Resistance Thermometer Devices (RTDs) are used to monitor the temperatures in sensitive locations of process piping, such as the hot and cold legs of the primary system of pressurized water reactor nuclear power plants. The RTDs are housed in thermowells that protrude in the cold and hot leg pipes. Eight of the twelve cold leg RTDs in the San Onofre Nuclear Plant have a direct safety function in the operation of the reactor. They provide the Core Protection Calculators with two temperature measurements from each of the cold legs. During the period 1997–2004, four RTD failures occurred in the cold legs of the San Onofre Nuclear Plant. Extensive investigations showed that all of the RTD failures were caused by cracking and fracturing of the platinum wires in the sensors. There was no damage to the nozzles or thermowells of the cold leg RTDs. Also, none of the RTDs in the hot leg were damaged. These failures occurred after the Cycle 9 refueling outage, when the Reactor Coolant System thermowells were replaced due to potential cracking of the Inconel 600 material. A root cause analysis was performed to identify the reasons for the failure of the RTDs in the reactor cold leg piping. The thermowells used for both the hot and cold leg RTDs are identical. The thermowells are 11.375” (289 mm) in length. Since the thickness of the cold leg pipe is 0.75” (19.1 mm) less than the thickness of the hot leg pipe, the cold leg thermowells protrude 0.75” (19.1 mm) more into the flow stream as compared to the hot leg thermowells. The protrusion lengths of the hot and cold leg thermowells in the pipe are 2.5” (63.5 mm) and 3.25” (82.6 mm), respectively. The increased protrusion length of the thermowells in the cold leg significantly lowers their natural frequency (as compared to the hot leg thermowells), which results in vortex-induced vibrations at lower flow velocities that cause failure. Also, the increased protrusion length results in increased amplitude of vortex-induced vibration of their tip that exceeds the capability of the installed RTDs. Thus, a recommendation was made to decrease the protrusion length into the cold leg pipe from 3.25” (82.6 mm) to 2.5” (63.5 mm). Since this recommendation was implemented more than eight months ago, there have been no RTD failures in the cold legs.

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