Chemical Sensors for Sodium Coolant Circuits
Liquid sodium, when pure, is chemically compatible with the structural steels of heat transfer circuits. But the presence of dissolved impurities like oxygen and carbon in it, even at a parts per million (ppm) level, can lead to corrosion and mass transfer in these circuits. When oxygen concentration in sodium is high, transport of radioactive nuclides such as 54Mn from the reactor core to other areas would be enhanced [32.1]. Similarly, high carbon activity levels in sodium can lead to carburization of the steels [32.2]. Hence, it is necessary to monitor these impurities in the coolant continuously using reliable sensors. In the steam generator section of the reactor, high-pressure steam (~150 bars) and liquid sodium at near-ambient pressure are separated by a ferritic steel tube with a wall thickness of ~4 mm. Although these steam generator components are subjected to very strict quality assurance examinations before their installation in circuit, development of a defect during their service that ultimately results in a steam leak into sodium is a possibility. Sodiumwater reaction is highly exothermic and produces gaseous hydrogen and corrosive molten NaOH. Molten sodium hydroxide would cause corrosion and erosion of steels and this would accelerate the spread of the defect. It would also initiate attack on nearby healthy ferritic steel tubes resulting in a cascade of failures [32.3,32.4]. Therefore, it is essential to detect a steam leak at its inception itself for initiating remedial actions. When the temperature of the sodium coolant is above 400°C, which would be the case when the fast reactor operates at full power, NaOH and hydrogen gas formed during sodium-water reaction dissolve into sodium. Since this causes increase in dissolved hydrogen concentration in sodium, continuous monitoring of hydrogen levels in the sodium is important for the detection of this type of leak. However, the temperature of the sodium coolant would be low when the reactor is under start-up conditions or under low-power operations. The reaction/dissolution of NaOH and hydrogen gas into sodium is kinetically hindered at low temperatures. Monitoring of dissolved hydrogen in liquid sodium would not be the reliable method to detect a steam leak under these conditions. At low temperatures, the gaseous hydrogen formed is transported as bubbles in the flowing sodium and gets collected in the argon cover gas of the coolant circuit. Since the volume of the argon cover gas plenum is low, the rate of increase in the hydrogen partial pressure in it would be significantly high and the event of a leak can be detected reliably by continuous monitoring of hydrogen in the cover gas.