Redundant Reactivity Control System

In the event a nuclear power reactor fails to scram when a transient occurs (an Anticipated Transient Without Scram or ATWS event), alternate means of reducing reactivity may prove desirable. Therefore, a Redundant Reactivity Control System (RRCS) was designed to prevent and, if necessary, mitigate the consequences of such an event. This system provides both an alternate method for the insertion of control rods and the means to flood the reactor core with a sodium pentaborate solution. The design includes a redundant pair of electronics cabinets which determine the presence of an ATWS event and, following pre-programmed logic, issue the necessary control initiation signals. In these cabinets, all essential logic and timing functions are implemented through the use of microprocessor based Universal Logic Cards. Other design features include highly modularized construction, microprocessor limit checking of internal power supply and bus voltages, and the switching of ten ampere loads across safety divisional boundaries. In order to provide a high assurance of operability, a microprocessor controlled self-test system is incorporated to continuously check operability of the safety related hardware in each cabinet. This paper describes the requirements for and the design of the Redundant Reactivity Control System (RRCS) cabinets and the cabinet environmental qualification testing performed.