Nuclear plant temperature instrumentation

Abstract

Most critical process temperatures in nuclear power plants are measured using resistance temperature detectors (RTDs) and thermocouples. In addition to excellent reliability and accident survivability, nuclear safety-related RTDs are expected to have good calibration and fast dynamic response time, as these characteristics are important to plant safety and economy. In plants where RTDs are installed in thermowells in the primary coolant pipes, response-time requirements have a range of 4.0–8.0 s versus the direct-immersion RTDs installed in bypass loops which have a required response range of 1.0–3.0 s. The variety of problems that can affect the accuracy and response time of RTDs is extensive: dynamic response problems, failure of extension leads, low-insulation resistance, premature failure, wrong calibration tables, loose or bad connections, large EMF effects, open elements, thinning of the platinum wire, lead-wire imbalance, seeping of chemicals from the connection head into the thermowell, cracking of the thermowell, and erroneous indication. The causes of core-exit thermocouples failure can take the form of large calibration shifts, erratic and noisy output, saturated output, accidental reverse connections, and response-time degradation. Several effective methods for detecting RTD and thermocouple performance failure while the plant is operating are available. To detect accuracy problems, the cross-calibration technique is effective for both RTDs and core-exit thermocouples. It involves recording the readings of redundant online RTDs, averaging these readings, and calculating the deviation of each RTD from the average, less any outliers. To detect response time degradation online, the loop current step response (LCSR) test is the most accurate method. However, the noise analysis technique remains the most popular for detecting response time degradation in core-exit thermocouples.