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Criticality Alarms and Nuclear Accident Dosimetry - doe-std-1128-98_ch10202
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Nuclear Accident Dosimetry - doe-std-1128-98_ch10204


DOE-STD-1128-98
Revision to the ANSI/ANS standard listed above will place the appropriate Section
of DOE Order 420.1A under immediate review by DOE. Revised ANSI standards
shall not be used unless an exemption is granted or it is incorporated into a DOE
Order.
As specified in ANSI/ANS-8.3, the need for criticality alarm systems shall be
evaluated for all activities in which the inventory of fissionable material in
individual unrelated work areas exceeds 700 g of 235U, 520 g of 233U, 450 g of 239Pu,
or 450 g of any combination of these three isotopes.
--
If the fissionable material mass exceeds the ANSI/ANS-8.3 limits and the
probability of criticality is greater than 10-6 per year, a criticality alarm
system shall (DOE Order 420.1A, Section 4.3.3) be provided to cover
occupied areas in which the expected dose exceeds 12 rad in free air. Nuclear
accident dosimetry shall also be provided, as required by 10 CFR 835.1304.
The criticality alarm system should include a criticality detection device and
a personnel evacuation alarm.
Note: In what follows, 10-6 per year is used as a measure of credibility, and does not
mean that a probabilistic risk assessment (PRA) has to be performed. Reasonable
grounds for incredibility may be presented on the basis of commonly accepted
engineering judgment.
--
If the fissionable material mass exceeds the ANSI/ANS-8.3 limits and the
probability of criticality is greater than 10-6 per year, but there are no
occupied areas in which the expected dose exceeds 12 rad in free air, then
only a criticality detector system (i.e., nuclear accident dosimetry) is needed
(DOE Order 420.1A, Section 4.3.3).
--
If the fissionable material mass exceeds the ANSI/ANS-8.3 limits, but a
criticality accident is determined to be impossible or less than 10-6 per year
(per a safety analysis report documentation), then neither a criticality alarm
system nor nuclear accident dosimetry is needed (DOE Order 420.1A,
Section 4.3.3).
ANSI/ANS-8.3 provides several additional requirements regarding criticality alarm
systems. The alarm signal shall be for immediate evacuation purposes only and of
sufficient volume and coverage to be heard in all areas that are to be evacuated.
Information on sound levels of the alarm can be found in ANSI/ANS-8.3. The
alarm trip point shall be set low enough to detect the minimum accident of concern.
The minimum accident of concern may be assumed to deliver the equivalent of an
absorbed dose in free air of 20 rad at a distance of 2 m from the reacting material
within 60 sec. The alarm signal shall activate promptly (i.e., within 0.5 sec) when
the dose rate at the detectors equals or exceeds a value equivalent to 20 rad/min at 2
m from the reacting material. A visible or audible warning signal shall be provided
at a normally occupied location to indicate system malfunction or loss of primary
power. Each alarm system should be tested at least once every three months. An
evacuation drill shall be conducted at least annually.
7-11


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