Regulatory Positions on Safety-Related Setpoints and Instrumentaion of Uncertainty in Wolsong Units

Abstract

This paper presents the regulatory positions on Special Safety Systems Instrumentation Uncertainty for Trip Setpoint and Allowable Values that were developed using the experience gained from Wolsong Units 2,3 &4. The equipment diversity method for the defense against common mode failure is applied to the transmitters of shutdown system (SDS) 2. However the Units experienced an unexpected drift problem with which the performance did not meet the Technical Specification (Tech Spec) Surveillance Requirements (SR). Discussed are the background, status and corrective actions for the drift problem. It is an instrument uncertainty methodology that the designer of safety system should have shown when the drift problem occurred. For deeper understanding of the problem, we present the background of Tech Spec SR for setpoints in Korean PWR and in CANDU reactors. The Setpoint Verification Test and Calibration Test shall be achieved by recording sufficient as-found data to determine the setpoint in terms of the measured process variables prior to any adjustment. We considered the problem using Canadian calibration practices and the as-found/ as-left method for drift surveillance. When an as-found value measured is between an Allowable Value and MAPS value in Setpoint Verification Test and Calibration Test on a single channel, plant operation is acceptable with one affected channel, only if the other channels are checked and reviewed according to the Tech Spec. Finally we present performance and diversity issues. The regulatory view for performance & diversity is that diversity should be applied to reduce common mode failure and enhance reliability. The diverse components can be used provided that there is a reasonable assurance for overall benefit. Equipment diversity should not be justified unless the performance of components of the SDS 2 is equivalent to that of the SDS 1 or is shown within the as-found limit criteria specified by designers. It is recommended that a consistency should be maintained between in-situ calibration procedures and instrument uncertainty methodology.