A new approach to the statistical setpoint drift management

The nuclear industry has widely adopted the approach suggested in ISA-RP67.04 for ensuring that the setpoints for safety-related instrumentation are established and maintained within the tech spec limits. The current setpoint drift analysis method corresponding to a dashed rectangle area in Figure 1, is closely related to the estimation of the maximum expected drift value, so-called 95/95 tolerance limits (TL). The 95/95 TL means that we are 95% sure that 95% of all past, present, and future drift values will be bounded by the limits. However, the current approach is inefficient when applied to the drift management, because it is no more than a guideline for determining a setpoint under uncertainty as well as it has some deficiencies such as the incompleteness of the analysis procedure (e.g., the scope bounded by the sample size of 30 or above), the absence of the drift pattern/trend analysis, etc. In this paper, we propose a new statistical analysis procedure for the setpoint drift management using the as-found (AF; prior to any adjustment) and as-left (AL; required if there is a need for adjustment) data. Some illustrations and the SeDA (Setpoint Drift Analysis) program to implement the new procedure are described in the technical report written by the authors