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Nuclear Accident Dosimetry - doe-std-1128-98_ch10204
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Fixed and Personnel Nuclear Accident Dosimeters - doe-std-1128-98_ch10206


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.
7-13


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