Paul Sirianni

Westinghouse is developing an In-Rod Sensor System (IRSS) [1, 2] capable of measuring key fuel rod operational parameters such as center-line fuel temperature, rod internal pressure, and axial fuel pellet stack elongation, all without the need of a penetration to the sealed fuel rod boundary. The sensor operates by utilizing a low frequency magnetic field to transmit data through numerous layers of metal barriers. When parameters are measured in real-time and combined with fuel performance model and atomistic modelling results, the time required to qualify new fuel technologies is greatly reduced, supporting Accelerated Fuel Qualification (AFQ) efforts. Furthermore, when applied to a commercial power reactor, operators may use the information gathered by the sensor to reduce safety margin uncertainty and operate closer to plant technical specification limits – thereby increasing economic benefits. The sensor system is under active development for Light Water Reactor (LWR) based fuel assemblies but can also be applied to Gen IV reactor fuel designs with minimal modifications. This paper will focus on recent test results from the Penn State Breazeale Reactor (PSBR). The test campaign demonstrates full system performance in a mock-up fuel rod bundle assembly adjacent to the PSBR core. The testing at PSBR is the latest in a campaign of prototype tests, including those conducted at the Massachusetts Institute of Technology Reactor (MITR) and Oak Ridge National Laboratory’s High Flux Isotope Reactor (ORNL HFIR). The results from those tests were covered in the previous NPIC-HMIT conference [3].

Westinghouse is developing an In-Rod Sensor (IRS) System [1, 2] capable of measuring critical fuel rod parameters such as center-line fuel temperature, rod internal pressure and axial fuel pellet stack elongation without having penetrations to the fuel rod. The technology is similar to the Halden Reactor instrumentation [3], but the IRS system focuses on commercial fuel rod instrumentation. Both systems circumvent fuel rod penetrations, however the IRS system overcomes unique geometric challenges found in commercial fuel rod assemblies. The IRS could also play a key role in accelerated fuel qualification by allowing real-time measurements during separate effects tests, integral fuel testing or during irradiation of lead test rods or lead test assemblies. Combining these measurements with data acquired from targeted separate effects testing and multi-scale modelling could significantly reduce the time required to qualify new fuel and reduce the safety margin uncertainty of operating plants. The IRS is actively in development for Light Water Reactors (LWR) based fuel assemblies but can also be applied to Gen IV reactor designs with minimal modifications.

This report summarizes the in situ data collected during three cycles of irradiation (~75 days) of the Wireless Instrumented Removable Beryllium Experiment 2021 (WIRE-21) in the removable beryllium (RB) positions of the High Flux Isotope Reactor (HFIR). The experiment was designed primarily to evaluate the effects of high neutron flux and fluence on the performance of wireless temperature and pressure sensors being developed by Westinghouse Electric Company (WEC). WEC’s sensors could provide critical data regarding the evolution of centerline temperatures and pressurization due to fission gas release during fuel operation in light-water reactors (LWRs) or various advanced reactor applications. In addition to WEC’s wireless sensors, many other sensors were interrogated, including thermocouples, self-powered neutron detectors (SPNDs), distributed fiber optic temperature sensors, and passive neutron flux wires and silicon carbide thermometry. WIRE-21 was irradiated for a total of 75 effective full-power days over five full or partial HFIR cycles, resulting in maximum thermal and fast neutron fluences of 5.8 × 1021 and 3.4 × 1021 n/cm2, respectively. Although two exposed thermocouples located inside WEC’s sensors failed during the first irradiation cycle, the remaining 10 thermocouples that were functional at the start of the irradiation survived for the entire duration of the experiment. Measured temperatures were within the range of typical LWR operating conditions: most thermocouples reading in the ~300–400°C range. The four SPNDs all showed a measurable response to changes in reactor power, but each SPND responded differently. The fiber optic sensors show interesting evolutions in reflected signal intensities that depend significantly on the fiber dopants, type of fiber Bragg gratings (FBGs), and the light guiding mechanism (total internal reflection vs. photonic band gap). The discrepancies in the SPND data and the detailed evolution of the fiber optic sensors are being investigated by the Advanced Sensors and Instrumentation program of the US Department of Energy, Office of Nuclear Energy. Multiple temperature and pressure transients were actuated to compare the response of WEC’s sensors with reference thermocouples or ex-core pressure transducers, respectively. WEC’s sensors successfully captured all actuated temperature transients, even after accumulating fast neutron fluence on the order of 2 × 1021 n/cm2. Significant reductions in the inductance in WEC’s six inductors caused some drift in the wireless sensors. These effects appear to be related to rate effects (i.e., higher neutron flux), as they were not observed during previous testing at lower neutron flux. However, WEC’s temperature sensor remained functional even after the inductances decreased. Unfortunately, WEC’s pressure sensor showed no response during pressurization of the metal bellows. Post-irradiation examination will help inform why the sensor did not respond to changes in applied pressure and determine the mechanism for the decrease in inductances.