Neutron Overpower (NOP) system coverage during Bruce NGS A channel turn-around

In the CANDU safety analysis process, a series of design basis accident are analyzed to confirm safety system effectiveness. Among all the postulated accidents, the Large Break Loss of Coolant (LBLOCA) sets the most demanding requirements on the speed and the reactivity depth of the shutdown reactivity devices. In such an accident, coolant discharge out of the fuel channels lead to a reactivity increase and power increase which is turned around by the shutdown systems action within seconds. In March 1993, the issue of fuel string relocation introducing additional positive reactivity for reactors fuelled against flow during a LBLOCA was identified. The combined reactivity effect could lead to power pulses much higher than would rise due to coolant voiding alone. To maintain adequate safety margins in the event of such postulated accident, the eight units of Bruce NGS A and B were derated to 60% full power