July-August 2018 NPJ

28 NuclearPlantJournal.com Nuclear Plant Journal, July-August 2018 NuScale Power... ( Continued from page 27) Once the NPM is restored in its operating bay within the reactor pool and the RCS has been brought to “hot shutdown” conditions, the cold tests shall be repeated. However, methods utilized for the “hot” tests must be hands-off or remote. The purpose of these repeated tests at “hot” conditions is to measure the in-situ performance of the RTDs at near normal operating conditions and, if necessary, extrapolate the results to actual operating conditions using the empirical correlations developed for the RTDs through previous laboratory experiments. It should be emphasized that the NPM cannot attain nuclear criticality until after it has been shown through in- situ measurements that the performance of the I&C sensors at the normal operating pressure and normal operating temperature (NOP/NOT) of the plant meets the plant technical specifications. If the NPM cannot reach NOP/NOT prior to achieving criticality, then laboratory correlations may be used in conjunction with the “hot” test results to account for the effects of higher pressure and temperature on sensor performance. 3. Mode 1 – “At Power” Tests (During Normal Operation) After the plant has achieved criticality and is operating, the actual “in-service” performance of the RTDs can be measured. At this point, if a sensor fails (e.g. cable or connector issue, poor response time, etc.); there is no opportunity for hands-on maintenance while the plant is operating, and the NPM must be shut down and disassembled to resolve the failure. Therefore, the cold tests are extremely important for saving time and reducing maintenance costs. Once baseline in-service performance has been established, OLM technologies can be used to monitor sensor performance and enable predictive maintenance of the RTDs and other I&C sensors or plant equipment. OLM enables plants to transition from time-based to condition- based maintenance which saves time and plant resources, especially during an outage. For integral SMRs like the NPM, incorporating OLM into the I&C architecture for sensor performance and aging management will be essential to the safe and efficient operation of the plant. Temperature Sensor Response Time Versus Fluid Flow Rate The response time of a temperature sensor is a function of its physical properties and of its surroundings including the process fluid flow rate and temperature. For iPWR SMRs that rely on natural circulation like the NPM, RCS flow velocities are expected to be much lower than conventional PWRs with forced circulation. The response time of a typical nuclear grade RTD is very sensitive to changes in flow at low flow velocities and generally increases as flow decreases. Response time data collected by AMS on a nuclear grade thermowell-mounted RTD at various flow velocities is plotted in Figure 2. From this data set, it is evident that the sensitivity of RTD response time to flow increases for low flows. In addition, at very low flow velocities below 2 feet per second, the minimum fluid transit time from the NPM reactor core to the hot leg section where RCS RTDs are located is Figure 2. Laboratory test results for response time of a typical well- mounted nuclear-grade RTD as a function of fluid flow rate. Figure 3. NuScale Vessel Internals with Location of Safety-Related RTDs.