IEC 60860-2014 pdf Radiation protection instrumentation – Warning equipment for criticality accidents

4 General requirements 4.1 General characteristics Criticality alarm systems are designed for the automatic and prompt detection of gamma radiation or neutrons from a criticality accident and to actuate immediate evacuation and warning alarms. The primary functions of the criticality alarm system shall be to: – detect a criticality accident as soon as it occurs within the monitoring zone of the detector(s); – actuate an alarm with minimal delay; – achieve a high degree of reliability required by its safety classification and low probability of false alarm; – fail safe by design and reveal failures (single failure shall be indicated but shall not disable the system and result in a potential non-detection of a criticality accident); – be secured against unauthorised adjustment. Secondary functions of the criticality alarm system should be established by agreement between the manufacturer and user. A recommended secondary function should include the ability to measure radiation levels during and following a criticality accident. It shall be possible to test the response and performance of the criticality alarm system without causing personnel evacuation. 4.2 Detection criterion The following detection criterion definition described in ISO 7753 is used. Criticality alarm systems shall be designed to detect promptly the minimum accident of concern. For this purpose, in typical unshielded process areas, the minimum accident may be assumed to deliver an equivalent absorbed neutron and gamma dose in free air of 0,2 Gy at a distance of 2 m from the reacting material within 60 s. Very slowly increasing excursions, while unlikely to occur, may not attain this value. Furthermore, excursions in unmoderated systems will probably occur much more rapidly. In the design of radiation detectors, it may be assumed that the minimum duration of the radiation transient is 1 ms FWHM (Full Width Half Maximum).
4.4 False alarms Particular consideration shall be given, during the design of the criticality alarm system, to minimize false alarms. A redundant system, requiring response from at least two detector channels out of three (2OO3) is one of the methods used in minimising false alarms. If a redundant system is used, alarm or failure of any single channel shall not activate the alarm or render the criticality alarm system inoperative. A warning signal of a detected malfunction shall be provided in this case and the system shall continue to operate as a one out of two (1 OO2) redundant system using the remaining healthy channels. The maintenance requirements shall be kept to the minimum practicable and the equipment shall be designed to facilitate maintenance without causing false alarms.