Molten Sodium Impurity Detection: Investigation of a Fiber-Optic Sensor as a Plugging Meter Alternative

We demonstrate the feasibility of an optical sensor that can monitor changes in dissolved sodium oxide levels [<10 parts per million (ppm)] in molten sodium. Sodium is used as a coolant in some Generation IV nuclear reactors known as sodium fast reactors (SFRs), where isolating the metal from oxygen is paramount for safe and reliable operation. The sensor presented here provides a significantly improved response time compared to the well-established plugging meter technique used in molten sodium systems for measuring dissolved oxygen concentrations of a few ppm. The sensing approach presented is promising as it can overcome several critical limitations of the plugging meter, such as equipment size, measurement time, and power consumption. It can also be a valuable tool in understanding impurity precipitation, deposition, and growth mechanisms in molten sodium. Index Terms— Corrosion tests, impurity detection, molten sodium, optical fiber, optical sensor, sodium fast reactors (SFRs), sodium oxide.