Criticality Alarm Systems.

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

Criticality Alarm Systems. ANSI/ANS 8.3 (ANSI, 1986a) discusses the

performance and design criteria for criticality accident alarm systems. The criteria

include the following:

-- Criticality alarm systems shall be designed to detect immediately the minimum

accident of concern; the minimum accident may be assumed to deliver the

equivalent of an absorbed dose in free air of 20 rad at a distance of 2 meters

from the reacting material within 60 seconds.

-- Systems shall be designed so that instrument response and alarm latching shall

occur as a result of radiation transients of 1-millisecond duration. The alarm

signal shall be for evacuation purposes only and of sufficient volume and

coverage to be heard in all areas that are to be evacuated. Very high audio

background noise in some areas may require that the alarm be supplemented

with visual signals; however, high background noise is a dangerous situation

that should be prevented by design. Instrument response to radiation shall be

calibrated periodically to confirm the continuing performance of the instrument.

The calibration interval may be determined on the basis of experience but shall

be no less frequent than annually. Tests should be performed at least monthly

and the results of testing should be documented.

The standard does not quantify criteria for reliability or the rejection of false

alarms. Consideration should be given to the avoidance of false alarms as

accomplished by providing reliable single detector channels or by requiring

concurrent response of two or more detectors to initiate the alarm. (ANSI 1986a).

Fixed Nuclear Accident Dosimeters. All DOE facilities that have sufficient

quantities and kinds of fissile material to potentially constitute a critical mass, such

that the excessive exposure of personnel to radiation from a nuclear accident is

possible, shall provide nuclear accident dosimetry for those personnel (10 CFR

835.1304). Requirements for fixed nuclear accident dosimeters are found in DOE

Order 420.1A (DOE, 2003a).

Effluent Monitors. Facilities that deal with unencapsulated plutonium should

have continuously operating effluent monitors to determine whether or not

plutonium is being released to the environment. Effluent monitor criteria is found

in IEC Publications 761-1 and 761-6 (IEC, 1983) and ANSI N42.18 (ANSI, 1974b)

and should be performed. Similar to airborne contamination monitors, effluent

monitors should be tested for air in-leakage at least annually or when seals or "O"

rings are replaced.

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