Design change No. 3418 Criticality Alarm System (CAS) 105 N

The changes, within the last ten years, in regulatory criteria for concept and design of a Criticality Alarm System (CAS) mandates the installation of a new measurement and alarm system to replace those channels previously assigned to the N-Reactor Tracerlab Area Radiation Monitoring System. The areas to be monitored for a criticality event will be the green fuel storage area, Room 181, the discharge pit area, the segregation basin, and the irradiated fuel storage area. Two identical systems will be installed on opposite sides of the reactor because of the large distance between the areas of survey. Each system will employ three General Electric Log Radiation Monitors (GE LRM) utilizing gamma sensitive ionization chambers as the detecting device. A digital comparator using a ''two of three'' majority minimum will be used to provide the decision for criticality alarm. The criticality alarm circuit will initiate audio and visual alarms. Annunciators will illuminate in the control room and the plant standard audible alarm (ah-oo-gah horn) for a criticality will sound in the area of coverage. The ah-oo-gah horns will be controlled by the Tracerlab Area Radiation Monitoring System as well as by the GE LRM. The ah-oo-gah horns thus will sound for a high level alarm condition as well as for a criticality event