Criticality Detection and Alarm Systems: Design Principles

This paper results from a review of the criticality detection and alarm arrangements adopted at the different Establishments of the U.K.A.E.A. and a distillation of the principles which should underlie the future provision and design of criticality detection and alarm systems. The discussions are confined to gamma detection which is used throughout the U.K.A.E.A. The basic tenets are: (a) The object is not the more rudimentary one of detecting any abnormal radiation, it is one of balancing the hazards from abnormal radiation against the hazards and costs associated with the instant evacuation of working plants. Therefore the alarm is not to be operated unless the radiation is of such a nature as could emanate from accidental criticality; but when the radiation is of such a nature then the alarm must be triggered reliably and quickly; and (b) where an alarm system is provided it should be designed to be operated by the minimum foreseeable initial excursion regardless of whether or not the circumstances may be that such an initial excursion of itself could result in unacceptable doses to men. Arguments, resting largely on analyses of data from past incidents as reported by Stratton, are deployed to derive those detailed technical criteria needed by designers of detection and alarm systems, and to quantify them where appropriate. Recommendations are made for a detection threshold for general use, a detection threshold for special use in areas of high ambient gamma background, dose/fission ratios and system response time. Reliabilities of detection systems and of alarm noise generation, transmission and propagation are discussed. Lastly, evacuation principles and facilities are discussed and recommendations made concerning evacuation area boundaries and methods. (author)