Operating experience with protection system logic in Dhruva

Instrumentation and safety logic of Dhruva protection system comprises of sensors, its processing circuit instrumentation, safety logic, and shutdown devices. The instrumentation monitors various parameters like neutron flux, pressures, temperatures, flows, radiation fields etc. For the instrumentation and associated hardware, three independent channels are used. The trip parameters are classified into three categories viz. absolute and conditional trips (AT/CT), auxiliary absolute trip (XT), and emergency trip (ET). The safety logic processes information received from instrumentation channel and performs various logics, using two out of three coincidence, to provide actuation signals to the shutdown devices. Over the last 11 years, based on operating experience various measures have been carried out to improve the performance and reliability of the protection logic such as improving the independence of moderator dumping in trip generation, addition of trip parameters in XT logic based in trip coverage of postulated initiating events etc. Also some new units were added for better information to operator and also maintenance staff. This paper describes the various aspects of operating experience gained and improvement measures incorporated in system logic to enhance its performance. (author)