Criticality Alarms and Nuclear Accident Dosimetry

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

7.4 CRITICALITY ALARMS AND NUCLEAR ACCIDENT DOSIMETRY

Guidelines for criticality alarm systems and nuclear accident dosimetry are presented in this section.

Criticality alarm systems provide rapid warning to individuals in the immediate accident location and

nearby locations to evacuate to a predesignated assembly location. Specific requirements for the criticality

alarm system are found in ANSI/ANS-8.3. Key requirements that may be of interest for the radiological

control staff are summarized in Section 7.4.1. Paxton noted lives have been saved in past criticality

accidents by radiation alarms coupled with effective evacuation procedures. Nuclear accident dosimetry,

discussed in Section 7.4.2, provides the means for determining the dose to workers in the vicinity of the

excursion.

7.4.1 Criticality Accident Alarm System (CAAS)

As specified in ANSI/ANS-8.3, the need for a CAAS 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.

a. 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 CAAS shall be provided to cover occupied areas in

which the expected dose exceeds 12 rad in free air. Nuclear accident dos imetry shall also be

provided, as otherwise required. The CAAS should include a criticality detection device and a

personnel evacuation alarm.

b. 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.

c. 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 Documented Safety Analysis),

then neither a criticality alarm nor nuclear accident dosimetry is needed.

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 meters from the reacting material within 60 seconds. The alarm signal

shall activate promptly (i.e., within 0.5 second) when the dose rate at the detectors equals or exceeds a

value equivalent to 20 rad/min at 2 meters 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.

Criticality accident alarm systems may consist of one to several detectors per unit. In multi-detector

units (e.g., three detectors), at least two detectors shall be at the alarm level before initiating the alarm; in

redundant systems, failure of any single channel shall not prevent the CAAS from functioning.

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