Luna has developed a modification of Luna’s high temperature fiber optic pressure sensor that can operate in the harsh environment of a nuclear reactor core.  Previous pressure sensors showed variability with reactor radiation exposure. Luna’s proprietary calibration methods minimize this limitation.  Future models will include active thermal compensation from an internal temperature sensor with minimal radiation sensitivity. A prototype sensor is shown below.

Prototype Status: Alpha Prototype  Luna recently completed long-term reactor endurance testing of fiber optic pressure sensors.  For this test, a reactor grade pressure vessel and heater control system was designed, built, and safely operated.  During active testing days, this system automatically generated pressure cycles to 500 psi, and heated the pressure sensor to 500 °C while the Ohio State University Laboratory Reactor was at full power (450kW).  Over a period of four months of daily reactor runs and weekly active tests, sensor response data was logged, and compared to calibrated pressure and temperature measurements.  As shown in the figure below, the pressure sensitivity of Luna’s pressure sensors did not show significant long-term drift due to reactor radiation exposure, even after a total neutron fluence of 5.15 E18 n/cm2.  Short term fluctuations in the pressure sensitivity are hypothesized to originate from variations in the local temperature of the sensor.  For this reason, the pressure sensors were equipped with a separate internal fiber optic temperature sensor; however, in the plot below thermal corrections were not applied, because the current fiber optic temperature sensors exhibited signs of radiation induced drift.  A separate project is currently developing radiation-insensitive fiber optic temperature sensors to meet this challenge.

Figure 1. The sensitivity of Luna’s pressure sensors was monitored during tests with heating (T=>600°C), without heating (0T), with reactor radiation (R), and without reactor radiation (0R). During each effective full power hour, the reactor at OSU produces the following fluence levels in the designated dry well locations: 1.93 E16 n-cm-2 total neutrons and 4.7 E7 Roentgens gamma in the AIF (a 2.4” dry well just inside the core), and 3.74 E14 n-cm-2 total neutrons and 10.5 E6 Roentgens gamma in the S2 location (a 6.6” dry well just outside the reactor core).