Criticality Alarm System cont'd

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.

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