Advanced surveillance of Resistance Temperature Detectors in Nuclear Power Plants

• A two time constant transfer function is proposed to describe the Resistance Temperature Detector dynamics.
• One constant is only related to the inner dynamics whereas the other one is related to the process and to the inner dynamics.
• The two time constants have been found in several RTDs from a Nuclear Power Plant.
• A Monte Carlo simulation is used to properly adjust the sampling time to find both constants.

The dynamic response of several RTDs located at the cold leg of a PWR has been studied. A theoretical model for the heat transfer between the RTDs and the surrounding fluid is derived. It proposes a two real poles transfer function. By means of noise analysis techniques in the time domain (autoregressive models) and the Dynamic Data System methodology, the two time constants of the system can be found. A Monte Carlo simulation is performed in order to choose the proper sampling time to obtain both constants. The two poles are found and they permit an advance in situ surveillance of the sensor response time and the sensor dynamics performance. One of the poles is related to the inner dynamics whereas the other one is linked to the process and the inner dynamics. So surveillance on the process and on the inner dynamics can be distinguished.