An Improved Methodology for Addressing Coolant Inlet Temperature Variations When Calculating NOP Trip Set-Points

Abstract

Throughout the operation of Pickering A, reactor inlet temperatures have varied with a general tendency upward. Current neutron over-power (NOP) analysis methods do not account for these variations and thus very conservative NOP detector calibration penalties are applied. The shutdown system enhancement (SDSE) to be installed on Pickering reactors, units I to 4, will involve the installation of an in-core NOP system with 29 flux detectors distributed throughout the core. An investigation of reactor coolant inlet temperatures and neutron over-power trip set-points was completed for Pickering A. This was done in order to more accurately account for systematic temperature variations between inlet headers and between channels within those headers, when calculating NOP trip set-points. Also, a method was devised for addressing changes in inlet temperatures when calibrating SDSE NOP detectors. The results of this investigation indicate that when calculating NOP trip set-points, scaling the critical channel powers before performing the Monte Carlo analysis is an effective method of accounting for systematic inlet header temperature variations. Using this technique, it was further shown that SDSE NOP detectors should be calibrated using a temperature correction to the channel power peaking factor (CPPF), where the hottest reactor inlet header (RIH) temperature (an average of FINCH temperatures) is used.