RTD

The response time of installed RTDs can be measured using the Loop Current Step Response (LCSR) method. This method is based on heating the RTD's sensing element with a small electric current (about 35 to 80 milliamperes) applied to the RTD's extension leads.

The current causes a temperature transient in the RTD as shown in this figure. This transient is sampled by a computer and analyzed to give the response time of the RTD. The analysis involves a mathematical fitting of the LCSR transient data to an exponential series that has been developed based on a general heat transfer model of RTDs.

The validity of the LCSR method has been established by numerous laboratory tests performed over a five year period in the mid-1970s. The LCSR method has been approved by the U. S. Nuclear Regulatory Commission (NRC) for in-situ response time testing of nuclear plant RTDs.

The advantage of the LCSR test is that it permits remote testing of an RTD as installed in an operating process, and thereby provides the actual "in-service" response time of the RTD. The test accounts for all installation and process condition effects on response time.

Cross calibration is a test that is performed on redundant RTDs in the primary coolant system of pressurized water reactors (PWRs). The test is performed to verify the calibration consistency of RTDs to ensure that accurate steady-state temperature measurements are provided to the plant control and safety systems. The test involves a systematic intercomparison of temperature or resistance readings of redundant RTDs to identify the ones that may have a larger-than-normal deviation from the average reading of the remaining RTDs.

In performing cross calibration, the resistance of all RTDs are measured sequentially and converted to equivalent temperatures using the most recent calibration tables available for the RTDs. The resulting temperatures are then averaged and the temperature deviation of each RTD from the average is calculated (see Table). Any RTD that exceeds a pre-specified temperature deviation is flagged and/or removed from the average and the process is repeated as necessary to identify the outliers (if any). In some cases, instead of reading the resistance of RTDs, their temperature readings are used for the cross calibration test.

Cross calibration tests can be performed at one or more temperature plateaus at isothermal conditions when the reactor coolant hot leg and cold temperatures are at approximately the same temperature. They can also be performed when the temperature is increasing or decreasing monotonically at a reasonably constant rate. The advantage of performing the cross calibration under temperature ramp conditions is that it saves test time as it does not require the plant to hold at plateau conditions for the test.

If cross calibration data is collected at three or more reasonably spaced temperature points during plant heat up or cool down, then a new calibration table can be generated for an outlier RTD using the cross calibration data, saving substantial time and cost by avoiding replacement of the RTD.

Most PWR plants, especially those which have removed their RTD by-pass manifolds, have large temperature fluctuations in their hot legs and to lesser extent in their cold legs. This is due to temperature stratification which is inherent in PWRs and originates in the core because different streams of water that exit the core are at different temperatures. This problem can interfere with accurate measurement of RTD response time using the Loop Current Step Response Method (LCSR). As such, AMS has developed new software, as well as a new procedure, to maintain high accuracy in response time testing results in spite of temperature stratification.