A Probabilistic Model for Assessing the Effectiveness of Shutdown System Trip Parameter Setpoints

Abstract

The shutdown systems in CANDU-PHW power reactors employ triplicated, independent channels of sensors and signals for the parameters that are employed to initiate a reactor trip. Two-out-of three rating is employed in the trip decision logic. An important part of assessing shutdown system capability is the determination of the effectiveness with which trip parameters prevent safety design limits from being exceeded under accident conditions. An appropriate means of incorporating uncertainties into this assessment is through the use of probabilistic modelling of the shutdown system trip function. One such probabilistic model is presented and the application of this model to the assessment of protection provided by neutron overpower trips for loss of reactor power regulation incidents is discussed.