MeV). A beta-gamma survey meter

is used to measure the 24Na activity in the blood by placing the detector

probe against the individual's abdomen and having the individual bend

forward to enclose the detector (Moe, 1988). Alternatively, the probe can

be positioned under the armpit with the open window facing the chest area.

Moe (1988) noted that this method is less sensitive than the use of indium

foils and even a small reading can indicate a significant exposure. An

approximate equation to calculate worker dose (D) based on body weight

and instrument reading is shown in Equation 7.1:

D ( Gy ) =

80 ( instrument reading in mR / h )

Body weight ( lb )

(7.1)

Differences in incident neutron energy spectrum, orientation, and

measurement techniques relative to conditions used to develop activity-

dose correlations can cause significant errors in estimated radiation dose

based on quick-sort surveys. Swaja and Oyan (1987) showed that radiation

doses estimated from induced body activity can vary by a factor of about 2

due to neutron energy spectrum or orientation effects and by as much as

30% due to probe position. Doses based on indium foil activity can vary by

a factor of about 9 due to neutron energy spectrum effects, a factor of 3

depending on foil orientation relative to the incident field, and a factor of

about 2 due to probe window setting. Swaja and Oyan (1987)

recommended that those count rates above background during quick-sort

techniques should be initially interpreted only as an indication that the

person has been exposed.

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