Modeling the aging effects of nuclear power plant resistance temperature detectors

Most of the critical process temperatures in nuclear power plants are measured using resistance temperature detectors (RTDs) and thermocouples. The primary coolant temperature and feedwater temperature are measured using RTDs. Primary coolant RTDs feed plant control and safety systems. As a result, these RTDs are usually subject to very stringent requirements for accuracy and response time performance. Aging of RTDs refers to decalibration or response time degradation of RTDs with normal environments and under normal operating conditions in a nuclear power plant. Calibration shift could occur due to stress, contamination or metallurgical changes in the sensing element or moisture in the insulation material of RTDs. The effect of temperature is the most critical as the RTD materials have different thermal expansion coefficients leading to stress on the RTD element when temperature changes. Over time, degradation of the response time of RTDs can be induced by changes of its heat transfer characteristics caused by high temperature, vibration and thermal cycling, etc. Past experience showed that air gaps between RTD/thermowell interface play an important role in controlling the response time of RTDs. In this paper, a finite-element multi-physics simulation tool, COMSOL 4.2a, is used to model the aging effects of nuclear power plant primary coolant RTDs. Response time of RTDs installed in the thermowell is characterized by inducing temperature changes on the surface of the thermowell. Key material properties such as the thermal conductivity of the thermowell and the insulation materials of RTDs are varied to represent the materials aging due to corrosion of the thermowell and moisture in the insulation materials of the RTD.