EDAC 21: Criticality Alarm System for detection, alarm and recording of criticality accidents

Features

• Unique immediate and dual detection of the emission of gamma and neutron flux
• Complies with the CEI 860 ed. 87, ISO 7753 ed. 87 and ANSI/ANS-8.3-1997- EEC standards certified
• Activates alarms for immediate evacuation of personnel (response time 300 ms)
• High reliability of technology and components redundancy of the circuitry
• Uninterrupted test allowing permanent control of proper operation
• 2 to 4 probes per monitored area
• Remote supervision
• Qualified under real excursions of criticality at SILENE reactor of the French Atomic Energy Commission (CEA).

Description

A criticity accident results from the accidental occurrence of conditions favourable to the generation of an uncontrolled chain reaction involving the emission of an intense flux of neutrons and photons.

With more than 1000 channels of detection units delivered over the last 15 years, CANBERRA is a keyactor in the field of criticality accident detection system.

The detection principle of the accident was worked out from the studies of the CEA (French Atomic Commission) on the CRAC and SILENE criticality equipments. These studies had shown that it was necessary to take as reference the neutron and gamma flux to which the personnel was exposed, these parameters being very dependent on the conditions of the accident (solid or liquid phase, fast or slow kinetic) and their respective effects which are of utmost importance for estimating consequences.

For all sites in which fissile materials in various forms are handled or stored (reprocessing, manufacturing fuel plant, research centres), it is necessary to detect as quickly as possible a criticality accident in so far the only possible protection for personnel is the immediate and fast exit from the critical zone as of the generation of the alarm.
The EDAC 21 criticality accident detection system consists of an uninterrupted follow-up, data recording and immediate evacuation of personnel.

Real time information on the accident kinetic constitutes an essential asset for managing the crisis.

Composition of the system
The system is composed of a cabinet with probes, processing unit, alarms, and includes:

• a monitoring cabinet managing 2, 3 or 4 detectors integrated into the processing unit or up to 200 m away from this one. The circuits, highly selected for their reliability, are redundant: tests can be carried out without interruption of the monitoring process
• a processing cabinet controlling the monitored zone. This cabinet also ensures both control and delay functions, an optional supervisor being located remotely, in safe zone up to 1 km from the system
• a power unit including generators of audible and visual alarms (1 audible alarm with 2 tones and 2 flashing lights)
• a power supply unit ensuring a 16 hours autonomy for monitoring and 30 minutes for alarm, in accordance with the international standards (second battery available as option).

On the front panel, indicators and control units are gathered so to ensure local control of detection devices and alarm generators:

• 4 green "correct operation" indicators
• 4 red “emergency” indicators
• 2 red “emergency” flashing indicator
• 2 indicators for mains
• Manual test of the cabinet with selection by key switches and release pushbuttons
• Individual or general tests of the probes (measurement displayed using bargraphs) These tests are achieved under control of an operator who can desactivate them in case of possible criticality accident
• Test of the cabinet indicator lights by key switches and pushbuttons
• Release of defects, audible and visual signals by pushbuttons
• As an option, the system is associated with 1 or 2 supervisor PCs on which the PANORAMA supervision software is implemented; this software is in charge of system monitoring, remote and real time follow-up of the events, remote control of the probe gain and edition of the logbook and probe test as described above.

• For a rate of false alarm of about 10 -7 :
  - Development of the alarm into two phases (on total neutron and gamma dose, and dose rate)
  - Application of the detection principle using a 2/n majority logic with n > 2 (at least 2 probes in alarm or alarm set-off)
• Confirmation of the accident and estimation of the ambient dose rate (on optional and external PC)
• Draft of all significant events which have occurred during the system operation by emission of a logbook giving the type of event (emergency, alarm or defect) and the date
• Remote operation of the supervisor thus operating as a remote cabinet for management of probe sensitivities (optional)
• Remote operation by supervisor located up to 1 km from processing unit by ETHERNET connection (optional).

Monitoring unit - Probe characteristics
Monitoring of the plant and surrounding buildings is ensured by a set of 2, 3 or 4 probes, each with:

• spectral response balanced in a mixed flux of referenced gamma and neutrons
• development of the alarm signal into 2 phases :
  - dose = 25 x 10 ^-6 Gy
  and
  - dose rate = 10 x 10 ^–3 Gy/h approximately (neutron + gamma)
• linear response and non saturation between 10 ^–3 Gy/h and 10 ² Gy/h
• possibility of detecting accidents with extremely slow dynamic corresponding to very low reactivities
• monitoring radius: 30 meters in the air for a threshold of 250 x 10 ^-6 Gy
• constant dose response of the doubling times: 40 ms < T< 40s
• continuous assessment by light source (LED), integrated to the probe
• estimation of the ambient dose rate (optional)
  The dynamic of the probe (10 ^–1 Gy/h by 10 Gy/h) is automatically adjusted when the accident occurs, and automatically return to the initial dynamic 10 ^-3 Gy/h to avoid signal saturation
• Dimensions : length 510 mm, diameter 100 mm
• Mass : 2 kg.

Remote supervision on external PC
The graphic interface makes it possible to know in real or delayed time:

• Data status for each probe (correct operation, Alert, Measurement)
• Probe parameters (ã and n sensitivity, dose rate threshold)
• History of the events (storage, histogram)
• Real time display in histogram mode of the last accidental events (recording mode)
• As an option, calls upon criticity alarms for operator obligation management (pager, mobile phone, e-mail, etc.)

This information is shown on specific screens and protected by password.

System validation test
Before system delivery and installation, probes are calibrated for accurate responses.
Optionally, each system can be tested and validated on the SILENE experimental equipment of CEA.

Choice of the probe location
Optionally, an expertise can be carried out to determine the best possible number and position of the detectors to ensure reliable detection and emergency functions (Please consult us).