Criticality Accident Alarm Systems (CAAS) are required in nuclear facilities where an accidental criticality excursion could result from operational processes. In general, most production-oriented nuclear fuel cycle facilities require a CAAS system because the fissile material is managed in quantities that exceed critically-safe mass limits.

The EDAC-21 unit is a 3rd generation system, continually building upon the 1970’s design that was introduced and co-developed by the CEA (the French Atomic Energy Commission). The 3rd generation system now available takes advantage of several significant design features that have evolved:

1. Three type of probes: a neutron probe, a gamma probe and a neutron-gamma probe.
2. A neutron and/or gamma-ray detector and alarm logic module reside in each detector head. This technology makes use of a neutron scintillator and a gamma-ray scintillator that provide high-reliability and performance. Because scintillation detectors are used, a lightemitting-diode with a feedback circuit allows continuous state of health monitoring of the detector. Any electronic failure of a probe is communicated back to the control unit and to the Ethernet network. This electronic feedback circuit checks complete functionality of the system, not just a portion of the system.
3. The alarm logic in each probe is sensitive to both criticality excursion scenarios: the “prompt-critical” metal system and the “slow-cooker” non-metal accident.
4. Each control unit, which supports up to four probes (same type), is a completely modular design that easily enables additional units to be placed directly onto the existing system network. All alarm functions and alarm latching mechanisms reside on each “independent and redundant” control unit. Local alarms on each control unit include redundant lights and horns. Remote horns and lights are driven from dry-contacts on each control unit. Alarms can also be broadcast over a network.
5. The system is autonomous. Each redundant set of probes and control-unit electronics will actuate local and remote alarms without the need for an operating system or software. However, the optional network and PANORAMA software that resides on each host computer, allows the facility operations managers and controllers to monitor state of health from the control room, as well as log data on system status and alarm functions.