Rod control cluster assembly misalignment detection in Westinghouse pressurized water reactors

This paper discusses the measurement problems associated with the detection of misaligned Rod Control Cluster Assemblies (RCCA's) using moveable in-core flux detectors at Westinghouse (W) Pressurized Water Reactors (PWR's). Typical measurements are presented to demonstrate the effects of detector/RCCA proximity and detector alignment on the RCCA detection reliability. Detection of misaligned RCCA's is an issue which reactor engineers face in connection with the Technical Specification Limiting Conditions for Operation (Tech Spec LCO's) associated with RCCA position. The Tech Specs LCO's require that all RCCA's be within 12 steps of their group demand, or step counter indicated position. In most Westinghouse PWR's the individual RCCA positions are monitored by the analog Rod Position Indication (RPI) system. The Tech Spec compliance is accomplished by periodically comparing the RPI positions to the step counters. The problem is that the accuracy of the analog RPI system may drift with time and temperature. The result is that there may be times when the RPI's indicate that an RCCA position deviates from that of the step counter by more than 12 steps