Criticality Alarm System - STD-1128-98master0215

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

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

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