Darlington NGS reactor operation with flux tilt impact on neutron overpower protection

This paper discusses the analysis performed to assess the Neutron Overpower (NOP) system trip coverage for Darlington NGS operation with initial side-to-side and bottom-to-top flux/power tilts. The required trip setpoints of the design basis set of flux shapes were calculated using the Monte Carlo statistical method. Initial flux tilt values of up to 20% were considered in the analyses. The analysis shows that the NOP system with the nominal trip setpoint of 126.0 percent provides adequate protection as long as the NOP detector signals are not re-calibrated after the occurrence of the tilt, or are re-calibrated using the CPPF (channel power peaking factor) corresponding to the tilted flux shape. The analysis also shows that for the case of re-calibrating the detector signals using the CPPF corresponding to the nominal un-tilted flux shape, the NOP system provides the required coverage for tilts of up to 6.8 percent. (author). 9 refs., 2 figs